Micro-, nano- and hierarchical structures for

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Phil. Trans. R. Soc. A (2009) 367, 1631–1672 doi:10.1098/rsta.2009.0014

Micro-, nano- and hierarchical structures for superhydrophobicity, self-cleaning and low adhesion B Y B HARAT B HUSHAN 1, * , Y ONG C HAE J UNG 1

AND

K ERSTIN K OCH 2

1

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics, Ohio State University, 201 West 19th Avenue, Columbus, OH 43210, USA 2 Nees Institute for Biodiversity of Plants, Rheinische Friedrich-Wilhelms University of Bonn, Meckenheimer Allee 170, 53115 Bonn, Germany Superhydrophobic surfaces exhibit extreme water-repellent properties. These surfaces with high contact angle and low contact angle hysteresis also exhibit a self-cleaning effect and low drag for fluid flow. Certain plant leaves, such as lotus leaves, are known to be superhydrophobic and self-cleaning due to the hierarchical roughness of their leaf surfaces. The self-cleaning phenomenon is widely known as the ‘lotus effect’. Superhydrophobic and self-cleaning surfaces can be produced by using roughness combined with hydrophobic coatings. In this paper, the effect of micro- and nanopatterned polymers on hydrophobicity is reviewed. Silicon surfaces patterned with pillars and deposited with a hydrophobic coating were studied to demonstrate how the effects of pitch value, droplet size and impact velocity influence the transition from a composite state to a wetted state. In order to fabricate hierarchical structures, a low-cost and flexible technique that involves replication of microstructures and self-assembly of hydrophobic waxes is described. The influence of micro-, nano- and hierarchical structures on superhydrophobicity is discussed by the investigation of static contact angle, contact angle hysteresis, droplet evaporation and propensity for air pocket formation. In addition, their influence on adhesive force as well as efficiency of selfcleaning is discussed. Keywords: superhydrophobic surfaces; biomaterials; biomimetics

1. Introduction Superhydrophobic surfaces with a high static contact angle above 1508 and contact angle hysteresis (the difference between the advancing and receding contact angles) below 108 exhibit extreme water repellence and self-cleaning properties (Bhushan & Jung 2008; Nosonovsky & Bhushan 2008a –c; Bhushan et al. 2009b). At a low value of contact angle hysteresis, the droplets may roll in addition to slide, which facilitate removal of contaminant particles. Surfaces with * Author for correspondence ( [email protected]). One contribution of 9 to a Theme Issue ‘Biomimetics II: fabrication and applications’.

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B. Bhushan et al. hydrophilic a rough surface liquid menisci formed around wetted asperities

a smooth surface

Figure 1. A schematic of condensed water vapour from the environment forming meniscus bridges at asperity contacts that lead to an intrinsic attractive force.

(a)

(i)

(ii)

10 µm

(iii)

2 µm

0.4 µm

(b)

Figure 2. (a) SEM images (shown at three magnifications (i)–(iii)) of lotus (N. nucifera) leaf surface, which consists of a microstructure formed by papillose epidermal cells covered with threedimensional epicuticular wax tubules on the surface, which create nanostructure. (b) Image of a water droplet sitting on a lotus leaf.

low contact angle hysteresis have a low water roll-off (tilt) angle, which denotes the angle to which a surface must be tilted for roll-off of water drops. Superhydrophobic and self-cleaning surfaces are of interest for various applications including self-cleaning windows, windshields, exterior paints for buildings and navigation of ships, utensils, roof tiles, textiles, solar panels and applications requiring antifouling and a reduction in drag in fluid flow, e.g. in micro/nanochannels. These surfaces can also be used in energy conversion and conservation (Nosonovsky & Bhushan 2008d, 2009). Condensation of water vapour from the environment and/or process liquid film can form menisci, leading to high adhesion in devices requiring relative motion (Bhushan 2002, 2003, 2008; figure 1). Superhydrophobic surfaces are needed to minimize adhesion between a surface and liquid. A model surface for superhydrophobicity and self-cleaning is provided by the leaves of the lotus plant, Nelumbo nucifera (figure 2a; Barthlott & Neinhuis 1997; Neinhuis & Barthlott 1997; Wagner et al. 2003; Bhushan & Jung 2006; Burton & Bhushan 2006; Koch et al. 2008a, 2009). The leaf surface is very rough due to Phil. Trans. R. Soc. A (2009)


Micro-, nano- and hierarchical structures

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so-called papillose epidermal cells, which form asperities or papillae. In addition to the microscale roughness, the surface of the papillae is also rough, with submicrometre-sized asperities composed of three-dimensional epicuticular waxes. The waxes of lotus exist as tubules, but on other leaves waxes exist also in the form of platelets or other morphologies (Koch et al. 2008a, 2009). Lotus leaves have hierarchical structures, which have been studied by Burton & Bhushan (2006) and Bhushan & Jung (2006). Water droplets on these surfaces readily sit on the apex of nanostructures because air bubbles fill in the valleys of the structure under the droplet. Therefore, these leaves exhibit considerable superhydrophobicity (figure 2b). Water droplets on the leaves remove any contaminant particles from their surfaces when they roll off, leading to self-cleaning. It has been reported that nearly all superhydrophobic and self-cleaning leaves consist of an intrinsic hierarchical structure (Neinhuis & Barthlott 1997; Koch et al. 2008a, 2009). Water on such a surface forms a spherical droplet, and both the contact area and adhesion to the surface are dramatically reduced (Nosonovsky & Bhushan 2007a, 2008a–c; Bhushan & Jung 2008). Based on the so-called lotus effect, one way to increase the hydrophobic property of the surface is to increase surface roughness; so roughness-induced hydrophobicity has become a subject of extensive investigations. Wenzel (1936) suggested a simple model predicting that the contact angle of a liquid with a rough surface is different from that with a smooth surface. Cassie & Baxter (1944) showed that a gaseous phase including water vapour, commonly referred to as ‘air’ in the literature, may be trapped in the cavities of a rough surface, resulting in a composite solid–liquid–air interface, as opposed to the homogeneous solid–liquid interface. These two models describe two possible wetting regimes or states: the homogeneous (Wenzel) and the composite (Cassie–Baxter) regimes. Many authors have investigated the stability of artificial superhydrophobic surfaces and showed that whether the interface is homogeneous or composite may depend on the history of the system, in particular whether the liquid was applied from the top or condensed at the bottom (Bico et al. 2002; He et al. 2003; Lafuma & Que´re´ 2003; Marmur 2003, 2004; Patankar 2004a). Extrand (2002) pointed out that whether the interface is homogeneous or composite depends on droplet size, due to gravity. It has also been suggested that the so-called two-tiered (or double) roughness, composed of superposition of two roughness patterns at different length scales (Herminghaus 2000; Patankar 2004b; Sun et al. 2005), and fractal roughness (Shibuichi et al. 1996) may enhance superhydrophobicity. Nosonovsky & Bhushan (2007a–d, 2008a–c) identified mechanisms that lead to destabilization of the composite interface, namely capillary waves, condensation and accumulation of nanodroplets, and surface inhomogeneity. These mechanisms are scale dependent, with different characteristic length scales. To effectively resist these scale-dependent mechanisms, a multiscale (hierarchical) roughness is required. High asperities resist capillary waves, while nanobumps prevent nanodroplets from filling the valleys between asperities and pin the triple line in the case of a hydrophilic spot. A number of artificial roughness-induced hydrophobic surfaces have been fabricated with hierarchical structures using electrodeposition, nanolithography, colloidal systems and photolithography (Shirtcliffe et al. 2004; Ming et al. 2005; Chong et al. 2006; del Campo & Greiner 2007). Moulding is a low-cost and Phil. Trans. R. Soc. A (2009)


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reliable way of surface structure replication and can provide a precision of the order of 10 nm (Madou 1997; Varadan et al. 2001). Bhushan et al. (2008a,b, 2009a, in press) and Koch et al. (in press) have developed a new method to fabricate hierarchical structured surfaces. Moulding was used to replicate leaf and silicon microstructures, and self-assembly of wax was used to create nanostructures on top of the microstructures to realize hierarchical structures. This two-step process provides flexibility in the fabrication of a variety of hierarchical structures. In this paper, in §2, numerical models that provide relationships between roughness and contact angle and contact angle hysteresis are discussed. Then, in §3 details are presented on micro- and nanopatterned polymers (hydrophobic and hydrophilic) fabricated to validate the models. These surfaces were examined by measuring their contact angle. Next, in §4, silicon surfaces patterned with pillars and deposited with a hydrophobic coating are studied to demonstrate how the effects of pitch value, droplet size and impact velocity influence the transition from a composite state to a wetted state. Finally, in §5, hierarchical structures consisting of microstructures with self-assembled hydrophobic plant waxes are described. In these studies, plant surfaces were used as models for the development of hierarchical structures. The hydrophobic waxes in plants exist in different morphologies, such as tubules on lotus and ginkgo (Ginkgo biloba) leaves, or as platelets in the taro plant (Colocasia esculenta) and in rice (Oryza sativa) leaves. Wax tubules have been reconstituted by self-assembly of the natural plant waxes, and platelets have been created using long-chain alkanes (n-hexatriacontane), which are common wax constituents. Based on this, hierarchical structures with different nanostructures and densities have been developed. A lotus surface was also replicated by fabricating a replica of the lotus leaf with the wax removed, followed by evaporation of the wax extracted from the lotus leaf. The influence of micro-, nano- and hierarchical structures on superhydrophobicity is discussed by the investigation of static contact angle, contact angle hysteresis, droplet evaporation and propensity for air pocket formation. In addition, their influence on adhesive force as well as efficiency of self-cleaning is discussed. The data are useful to validate the models and to provide design guidelines for superhydrophobic and self-cleaning surfaces. 2. Modelling of contact angle for a liquid in contact with a rough surface As stated in §1, superhydrophobic and self-cleaning surfaces should have both high contact angle and low contact angle hysteresis. Liquid may form either a homogeneous interface with a solid or a composite interface with air pockets trapped between the solid and the liquid. In this section, mathematical models that provide relationships between roughness and contact angle are discussed. (a ) Homogeneous (Wenzel ) interface Consider a rough solid surface with a typical size of roughness details smaller than the size of the droplet as shown in figure 3a. For a droplet in contact with a rough surface without air pockets, referred to as a homogeneous interface, the contact angle is given as (Wenzel 1936) cos q Z R f cos q0 ; Phil. Trans. R. Soc. A (2009)

ð2:1Þ


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Micro-, nano- and hierarchical structures (a) (i)

(ii) air liquid

liquid

solid

solid

(b)

air

(c) 180 150

(°)

120 90 60 30 0 1.0

2.0

1.5 Rf

Figure 3. (a) Schematic of a liquid droplet in contact with (i) a smooth solid surface (contact angle, q0) and (ii) a rough solid surface (contact angle, q), (b) contact angle for a rough surface (q) as a function of the roughness factor (R f) for various contact angles of the smooth surface (q0) and (c) schematic of hemispherically topped pyramidal asperities with complete packing ( Nosonovsky & Bhushan 2008b).

where q is the contact angle for the rough surface; q0 is the contact angle for a smooth surface; and R f is a roughness factor defined as the ratio of the solid– liquid area ASL to its projection on a flat plane, AF, Rf Z

ASL : AF

ð2:2Þ

The dependence of the contact angle on the roughness factor is presented in figure 3b for different values of q0, based on equation (2.1). The model predicts that a hydrophobic surface (q0O908) becomes more hydrophobic with an increase in R f, and that a hydrophilic surface (q0!908) becomes more hydrophilic with an increase in R f (Nosonovsky & Bhushan 2005; Jung & Bhushan 2006). As an example, figure 3c shows a geometry with hemispherically topped pyramidal asperities, which has complete packing. The size and shape of the asperities can be optimized for a desired roughness factor. (b ) Composite (Cassie–Baxter ) interface For a rough surface, a wetting liquid will be completely absorbed by the rough surface cavities, while a non-wetting liquid may not penetrate into surface cavities, resulting in the formation of air pockets, leading to a composite solid– liquid–air interface as shown in figure 4a. Cassie & Baxter (1944) extended Phil. Trans. R. Soc. A (2009)


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(b) 1.0 hydrophobic region 0.8 solid air

fLA

liquid

0.6 0.4 0.2 0

1

2

3

4

Rf Figure 4. (a) Schematic of the formation of a composite solid–liquid–air interface for a rough surface and (b) fLA requirement for a hydrophilic surface to become hydrophobic as a function of the roughness factor (R f) and q0 ( Jung & Bhushan 2006).

the Wenzel equation, which was originally developed for a homogeneous solid– liquid interface, to the case of a composite interface. For this case, there are two sets of interfaces: a solid–liquid interface with the ambient environment surrounding the droplet, and a composite interface involving the liquid–air and solid–air interfaces. In order to calculate the contact angle for the composite interface, Wenzel’s equation can be modified by combining the contribution of the fractional area of wet surfaces and the fractional area with air pockets (qZ1808), cos q Z R f cos q0 K fLA ðR f cos q0 C 1Þ;

ð2:3Þ

where fLA is the fractional flat geometric area of the liquid–air interfaces under the droplet. According to equation (2.3), even for a hydrophilic surface, the contact angle increases with an increase in fLA. At a high value of fLA, a surface can become hydrophobic; however, the value required may be unachievable or the formation of air pockets may become unstable. Using the Cassie–Baxter equation, the value of fLA at which a hydrophilic surface could turn into a hydrophobic one is given as (Jung & Bhushan 2006) fLA R

R f cos q0 R f cos q0 C 1

for q0 ! 908:

ð2:4Þ

Figure 4b shows the value of this fLA requirement as a function of R f for four surfaces with different contact angles q0. Hydrophobic surfaces can be achieved above a certain fLA value as predicted by equation (2.4). The upper part above each contact angle line is the hydrophobic region. For the hydrophobic surface, the contact angle increases with an increase in fLA for both smooth and rough surfaces. (c ) Contact angle hysteresis The contact angle hysteresis is another important characteristic of a solid– liquid interface. If a drop sits on a tilted surface (figure 5), the contact angles at the front and back of the droplet correspond to the advancing and receding contact angle, respectively. The advancing angle is greater than the receding angle, which results in contact angle hysteresis. Contact angle hysteresis occurs due to surface roughness and heterogeneity. Although, for surfaces with Phil. Trans. R. Soc. A (2009)



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Figure 5. Tilted surface profile (tilt angle, a) with a liquid droplet; advancing and receding contact angles are qadv and qrec, respectively.

roughness carefully controlled on the molecular scale, it is possible to achieve a contact angle hysteresis as low as 18 or less (Gupta et al. 2005), hysteresis cannot be eliminated completely, since even atomically smooth surfaces have a certain roughness and heterogeneity. Contact angle hysteresis is a measure of energy dissipation during the flow of a droplet along a solid surface. Surfaces with low contact angle hysteresis have a very low water roll-off angle, which is the angle to which a surface must be tilted for roll-off of water drops (Nosonovsky & Bhushan 2007a, 2008a–c; Bhushan & Jung 2008). Low water roll-off angle is important in liquid flow applications, such as in micro/nanochannels and surfaces with selfcleaning ability. Nosonovsky & Bhushan (2007b) derived a relationship for contact angle hysteresis as a function of roughness, given as cos qa0 Kcos qr0 Ksin q pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi cos qr0 Kcos qa0 ffi: Z 1K fLA R f pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ðR f cos q0 C 1Þ

qadv K qrec Z ð1K fLA ÞR f

ð2:5Þ

For a homogeneous interface fLAZ0, whereas for a composite interface fLA is a non-zero number. It is observed from equation (2.5) that, for a homogeneous interface, increasing roughness (high R f) leads to increasing contact angle hysteresis (high values of qadvKqrec), while for a composite interface, an approach to unity of fLA provides both a high contact angle and a low contact angle hysteresis (Jung & Bhushan 2006; Bhushan et al. 2007; Nosonovsky & Bhushan 2007b,d ). Therefore, a composite interface is desirable for superhydrophobicity and self-cleaning. (d ) Stability of a composite interface and role of hierarchical structure Formation of a composite interface is a multiscale phenomenon that depends on the relative sizes of the liquid droplet and roughness details. A composite interface is metastable, and its stability is an important issue. Even though it may be geometrically possible for the system to become composite, it may be energetically profitable for the liquid to penetrate into the valleys between asperities and form a homogeneous interface. A composite interface is fragile and can be irreversibly transformed into a homogeneous interface, thus damaging superhydrophobicity. In order to form a stable composite interface with air pockets between solid and liquid, destabilizing factors, such as capillary waves, nanodroplet condensation, surface inhomogeneities and liquid pressure, should be avoided. For high fLA, Phil. Trans. R. Soc. A (2009)


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B. Bhushan et al. h d H D P

Figure 6. Schematic of the structure of an ideal hierarchical surface. Microasperities consist of circular pillars with diameter D, height H and pitch P. Nanoasperities consist of pyramidal nanoasperities of height h and diameter d with rounded tops (Bhushan & Jung 2008).

a nanopattern is desirable because whether the liquid–air interface is generated depends on the ratio of the distance between two adjacent asperities and droplet radius. Furthermore, nanoscale asperities can pin liquid droplets and thus prevent liquid from filling the valleys between asperities. High R f can be achieved by both micro- and nanopatterns. Nosonovsky & Bhushan (2007b) studied the destabilizing factors for a composite interface and found that a convex surface leads to a stable interface and high contact angle. Also, they suggested that the effects of a droplet’s weight and curvature were among the factors that affect the transition. Nosonovsky & Bhushan (2007a–d, 2008a–d ) demonstrated that a combination of micro- and nanoroughness (multiscale roughness) with convex surfaces can help to resist destabilization by pinning the interface. It also helps in preventing the gaps between the asperities from filling with liquid, even in the case of a hydrophilic material. The effect of roughness on wetting is scale dependent, and the mechanisms that lead to destabilization of a composite interface are also scale dependent. To effectively resist these scale-dependent mechanisms, it is expected that a multiscale roughness is optimum for superhydrophobicity. (e ) Ideal surfaces with hierarchical structure For stability of a composite interface, Bhushan & Jung (2008) suggested the structure of an ideal hierarchical surface as shown in figure 6. The asperities should be high enough so that the droplet does not touch the valleys. As an example, for a structure with circular pillars, the following relationship should hold for a composite interface. For a droplet with a radius of the order of 1 mm or larger, values of H, D and P of the order of 30, 15 and 130 mm, respectively, are optimum. Nanoasperities can pin the liquid–air interface and thus prevent liquid from filling the valleys between asperities. They are also required to support nanodroplets, which may condense in the valleys between high asperities. Therefore, nanoasperities should have a low pitch to handle nanodroplets, less than 1 mm down to a few nanometres radius. Values of h and d of the order of 10 and 100 nm, respectively, can be easily fabricated. 3. Fabrication and characterization of micro- and nanopatterned polymer surfaces In this section and the following two, we will discuss experimental measurements of the wetting properties of micro-, nano- and hierarchical structured surfaces fabricated using different techniques. Phil. Trans. R. Soc. A (2009)



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In order to validate the models and to provide design guidelines for superhydrophobic surfaces, in this section we first study micro- and nanopatterned polymers fabricated using soft lithography and show experimental measurements of the wetting properties on the surface. (a ) Fabrication of micro- and nanopatterned polymers using soft lithography Jung & Bhushan (2006) studied two types of polymers: polymethyl methacrylate (PMMA) and polystyrene (PS). PMMA and PS were chosen because they are widely used in microelectromechanical systems and nanoelectromechanical systems devices. Both hydrophilic and hydrophobic surfaces can be produced using these two polymers, as PMMA has polar (hydrophilic) groups with high surface energy while PS has electrically neutral and non-polar (hydrophobic) groups with low surface energy. Furthermore, a PMMA structure can be made hydrophobic by treating it appropriately, e.g. by coating with a hydrophobic self-assembled monolayer (SAM). Four types of surface patterns were fabricated from PMMA: a flat film; low aspect ratio (LAR) asperities (1 : 1 height-to-diameter ratio); high aspect ratio (HAR) asperities (3 : 1 height-to-diameter ratio); and a replica of the lotus leaf (the lotus pattern). Two types of surface patterns were fabricated from PS: a flat film; and the lotus pattern. Figure 7 shows scanning electron microscope (SEM) images of two types of nanopatterned structures, LAR and HAR, and one type of micropatterned structure, the lotus pattern, all on a PMMA surface (Burton & Bhushan 2005; Jung & Bhushan 2006). Both micro- and nanopatterned structures were manufactured using soft lithography. For nanopatterned structures, a PMMA film was spin coated on a silicon wafer. A UV-cured mould (PUA mould) with the nanopatterns of interest was made, which enables one to create sub-100 nm patterns with high aspect ratio (Choi et al. 2004). The mould was placed on the PMMA film, and a slight pressure of approximately 10 g cmK2 (approx. 1 kPa) was applied and annealed at 1208C. Finally, the PUA mould was removed from the PMMA film. For micropatterned structures, a polydimethylsiloxane (PDMS) mould was first made by casting PDMS against a lotus leaf followed by heating. Then, the mould was placed on the PMMA and PS film to create a positive replica of the lotus leaf. As shown in figure 7, it can be seen that only microstructures exist on the surface of the lotus pattern ( Jung & Bhushan 2006). Since PMMA by itself is hydrophilic, in order to obtain a hydrophobic sample, a SAM of perfluorodecyltriethoxysilane (PFDTES) was deposited on the sample surfaces using the vapour-phase deposition technique. PFDTES was chosen because of its hydrophobic nature. The deposition conditions for PFDTES were 1008C temperature, 400 Torr pressure, 20 min deposition time and 20 min annealing time. The polymer surface was exposed to an oxygen plasma treatment (40 W, O2 187 Torr, 10 s) prior to coating (Bhushan et al. 2006). The oxygen plasma treatment is necessary to oxidize any organic contaminants on the polymer surface and also to alter the surface chemistry to allow for enhanced bonding between the SAM and the polymer surface. Phil. Trans. R. Soc. A (2009)


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(a) (ii)

(i)

1 µm

200 nm

(iv)

(iii)

1 µm

200 nm (b) (i)

(ii)

10 µm

30 µm

Figure 7. SEM images of (a) the two nanopatterned polymer surfaces (shown using two magnifications to show both the asperity shape and the asperity pattern on the surface; (i)(ii) PMMA low aspect ratio (LAR) and (iii)(iv) PMMA high aspect ratio (HAR)) and (b) the micropatterned polymer surface (lotus pattern, which has only microstructures on the surface; (i)(ii) PMMA lotus replica) (Burton & Bhushan 2005; Jung & Bhushan 2006).

(b ) Contact angle measurements Jung & Bhushan (2006) measured the static contact angle of water with the patterned PMMA and PS structures (figure 8). For static contact angle, droplets approximately 5 ml in volume were gently deposited on the surface using a microsyringe. Since the Wenzel roughness factor is the parameter that often determines wetting behaviour, the roughness factor was calculated, and it is presented in table 1 for various samples. The data show that the contact angle of the hydrophilic materials decreases with an increase in the roughness factor, as predicted by the Wenzel model. When the polymers were coated with PFDTES, the film surface became hydrophobic. Figure 8 also shows the contact angle for various PMMA samples coated with PFDTES. For a hydrophobic surface, the standard Wenzel model predicts an increase in contact angle with roughness factor, which is what happens in the case of patterned samples. The calculated values of contact angle for various patterned samples based on the contact angle of the smooth film and Wenzel equation are also presented. The measured contact angle values for the lotus pattern were comparable with the calculated Phil. Trans. R. Soc. A (2009)


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contact angle (°)

(a) 150

hydrophobic calculated

hydrophobic

(b)

120 90 60

s tu

PS on

S D

TE

TE PF

D PF

lo

fil m

s tu

PS

lo S

on

PS

PM

PS

fil m

M PM A M film PM A l M otu PF A s P D M LA TE M R PF S on A H D TE PM AR S PF on M A D f TE PM M ilm S A on PF D PM lotu TE s M S A on PM LA R M A H A R

30

Figure 8. Contact angles for various patterned surfaces on (a) PMMA and (b) PS polymers and values calculated using the Wenzel equation ( Jung & Bhushan 2006). Table 1. Roughness factor for micro- and nanopatterned polymers ( Jung & Bhushan 2006).

Rf

LAR

HAR

lotus

2.1

5.6

3.2

values, whereas for the LAR and HAR patterns they are higher. This suggests that nanopatterns benefit from air pocket formation. For the PS material, the contact angle of the lotus pattern also increased with increased roughness factor. 4. Fabrication and characterization of micropatterned Si surfaces In order to investigate how the effects of pitch value, droplet size and impact velocity influence the transition from a composite state to a wetted state, we studied micropatterned Si surfaces with varying pitch values fabricated using photolithography. (a ) Fabrication of micropatterned Si surfaces using photolithography Micropatterned surfaces produced from single-crystal silicon (Si) by photolithography and coated with a SAM were used by Jung & Bhushan (2007, 2008a,b) in their studies. One of the purposes of this investigation was to study the transition from Cassie–Baxter to Wenzel regimes by changing the distance between the pillars. To create a patterned Si, 5 mm diameter and 10 mm height, flat-topped, cylindrical pillars with different pitch values (7, 7.5, 10, 12.5, 25, 37.5, 45, 60 and 75 mm) were fabricated using photolithography (Barbieri et al. 2007). The pitch is the spacing between the centres of two adjacent pillars. The SAM of 1,1,2,2-tetrahydroperfluorodecyltrichlorosilane (PF3) was deposited on the Si sample surfaces using the vapour-phase deposition technique. PF3 was chosen because of its hydrophobic nature. The thickness and standard deviation of the SAM of PF3 were 1.8 and 0.14 nm, respectively (Kasai et al. 2005). Phil. Trans. R. Soc. A (2009)


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(ii) 90 µm

10 µm 0

100 µm

H P D

90 µm 0 (b)

(i)

100 µm

(ii) section A–A

A

maximum droop of droplet

A

H

D 2

D

P

2P

D

P Figure 9. (a) Surface height maps of the patterned Si surface (5 mm diameter, 10 mm height, 10 mm pitch pillars) using an optical profiler (Bhushan & Jung 2007) and (b) a liquid droplet suspended on a hydrophobic surface consisting of a regular array of circular pillars. The maximum droop of the droplet (d) can be found in the middle of two pillars that are diagonally adjacent ( Jung & Bhushan 2007).

An optical profiler was used to measure the surface topography of the patterned surfaces (Bhushan & Jung 2007, 2008). A surface height map can be seen for the patterned Si in figure 9a. A three-dimensional map and a flat map are shown. A scan size of 100!90 mm was used to obtain a sufficient amount of pillars to characterize the surface but also to maintain enough resolution to get an accurate measurement. The images found with the optical profiler show that the flat-topped, cylindrical pillars on the Si surface are distributed over the entire surface in a square grid with different pitch values. (b ) Cassie–Wenzel transition criteria A stable composite interface is essential for the successful design of superhydrophobic surfaces. However, a composite interface is fragile, and it may transform into a homogeneous interface. Bhushan & Jung (2007, 2008) and Jung & Bhushan (2007, 2008a,b) investigated the effect of droplet curvature on the Cassie–Wenzel regime transition. First, they considered a small water droplet suspended on a superhydrophobic surface consisting of a regular array of circular pillars with diameter D, height H and pitch P as shown in figure 9b. The local deformation for small droplets is governed by surface effects rather than Phil. Trans. R. Soc. A (2009)



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gravity. The curvature of a droplet is governed by the Laplace equation, which relates the pressure inside the droplet to its curvature (Adamson 1990). Therefore, the curvature is the same at the top and at the bottom of the droplet (Nosonovsky & Bhushan 2007d ). The maximum droop of the droplet (d) in the recessed region occurs at the midpoint between diagonally adjacent pillars, as pffiffiffi shown in figure 9b, which is ð 2P KDÞ2 =ð8RÞ, where R is the droplet radius. If the droop is greater than the depth of the cavity, then the droplet will just contact the bottom of the cavity between the pillars. If it is much greater, transition occurs from the Cassie–Baxter to Wenzel regimes, pffiffiffi ð 2P KDÞ2 =RR H : ð3:1Þ To investigate the dynamic effect of a bouncing water droplet on the Cassie– Wenzel regime transition, Jung & Bhushan (2008b) considered a water droplet hitting a superhydrophobic surface as shown in figure 9b. As the droplet hits the surface at velocity V, the liquid–air interface below the droplet is formed when the dynamic pressure is less than the Laplace pressure. The Laplace pressure can be written as pffiffiffi pL Z 2g=R Z 16gd=ð 2P KDÞ2 ; ð3:2Þ where g is the surface tension of the liquid–air interface, and the dynamic pressure of the droplet is equal to 1 pd Z rV 2 ; ð3:3Þ 2 where r is the mass density of the liquid droplet. If the maximum droop of the droplet (d) is larger than the height of the pillar (H ), the droplet contacts the bottom of the cavity between the pillars. Determination of the critical velocity at which the droplet touches the bottom is obtained by equating the Laplace pressure to the dynamic pressure. To develop a composite interface, the velocity should be lower than the critical velocity given as sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 32gH V! ð3:4Þ pffiffiffi 2 : r 2P KD Furthermore, in the case of large distances between the pillars, the liquid–air interface can be easily destabilized due to dynamic effects. This leads to the formation of a homogeneous solid–liquid interface (Nosonovsky & Bhushan 2007b). (c ) Contact angle measurements In order to study the effect of pitch value on the transition from the Cassie– Baxter to Wenzel regimes, the static contact angles were measured on patterned Si coated with PF3, and the data are plotted as a function of pitch between the pillars in figure 10 (Jung & Bhushan 2007; Bhushan & Jung 2008). The dotted line represents the transition criterion range obtained using equation (3.1). The flat Si coated with PF3 showed a static contact angle of 1098. The contact angle of selected patterned surfaces is much higher than that of the flat surfaces. It first increases with an increase in the pitch values, then drops rapidly to Phil. Trans. R. Soc. A (2009)


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120 90

0

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100 150 pitch (µm)

200

250

Figure 10. (a) Static contact angle (circles; the dotted line represents the transition criterion range obtained using equation (3.1)) and (b) contact angle hysteresis (circles) and tilt angle (triangles) as a function of pitch values for patterned surfaces (5 mm diameter, 10 mm height pillars) with different pitch values for a droplet with 1 mm radius (5 ml volume). Data at zero pitch correspond to a flat sample (Bhushan & Jung 2007; Jung & Bhushan 2007).

a value slightly higher than that of flat surfaces. In the first portion, it jumps to a high value of 1528 corresponding to a superhydrophobic surface and continues to increase to 1708 at a pitch of 45 mm because open air space increases with an increase in pitch responsible for the propensity for air pocket formation. The sudden drop at a pitch value of approximately 50 mm corresponds to the transition from the Cassie–Baxter to Wenzel regimes. The value predicted from the curvature transition criterion (equation (3.1)) is a little higher than the experimental observations for the Cassie–Wenzel transition. Contact angle hysteresis and tilt angle were also measured. For contact angle hysteresis, the advancing and receding contact angles were measured at the front and back of a droplet moving along the tilted surface, respectively. The tilt angle was measured by using a simple tilting stage. Figure 10 also shows contact angle hysteresis and tilt angle as a function of pitch. The two angles are comparable. The flat Si coated with PF3 showed a contact angle hysteresis of 348 and a tilt angle of 378. The angle first increases with an increase in pitch value, which has to do with the pinning of the droplet at the sharp edges of the micropillars. As the pitch increases, there is higher propensity for air pocket formation and fewer numbers of sharp edges per unit area, which is responsible for the sudden drop in the angle. The lowest contact angle hysteresis and tilt angle are 58 and 38, respectively, which were observed on patterned Si with 45 mm pitch. Above a pitch value of 50 mm, the angle increases very rapidly because of the transition to the Wenzel regime. These results suggest that air pocket formation and reduction in pinning in the patterned surface play an important role for a surface with both low contact angle hysteresis and tilt angle (Bhushan & Jung 2007, 2008). Hence, to create superhydrophobic surfaces, it is important that they are able to form a stable composite interface with air pockets between solid and liquid. (d ) Observation of transition during droplet evaporation In order to study the effect of droplet size on the transition from a composite to a wetted state, Jung & Bhushan (2007, 2008a) performed droplet evaporation experiments to observe the Cassie–Wenzel transition on patterned Si surfaces Phil. Trans. R. Soc. A (2009)


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radius: 703 µm

483 µm

(ii)

(iii)

360 µm

(iv)

474 µm

transition 300 µm

no air pocket

air pocket

radius of droplet (µm)

(b) 1000 800

Cassie and Baxter regime

600 400

Wenzel regime

200 0

100 50 pitch (µm)

150

Figure 11. (a) Evaporation of a droplet on a patterned surface. The initial radius of the droplet is approximately 700 mm, and the time interval between (i) and (ii) was 180 s and between (ii) and (iv) was 60 s. As the radius of the droplet reaches 360 mm on this surface with 5 mm diameter, 10 mm height and 37.5 mm pitch pillars, the transition from the Cassie–Baxter to Wenzel regimes occurs, as indicated by the arrow. Before the transition, an air pocket is clearly visible at the bottom area of the droplet, but after the transition, no air pocket is found at the bottom area of the droplet. (b) Radius of droplet as a function of pitch values for the experimental results (circles) compared with the transition criterion from the Cassie–Baxter to Wenzel regimes (solid line) for the patterned surface with 5 mm diameter and 10 mm height pillars and different pitch values (Jung & Bhushan 2008a).

coated with PF3. Figure 11a(i–iv) shows successive photographs of a droplet evaporating on the patterned surface. The initial radius of the droplet was approximately 700 mm, and the time interval between the first two photographs was 180 s and between the latter was 60 s. In the first three photographs, the droplet is shown in the Cassie–Baxter state, and its size gradually decreases with time. However, when the radius of the droplet reached 360 mm on the surface with 5 mm diameter, 10 mm height and 37.5 mm pitch pillars, the transition from the Cassie–Baxter to Wenzel regimes occurred, as indicated by the arrow. Light passes below the first droplet, indicating that air pockets exist, so that the droplet is in the Cassie–Baxter state. However, no air pocket is visible below the last droplet, so it is in the Wenzel state. This could result from an impalement of the droplet in the patterned surface, characterized by a lower contact angle. To prove the validity of the transition criterion in terms of droplet size, the critical radius of a droplet deposited on the patterned Si with different pitch values coated with PF3 is measured during the evaporation experiment ( Jung & Bhushan 2007, 2008a). Figure 11b shows the radius of a droplet as a function of geometric parameters for the experimental results (circles) compared with the Phil. Trans. R. Soc. A (2009)


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transition criterion (equation (3.1)) from the Cassie–Baxter to Wenzel regimes (solid line) for patterned Si with different pitch values coated with PF3. It is found that the critical radius of impalement is in good quantitative agreement with our predictions. The critical radius of the droplet increases linearly with the geometric parameter (pitch). For the surface with low pitch, the critical radius of a droplet can become quite small. (e ) Observation of transition for a bouncing droplet Jung & Bhushan (2008b) performed bouncing droplet experiments to observe how impact velocity influences the Cassie–Wenzel transition for a droplet hitting the surface of micropatterned Si coated with PF3. Figure 12a shows snapshots of a droplet with 1 mm radius hitting the surface. The impact velocity was obtained just prior to the droplet hitting the surface. As shown in figure 12a(i), the droplet hitting the surface at an impact velocity of 0.44 m sK1 first deformed and then retracted, and bounced off the surface. Finally, the droplet sat on the surface and had a high contact angle, which suggests the formation of a solid–air–liquid interface. Next, they repeated the impact experiment with increasing impact velocity. Bounce-off does not occur, and wetting of the surface (and possibly pinning of the droplet) occurred at an impact velocity of 0.88 m sK1, referred to as the critical velocity (described earlier). Figure 12a(ii) shows images of the droplet at the critical velocity. After the droplet hits the surface, it wetted the surface (possibly the droplet was also pinned) after the deformation of the droplet. This is because air pockets do not exist below the droplet as a result of droplet impalement by the pillars, characterized by a lower contact angle. These observations indicate the transition from the Cassie–Baxter to Wenzel regimes. To study the validity of the transition criterion (equation (3.4)), the critical impact velocity at which wetting of the surface (possibly pinning of the droplet) occurs was measured (Jung & Bhushan 2008b). For calculations, the surface tension of the water–air interface (g) was taken as 0.073 N mK1, the mass density (r) is 1000 kg mK3 for water and 1 kg m sK2Z1 N (Adamson 1990). Figure 12b shows the measured critical impact velocity of a droplet with 1 mm radius as a function of geometric parameters (triangles). The trends are compared with the predicted curve (solid line). It is found that the critical impact velocity at which wetting occurs is in good quantitative agreement with our predictions. The critical impact velocity of the droplet decreases with the geometric parameter (pitch). For a surface with low pitch, the critical impact velocity of a droplet can be high.

5. Fabrication and characterization of hierarchical structured surfaces In this section, we introduce fabrication techniques that involve a low-cost moulding process to replicate microstructures with self-assembly of wax platelets and tubules to create different nanostructures. This two-step process provides flexibility in the fabrication of a variety of hierarchical structures. Then, experimental measurements of the wetting properties of micro-, nano- and hierarchical structures fabricated using these techniques will be discussed. Phil. Trans. R. Soc. A (2009)


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0s

8 ms

16 ms

24 ms

32 ms

1.8 s

1 mm

(ii)

0s

8 ms

16 ms

24 ms

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1 mm

critical impact velocity (m s–1)

(b) 2.0

1.5

1.0

0.5

0

50

150

100

200

250

pitch (µm) Figure 12. (a) Snapshots of a droplet with 1 mm radius hitting a patterned surface. The reported impact velocity is that just prior to the droplet hitting the surface. The bouncing and pinning of the droplet on this surface with 5 mm diameter, 10 mm height and 10 mm pitch pillars occurred at an impact velocity of (i) 0.44 m sK1 and (ii) 0.88 m sK1, respectively. (b) Measured critical impact velocity of a droplet with 1 mm radius as a function of geometric parameters (triangles, experimental; 5 mm diameter and 10 mm height pillars). The data are compared with the criterion of impact velocity for pinning of the droplet (solid line, predicted) for patterned Si with different pitch values ( Jung & Bhushan 2008b).

(a ) Fabrication of hierarchical structured surfaces Hierarchical structures are composed of at least two levels of structuring in different length scales. Bhushan et al. (2008a,b, 2009a, in press) and Koch et al. (in press) fabricated surfaces with a hierarchical structure with micropatterned epoxy replicas, and lotus leaf microstructure, and created a second level of structuring with wax platelets and wax tubules. Platelets and tubules are the Phil. Trans. R. Soc. A (2009)


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most common wax morphologies found in plant surfaces, and exist on superhydrophobic leaves, such as lotus and C. esculenta leaves. The structures developed mimic the hierarchical structures of superhydrophobic leaves. The two steps of the fabrication process include the production of microstructured surfaces by soft lithography and subsequent development of nanostructures on top by self-assembly of plant waxes and artificial wax components. (i) Fabrication of microstructures by soft lithography A two-step moulding process was used to fabricate several structurally identical copies of the micropatterned Si surface and lotus leaves. The technique used is a fast, precise and low-cost moulding process for biological and artificial surfaces (Koch et al. 2007, 2008b). The technique was used to mould a microstructured Si surface with pillars of 14 mm diameter and 30 mm height with 23 mm pitch (Bhushan et al. 2008a,b, 2009a, in press; Koch et al. in press), fabricated by photolithography (Barbieri et al. 2007). Before replication of the lotus leaf, the epicuticular wax tubules were removed in areas of approximately 6 cm2. Therefore, a two-component fast hardening glue was applied on the upper side of the leaves and was carefully pressed onto the leaf. After hardening, the glue with the embedded waxes was removed from the leaf, and the same procedure was repeated (Koch et al. in press). The replication is a two-step moulding process, in which first a negative replica of a template is generated and then a positive replica, as shown in figure 13a. A polyvinylsiloxane dental wax was applied on the surface and immediately pressed down with a glass plate. After complete hardening of the moulding mass (at room temperature for approximately 5 min), the silicon master surface and the mould (negative) were separated. After a relaxation time of 30 min for the moulding material, the negative replicas were filled with a liquid epoxy resin. Specimens were immediately transferred to a vacuum chamber at 750 mTorr (100 Pa) pressure for 10 s to remove trapped air and to increase resin infiltration through the structures. After hardening at room temperature (228C for 24 hours), the positive replica was separated from the negative replica. The second step can be repeated to generate a number of replicas. The pillars of the master surface have been replicated without any morphological changes as shown in figure 13b. Nanogrooves of a couple of hundred nanometres in lateral dimension present on the pillars of the master surface are shown to reproduce faithfully in the replica. (ii) Fabrication of nanostructures by self-assembly Bhushan et al. (2008a,b, 2009a, in press) and Koch et al. (in press) created a nanostructure by self-assembly of synthetic and plant waxes deposited by thermal evaporation. The alkane n-hexatriacontane (C36H74) has been used for the development of platelet nanostructures. Tubule-forming waxes, which were isolated from the leaves of Tropaeolum majus (L.) and N. nucifera, in the following referred to as T. majus and lotus, were used to create tubule structures. The chemical structure of the major components of the tubule-forming wax and alkane n-hexatriacontane are shown in table 2. The complete chemistry of the plant waxes used is presented in Koch et al. (2006a). Phil. Trans. R. Soc. A (2009)


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Micro-, nano- and hierarchical structures (a)

(i)

(ii)

template

dental wax

apply dental wax

apply epoxy resin

dental wax

epoxy resin

template after hardening, separate negative from template

dental wax after hardening, separate positive from negative

dental wax

epoxy resin

negative moulding process

positive moulding process

(b) (i)

(ii)

10 µm

2 µm

(iv)

(iii)

10 µm

2 µm

Figure 13. (a) Schematic of the two-step moulding process used to fabricate microstructure, in which at first (i) a negative is generated and then (ii) a positive. (b) SEM images of (i)(ii) the master of the micropatterned Si surface and (iii)(iv) a positive replica fabricated from the master surface measured at 458 tilt angle (shown using two magnifications).

For a homogeneous deposition of the waxes and alkane, a thermal evaporation system, as shown in figure 14, has been used. Specimens of smooth surfaces (flat silicon replicas) and microstructured replicas were placed in a vacuum chamber at 30 mTorr (4 kPa), 20 mm above a heating plate loaded with waxes of n-hexatriacontane (300, 500 or 1000 mg), T. majus (500, 1000, 1500 or 2000 mg) and lotus (2000 mg) (Bhushan et al. 2008a,b, 2009a, in press; Koch et al. in press). The wax was evaporated by heating it up to 1208C. In a vacuum chamber, evaporation from the point source to the substrate occurs in a straight line; thus, the amount of sublimated material is equal in a hemispherical region over the point source (Bunshah 1994). In order to estimate the amount of sublimated mass, the Phil. Trans. R. Soc. A (2009)


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Table 2. Chemical structure of the major components of n-hexatriacontane, T. majus and lotus waxes. (The major component is shown first.) C36H74

n-hexatriacontane

Tropaeolum majus

lotus

nonacosan-10-ol

OH j CH3 –ðCH2 Þ8 –CH–ðCH2 Þ18 –CH3

nonacosane-4,10-diol

OH OH j j CH3 –ðCH2 Þ2 –CH–ðCH2 Þ5 –CH–ðCH2 Þ18 –CH3

nonacosane-10,15-diol nonacosan-10-ol

OH OH j j CH3 –ðCH2 Þ8 –CH–ðCH2 Þ4 –CH–ðCH2 Þ13 –CH3 OH j CH3 –ðCH2 Þ8 –CH–ðCH2 Þ18 –CH3

vacuum chamber

specimen

wax heating plate

mechanical pump Figure 14. Schematic of the thermal evaporation system for self-assembly of a wax. Evaporation from the point source to the substrate occurs over a hemispherical region (dotted line) (Bhushan et al. in press).

surface area of the half sphere was calculated by using the formula 2pr 2, whereby the radius (r) represents the distance between the specimen to be covered and the heating plate with the substance to be evaporated. The amount of wax deposited on the specimen surfaces was 0.12, 0.2 and 0.4 mg mmK2 for n-hexatriacontane, 0.2, 0.4, 0.6 and 0.8 mg mmK2 for T. majus and 0.8 mg mmK2 for lotus waxes, respectively. After coating, the specimens with n-hexatriacontane were placed in a desiccator at room temperature for 3 days for crystallization of the alkanes. A stable stage was indicated by no further increase in crystal sizes. Phil. Trans. R. Soc. A (2009)



1651

filter paper

vapour circulation

specimen

filter paper + solution Figure 15. Schematic of the glass recrystallization chamber used for wax tubule formation. The filter paper placed at the bottom of the chamber was wetted with 20 ml of the solvent, and slow evaporation of the solvent was enabled by placing a thin filter paper between the glass body and the cap placed above. The total volume of the chamber is approximately 200 cm3 (Bhushan et al. in press).

(b)

(a)

0.8 µm

0.8 µm

Figure 16. SEM images of the morphology of lotus wax deposited on the flat epoxy replica surface after two treatments of specimens measured at 458 tilt angle: (a) after 7 days at 218C, nanostructure on flat epoxy replica was found with no tubules; and (b) after 7 days at 508C with ethanol vapour, tubular nanostructures with random orientation were found on the surface (Koch et al. in press).

For the plant waxes, which are a mixture of aliphatic components, different crystallization conditions have been chosen. It has been reported by Niemietz et al. (submitted) that an increase in temperature from 218C (room temperature) to 508C had a positive effect on the mobilization and diffusion of wax molecules, required for the separation of the tubule-forming molecules. It is also known that chemical environment has an influence on the propensity for wax crystallization; thus, the specimens with evaporated plant waxes were stored for 3 days at 508C in a crystallization chamber, where they were exposed to a solvent (ethanol) in vapour phase (figure 15). Specimens were placed on metal posts and a filter paper wetted with 20 ml of the solvent was placed below the specimens. Slow diffusive loss of the solvent in the chamber was enabled by placing a thin filter paper between the glass body and the lid. After evaporation of the solvent, specimens were left in the oven at 508C in total for 7 days. Figure 16 shows the nanostructures formed by lotus wax, 7 days after wax deposition on flat surfaces. Figure 16a shows the nanostructure after storage at 218C, in which no tubules Phil. Trans. R. Soc. A (2009)


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were grown. Figure 16b shows that the lotus waxes exposed to ethanol vapour at 508C for 3 days formed wax tubules. A detailed description of the nanostructure sizes and nanoroughness is given in the following. Flat films of n-hexatriacontane and wax tubules were made by heating the substances above their melting point and rapidly cooling down. This procedure interrupts crystallization and leads to smooth films (Bhushan et al. 2008a,b, 2009a, in press; Koch et al. in press). (b ) The influence of wax platelets on superhydrophobicity (i) Morphological characterization of various structures For the development of nanostructures by platelet formation, n-hexatriacontane was used. Figure 17a shows SEM micrographs of a flat surface and nanostructures fabricated with various masses of n-hexatriacontane and different densities of crystals (Bhushan et al. 2008a). The nanostructures formed by three-dimensional platelets of n-hexatriacontane are shown in detail in figure 17b. These platelets are flat structures, grown perpendicular to the substrate surface. The platelet thickness varied between 50 and 100 nm and their length varied between 500 and 1000 nm. On specimens with higher wax masses (0.2 and 0.4 mg mmK2), the platelets are connected to their neighbouring crystals at their lateral ends. They are randomly distributed, and their arrangement leads to a kind of cross-linking of the single platelets. The created platelets are comparable to the wax crystal morphology found on superhydrophobic leaves, such as C. esculenta (Neinhuis & Barthlott 1997) and Triticum aestivum (wheat; Koch et al. 2006b). Atomic force microscopy (AFM) was used to characterize the nanostructures. The statistical parameters of nanostructures (root mean square (r.m.s.) height, peak-to-valley height and summit density (h)) were calculated (Bhushan 1999, 2002) and are presented in table 3. (ii) Wettability and adhesion forces of flat and nanostructured surfaces To study the effect of nanostructures with different crystal density on superhydrophobicity, the static contact angle, hysteresis angle, tilt angle and adhesive forces were measured (Bhushan et al. 2008a). The data are shown in figure 18a. The static contact angle of a flat surface coated with a film of n-hexatriacontane was 918, with a hysteresis angle of 878, and the droplet still adhered to the surface at a tilt angle of 908. Nanostructuring of flat surfaces with n-hexatriacontane platelets creates superhydrophobic surfaces with high static contact angle and a reduction in hysteresis and tilt angles. The values are a function of crystal density. Figure 18b shows a plot of static contact angle and hysteresis angle as a function of the mass of n-hexatriacontane deposited (Bhushan et al. 2008a). As the mass of n-hexatriacontane increased, the static contact angle first increased followed by a decrease with 0.2 mg mmK2 mass. The reverse trend was found for the hysteresis angle. The highest static contact angle and lowest hysteresis angle are 1588 and 238 with 0.2 mg mmK2 mass. As shown in figure 18a, the adhesive force measured using a 15 mm radius borosilicate tip in an AFM on the nanostructured surfaces was lower than on a flat surface because the contact between the tip and the surface was reduced by surface structuring (Bhushan 1999, 2002). Phil. Trans. R. Soc. A (2009)


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(ii)

10 µm

(iii)

2 µm

(iv)

10 µm

(v)

2 µm

(vi)

2 µm

10 µm

(vii)

(viii)

10 µm

2 µm

(b)

0.4 µm

Figure 17. (a) SEM images of the flat surface and nanostructures fabricated with various masses of n-hexatriacontane on epoxy resin measured at 458 tilt angle (shown using two magnifications; (i)(ii) flat, (iii)(iv) nanostructure (0.12 mg mmK2), (v)(vi) nanostructure (0.2 mg mmK2) and (vii)(viii) nanostructure (0.4 mg mmK2)). (b) SEM image of three-dimensional platelets forming nanostructures on the surface fabricated with 0.2 mg mmK2 mass of n-hexatriacontane measured at 458 tilt angle (Bhushan et al. 2008a).

In order to identify wetting regimes (Wenzel or Cassie–Baxter), as well as to understand the effect of crystal density on propensity for air pocket formation for the nanostructured surfaces, the roughness factor (R f) and fractional liquid–air interface (fLA) are needed. Bhushan et al. (2008a) calculated R f for the nanostructures using the AFM map. The R f values for nanostructured surfaces Phil. Trans. R. Soc. A (2009)


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Table 3. Roughness statistics for nanostructure surface measured using an AFM (scan size 10! 10 mm). Nanostructures were fabricated with n-hexatriacontane, T. majus and lotus waxes (Bhushan et al. 2008a, 2009a; Koch et al. in press); r.m.s., root mean square; h, summit density.

r.m.s. height (nm) n-hexatriacontane nanostructure (0.12 mg mmK2) nanostructure (0.2 mg mmK2) nanostructure (0.4 mg mmK2) Tropaeolum majus wax nanostructure (0.8 mg mmK2) lotus wax nanostructure (0.8 mg mmK2)

peak-to-valley height (nm)

h (mmK2)

46 65 82

522 663 856

0.78 1.39 1.73

180

1570

0.57

187

1550

1.47

with masses of 0.12, 0.2 and 0.4 mg mmK2 were found to be 3.4, 4.9 and 6.8, respectively. For calculation of fLA of the nanostructures, only the larger crystals are assumed to come into contact with a water droplet. The fractional geometric area of the top surface for the nanostructures was calculated from the SEM images with top view (08 tilt angle). The SEM images were converted to high-contrast black and white images. The increase in contrast eliminates the smaller platelet structures, and the larger crystals led to white signals in the SEM image. The fractional geometric area of the top nanostructured surfaces with masses of 0.12, 0.2 and 0.4 mg mmK2 was found to be 0.07, 0.15 and 0.24, leading to fLA of 0.93, 0.85 and 0.76, respectively. The values of static contact angle in the Wenzel and Cassie–Baxter regimes for the nanostructured surfaces were calculated using the values of R f and fLA and are presented in figure 18a. The values of hysteresis angle in the Cassie–Baxter regime for various surfaces were calculated using equation (2.5). The experimental static contact angle and hysteresis angle values for the two nanostructured surfaces with 0.2 and 0.4 mg mmK2 were comparable to the calculated values in the Cassie–Baxter regime. The results suggest that droplets on these two nanostructured surfaces should exist in the Cassie–Baxter regime. However, the experimental static contact angle and hysteresis angle values for the nanostructured surface with 0.12 mg mmK2 were lower and higher than the calculated values in the Cassie–Baxter regime, respectively. It is believed that, since neighbouring crystals are separated at lower crystal density, any air between the platelets can be squeezed out, whereas on surfaces with higher densities neighbouring crystals are interconnected and air remains trapped. At the highest crystal density with 0.4 mg mmK2 mass, there is less open volume compared with that at 0.2 mg mmK2, and that explains the droplet static contact angle going from 158 to 1508. (iii) Wettability and adhesion forces of hierarchical structures with platelet nanostructure In order to study the influence of hierarchical structure on the static contact angle, contact angle hysteresis, tilt angle and adhesive forces, Bhushan et al. (2008b, in press) created surfaces with n-hexatriacontane. For nanostructured Phil. Trans. R. Soc. A (2009)


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mass of n-hexatriacontane (µg mm–2) Figure 18. (a) (i)–(iii) Bar charts showing the measured static contact angle, contact angle hysteresis and tilt angle, calculated contact angles obtained using Wenzel and Cassie–Baxter equations with a given value of q0, and calculated contact angle hysteresis using the Cassie–Baxter equation on a flat surface and nanostructures fabricated with various masses of n-hexatriacontane. The droplet on a flat surface does not move along the surface even at a tilt angle of 908. (iv) Bar chart showing the adhesive forces for various structures measured using a 15 mm radius borosilicate tip. (b) Static contact angle and contact angle hysteresis as a function of mass of n-hexatriacontane deposited on the nanostructures (Bhushan et al. 2008a).

surfaces, the amount of 0.2 mg mmK2 n-hexatriacontane has been identified to create optimized crystal densities with the highest static contact angle, low hysteresis and low adhesion. Figure 19 shows SEM micrographs of the flat surface and micro-, nano- and hierarchical structures. The data are shown in figure 20. The static contact angle of a flat surface coated with a film of n-hexatriacontane was 918, and increased to 1588 when n-hexatriacontane formed a nanostructure of platelets on it. The static contact angle on a flat specimen with a microstructure was 1548, but it increased to 1698 for the hierarchical surface structure. Contact angle hysteresis and tilt angle for flat, micro- and nanostructured surfaces show similar trends. The flat surface showed contact angle hysteresis of 878, and the droplet still adhered at a tilt angle of 908. The micro- and nanostructured surfaces showed a reduction in contact angle hysteresis and tilt angle, but a water droplet still needs a tilt angle of 26 and 518, respectively, before sliding. Only the hierarchical surface structure with static contact angle of 1698 and low contact angle hysteresis of 28 Phil. Trans. R. Soc. A (2009)


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(b) (i)

(ii)

2 µm

10 µm

(c) (i)

(ii)

2 µm

10 µm

(d ) (i)

(ii)

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2 µm

Figure 19. SEM images of (a) the flat surface, (b) nanostructure, (c) microstructure and (d ) hierarchical structure measured at 458 tilt angle (shown using two magnifications (i)(ii)). All samples are fabricated with epoxy resin coated with 0.2 mg mmK2 mass of n-hexatriacontane (Bhushan et al. in press).

exceeds the basic criterion for superhydrophobic and self-cleaning surfaces (Koch et al. 2008a). The adhesion force of the hierarchical surface structure was lower than that of both micro- and nanostructured surfaces because the contact between the tip and the surface was lower as a result of the contact area being reduced. In order to identify wetting regimes (Wenzel or Cassie–Baxter) for the various surfaces, the roughness factor (R f) and fractional liquid–air interface (fLA) are needed. The R f value for the nanostructure was described earlier. The R f value for the microstructure was calculated for the geometry of flat-topped, cylindrical Phil. Trans. R. Soc. A (2009)


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nanostructure microstructure

(d )

400

adhesive force (nN)

tilt angle (°)

(c)

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hierarchical structure

200 100 0

flat

nanostructure microstructure hierarchical structure

Figure 20. (a)–(c) Bar charts showing the measured static contact angle, contact angle hysteresis and tilt angle, calculated contact angles obtained using Wenzel and Cassie–Baxter equations with a given value of q0, and calculated contact angle hysteresis using the Cassie–Baxter equation on various structures. The droplet on a flat surface does not move along the surface even at a tilt angle of 908. (d ) Bar chart showing the adhesive forces for various structures, measured using a 15 mm radius borosilicate tip. The error bar represents G1 s.d. (Bhushan et al. in press).

Table 4. Summary of static contact angles and contact angle hysteresis measured and calculated for droplets in Wenzel and Cassie–Baxter regimes on the various surfaces with n-hexatriacontane using the calculated values of R f and fLA (Bhushan et al. 2008b, in press). static contact angle (8)

flat nanostructure microstructure hierarchical structure

Rf

fLA

calculated in Wenzel measured regime

4.9 3.5 8.4

0.85 0.71 0.96

91 158 154 169

95 94 98

contact angle hysteresis (8) calculated in Cassie– Baxter regime measured

149 136 164

87a 23 36 2

calculated in Cassie– Baxter regime

24 34 12

a

Advancing and receding contact angles are 141 and 548, respectively.

pillars of diameter D, height H and pitch P distributed in a regular square array. For this case, the roughness factor for the microstructure is (R f)microZ 1C(pDH/P 2). The roughness factor for the hierarchical structure is the sum of (R f)micro and (R f)nano. The values calculated for various surfaces are summarized in table 4. For calculation of fLA for the microstructure it was considered that a droplet in size much larger than the pitch P contacts only the flat top of the pillars in the composite interface, and the cavities are filled with air. For the Phil. Trans. R. Soc. A (2009)


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radius: 758 µm

(i)

568 µm

(ii)

501 µm

396 µm

(iv)

(iii)

transition 300 µm

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radius: 738 µm

465 µm

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(i)

367 µm

(iii)

223 µm

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air pocket

Figure 21. Evaporation of a droplet on (a) microstructured and (b) hierarchical structured surfaces. The initial radius of the droplet was approximately 750 mm, and the time interval between successive photos was 60 s. As the radius of the droplet reached 396 mm (footprintZ785 mm) on the microstructured surface, the transition from Cassie–Baxter to Wenzel regimes occurred, as indicated by the arrow. On the hierarchical structured surface, air pockets, visible at the bottom area of the droplet, exist until the droplet evaporated completely (Bhushan et al. in press).

microstructure, the fractional flat geometric area of the liquid–air interface under the droplet is (fLA)microZ1K(pD2/4P 2) (Bhushan & Jung 2008). For the hierarchical structure, the fractional flat geometric area of the liquid–air interface is ðfLA Þhierarchical Z 1KðpD 2 =4P 2 Þ½1KðfLA Þnano . The values of contact angle hysteresis in the Cassie–Baxter regime for various surfaces were calculated using equation (2.5). The values calculated for various surfaces are summarized in table 4. The values of static contact angle in the Wenzel and Cassie–Baxter regimes for various surfaces were calculated using the values of R f and fLA. As shown in figure 20, the experimental static contact angle values for the three structured surfaces were higher than the calculated values in the Cassie–Baxter regime. The results suggest that the droplets on the micro-, nano- and hierarchical structured surfaces were in the Cassie–Baxter regime. For contact angle hysteresis, there is a good agreement between the experimental data and the theoretically predicted values for the Cassie–Baxter regime. These results show that air pocket formation in the micro- and nanostructures decreases the solid–liquid contact. In hierarchical structured surfaces, air pocket formation occurs, which further decreases the solid– liquid contact and thereby reduces contact angle hysteresis and tilt angle. To further verify the effect of hierarchical structure on propensity for air pocket formation, evaporation experiments with a droplet on a microstructure and a hierarchical structure were performed (Bhushan et al. in press). Phil. Trans. R. Soc. A (2009)



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Figure 22. The remaining dust trace after droplet evaporation on (a) the microstructured and (b) the hierarchical structured surfaces. In (a) the transition occurred at a footprint of approximately 806 mm comparable to that in figure 20, and in (b) the dirt remained on the top of the structure, indicating that no transition occurred during the process of droplet evaporation as reported in figure 20 (Bhushan et al. in press).

Figure 21 shows successive photographs of a droplet evaporating on two structured surfaces. On the microstructured surface, light passes below the droplet and air pockets can be seen, so to start with the droplet is in the Cassie– Baxter regime. When the radius of the droplet decreased to 396 mm, air pockets are no longer visible, and the droplet is in the Wenzel regime. This transition results from an impalement of the droplet in the patterned surface, characterized by a lower contact angle. For the hierarchical structure, an air pocket was clearly visible at the bottom area of the droplet throughout, and the droplet was in a hydrophobic state until the droplet evaporated completely. It suggests that a hierarchical structure with nanostructures prevents the gaps between the pillars from being filled with liquid. Nosonovsky & Bhushan (2008a) reported that the liquid–air interface in the Cassie–Baxter regime is metastable. A hierarchical structure with convex-shaped nanostructures resists the destabilization of the liquid–air interface by pinning the triple line (line of contact between solid, liquid and air), and thus leading to stable equilibrium. In another experiment, to verify the transition, dust mixed in water was used, and the footprint and location of dirt were observed (Bhushan et al. in press). By looking at the footprint, one can calculate the droplet radius at which air pockets cease to exist. Figure 22 presents the dust trace remaining after a droplet with 1 mm radius (5 ml volume) was evaporated from the micro- and hierarchical structured surfaces. On the microstructured surface, shown in figure 22a, the dust particles remained at the top and also between the pillars, with a footprint size of approximately 806 mm, which corresponds to a droplet radius of 396 mm. On the hierarchical surface, shown in figure 22b, the dust particles remained on only a few pillars with a footprint size of approximately 250 mm, which corresponds to a droplet radius of 120 mm. It is observed that, as the transition occurred, the radius of the droplet on the hierarchical surface was smaller than that on the microstructured surface. This indicates that the hierarchical structure remains in the Cassie– Baxter regime and prevents the droplet from sinking, as observed in the evaporation experiments. Phil. Trans. R. Soc. A (2009)


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0.4 µm Figure 23. (a) SEM images taken at 458 tilt angle (shown using two magnifications) of the (i–iv) nanostructure and (v–viii) hierarchical structure fabricated with two different masses ((i)(ii) and (v)(vi) 0.6 and (iii)(iv) and (vii)(viii) 0.8 mg mmK2) of T. majus wax after storage at 508C with ethanol vapour. (b) SEM image taken at 458 tilt angle of three-dimensional tubule-forming nanostructures on the surface fabricated with 0.8 mg mmK2 mass of T. majus wax after storage at 508C with ethanol vapour (Bhushan et al. 2009a).

(c ) The influence of wax tubules on superhydrophobicity (i) Morphological characterization, wettability and adhesion forces of T. majus tubules For the development of nanostructures by tubule formation of T. majus wax, specimens were stored at 508C with ethanol vapour (Bhushan et al. 2009a). Figure 23a shows SEM micrographs of the nano- and hierarchical structures fabricated with two different masses (0.6 and 0.8 mg mmK2) of T. majus. The SEM images show an increase in amount of tubules on flat and microstructure surfaces after deposition of higher masses of wax. The tubules of T. majus wax grown are comparable to the wax morphology found on the leaves of T. majus. SEM images show a homogeneous distribution of wax mass on the specimen surfaces. As shown in figure 23b, the tubular waxes are hollow structures, randomly orientated on the surface. Their shapes and sizes show some variations, the tubular diameter varying between 100 and 300 nm and the length varying between 300 and 1200 nm. Recrystallization experiments showed that, for T. majus wax, the secondary alcohol nonacosan-10-ol and at least 2 per cent of diols form the tubules (Dommisse 2007). The remaining approximately 30 per cent of wax mass are long-chain hydrocarbons (Koch et al. 2006a), which might not contribute to the structure formation, but form a basal wax layer. Phil. Trans. R. Soc. A (2009)


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Figure 24. (a) (i)–(iii) Bar charts showing the measured static contact angle, contact angle hysteresis and tilt angle on various structures fabricated with 0.8 mm mmK2 mass of T. majus wax after storage at 508C with ethanol vapour. (iv) Bar chart showing the adhesive forces for various structures, measured using a 15 mm radius borosilicate tip. The error bar represents G1 s.d. (b) A plot of static contact angle and contact angle hysteresis as a function of mass of T. majus wax (circles, nanostructure; crosses, hierarchical structure) (Bhushan et al. 2009a).

AFM was used to characterize the nanostructure fabricated using T. majus wax of 0.8 mg mmK2. Statistical parameters of nanostructure (r.m.s. height, peak-to-valley height and summit density (h)) were calculated and are presented in table 3. To study the effect of nano- and hierarchical structures with different crystal densities on superhydrophobicity, Bhushan et al. (2009)a measured static contact angle and contact angle hysteresis. The data in figure 24a show that the static contact angle of a flat surface coated with a film of T. majus wax is 1128, but increased to 1648 when the wax formed a nanostructure of tubules. On a specimen with only a microstructure, the static contact angle was 1548, but increased to 1718 for the hierarchical structure. Contact angle hysteresis and tilt angle for flat, micro- and nanostructured surfaces show similar trends. The flat surface showed a contact angle hysteresis of 618 and a tilt angle of 868. The microstructured surface shows a reduction in contact angle hysteresis and tilt angle, but a water droplet still needs a tilt angle of 318 before sliding. As tubules are formed on the flat and microstructured surfaces, the nanostructured and hierarchical structure surfaces have low contact angle hysteresis of 5 and 38, respectively. Phil. Trans. R. Soc. A (2009)


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Table 5. Summary of the calculated roughness factor (R f) and fractional liquid–air interface ( fLA), and measured and calculated static contact angles and contact angle hysteresis on the various surfaces. Nanostructures and hierarchical structures were fabricated with 0.8 mg mmK2 of T. majus wax after storage at 508C with ethanol vapour. The variation represents G1 s.d. (Bhushan et al. 2009a). Rf flat nanostructure microstructure hierarchical structure a

11 3.5 14.5

fLA

static contact angle (8) contact angle hysteresis (8)

0.86 0.71 0.96

112G2.1 164G2.4 154G1.9 171G1.3

61G2.1a 5G1.1 27G1.5 3G0.7

Advancing and receding contact angles are 132 and 718, respectively.

Adhesive force measured by using a 15 mm radius borosilicate tip in an AFM also shows a similar trend as the wetting properties. The adhesion force of the hierarchical surface structure was lower than that of either micro- or nanostructured surfaces because the contact between the tip and the surface was lower as a result of the contact area being reduced (Bhushan 1999, 2002). Figure 24b shows a plot of the mean values of static contact angle and contact angle hysteresis as a function of the mass of T. majus wax deposited. As the mass of wax increased, the amount of tubules increased, and the static contact angle gradually increased and contact angle hysteresis decreased. The highest static contact angle and lowest contact angle hysteresis for the nanostructure are approximately 161 and 68, respectively, with 0.8 mg mmK2 mass. The highest static contact angle and lowest contact angle hysteresis for the hierarchical structure are approximately 171 and 38, respectively, with 0.8 mg mmK2 mass. As indicated earlier, the hierarchical structure can lead to air pocket formation, which allows desirable contact angles. In order to identify propensity for air pocket formation for the various surfaces, the roughness factor (R f) and fractional liquid–air interface (fLA) are needed. The R f and fLA values for the nano- and microstructures were calculated using the methods described earlier. The roughness factor for the hierarchical structure is the sum of (R f)micro and (R f)nano. The fractional geometric area of the top surface with T. majus wax for the nanostructures was found to be 0.14 and 0.13, leading to fLA of 0.86 and 0.87, respectively. For the hierarchical structure, the fractional flat geometric area of the liquid–air interface is ðfLA Þhierarchical Z 1KðpD 2 =4P 2 Þ½1KðfLA Þnano . The values calculated for various surfaces are summarized in table 5. The roughness factor and fractional liquid–air interface of the hierarchical structure are higher than those of micro- and nanostructures. These results show that air pocket formation occurs in hierarchical structured surfaces, which further decreases the solid–liquid contact and thereby reduces contact angle hysteresis and tilt angle. (ii) Morphological characterization, wettability and adhesion forces of lotus tubules For the development of nanostructures by tubule formation, lotus wax was used (Koch et al. in press). Figure 25a shows SEM images of flat surfaces with tubular nanostructures. The microstructures shown in figure 25b are the lotus Phil. Trans. R. Soc. A (2009)


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Figure 25. SEM images taken at 458 tilt angle (shown using three magnifications) of (a(i–iii)) nanostructure on flat replica, (b(i–iii)) microstructures in lotus replica and (iv–vi) micropatterned Si replica and (c(i–iii)) hierarchical structure using lotus replica and (iv–vi) micropatterned Si replica. Nano- and hierarchical structures were fabricated with 0.8 mg mmK2 mass of lotus wax after storage for 7 days at 508C with ethanol vapour (Koch et al. in press).

leaf and micropatterned Si replica covered with a lotus wax film. Hierarchical structures were fabricated with microstructured lotus leaf replicas and micropatterned Si replicas covered with a nanostructure of lotus wax tubules as shown in figure 25c. SEM images show an overview (figure 25a–c(i)(iv)), a detail in higher magnification (figure 25a–c(ii)(v)) and a high magnification of the created flat wax layers and tubule nanostructures (figure 25a–c(iii)(vi)). The grown tubules provide the desired nanostructure on flat and microstructured surfaces. The recrystallized lotus wax shows tubular hollow structures, with Phil. Trans. R. Soc. A (2009)


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Table 6. Summary of the calculated roughness factor (R f) and fractional liquid–air interface (fLA), and measured and calculated static contact angles and contact angle hysteresis on the various surfaces. Nanostructures and hierarchical structures were fabricated with 0.8 mg mmK2 of lotus wax after storage at 508C with ethanol vapour. The variation represents G1 s.d. (Koch et al. in press).

Rf flat nanostructure 18 lotus replica microstructure 6.9 hierarchical structure 24.9 micropatterned Si replica microstructure 3.5 hierarchical structure 21.5 lotus leaf hierarchical structure a

fLA

static contact angle (8)

contact angle hysteresis (8)

0.83

119G2.4 167G0.7

71G3.2 (162a, 91b) 6G1.1 (170a, 164b)

0.78 0.96

158G2.7 171G0.8

29G2 (165a, 136b) 2G0.7 (172a, 170b)

0.71 0.95

160G1.8 173G0.8

27G2.1 (169a, 142b) 1G0.6 (174a, 173b)

164G0.9

3G1.0 (166a, 163b)

Advancing contact angle. Receding contact angle.

b

random orientation on the surfaces. Their shapes and sizes show only a few variations. The tubular diameter varied between 100 and 150 nm, and their length varied between 1500 and 2000 nm. The lotus wax chemical composition shows as dominant components 67 per cent diols, mainly nonacosane-10,15-diol (34%), and additionally 19 per cent of the secondary alcohol nonacosan-10-ol (Koch et al. 2006a). The molecular formulae of the tubule-forming components are given in table 2. The remaining approximately 14 per cent of the wax is a small amount of aliphatic acids, and 13 per cent unidentified components, which are assumed to belong to the basal wax layer described for several plant species (Koch et al. 2006a; Koch & Ensikat 2008). AFM was used to characterize the nanostructure of the lotus wax tubules. The statistical parameters of the nanostructure (r.m.s. height, peak-to-valley height and summit density (h)) were calculated and are presented in table 3. To study the effect of lotus wax tubule nanostructures on superhydrophobicity, static contact angle, contact angle hysteresis and tilt angle were measured on flat, microstructured lotus replica, micropatterned Si replica and hierarchical surfaces. Hierarchical surfaces were made of the lotus leaf replica and micropatterned Si replica with a nanostructure of wax tubules on top. Additionally, fresh lotus leaves were investigated to compare the properties of the fabricated structures with the original biological model. For static contact angle, droplets of approximately 5 ml in volume were gently deposited on the surface using a microsyringe. The median values and standard deviation are given in table 6. Figure 26 shows that the highest static contact angles of 1738, the lowest contact angle hysteresis of 18, and tilt angle varying between 1 and 28 were found for the hierarchical structured Si replica. The hierarchical structured lotus leaf replica showed a static contact angle of 1718, and the same contact angle hysteresis (28) and tilt angles (1–28) as the hierarchical Si replica. Both artificial hierarchical structured surfaces show Phil. Trans. R. Soc. A (2009)


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Figure 26. (a)–(c) Bar charts showing the measured static contact angle, contact angle hysteresis and tilt angle on various structures fabricated with 0.8 mg mmK2 mass of lotus wax after storage at 508C for 7 days with ethanol vapour. (d ) Bar chart showing the adhesive forces for various structures, measured using a 15 mm radius borosilicate tip. Hierarchical structures were fabricated using lotus and micropatterned Si replicas superimposed with nanostructures of lotus wax tubules. The error bar represents G1 s.d. (Koch et al. in press).

similar values as the fresh lotus leaf surface investigated here, with static contact angle of 1648, contact angle hysteresis of 38 and a tilt angle of 38. However, the artificial hierarchical surfaces showed higher static contact angle and lower contact angle hysteresis. Structural differences between the original lotus leaf and the artificial lotus leaf produced here are limited to a difference in length of wax tubules, which are 0.5–1 mm longer in the artificial lotus leaf (Koch et al. in press). The melting of the wax led to a flat surface with a flat wax film, a lower static contact angle (1198), higher contact angle hysteresis (718) and high tilt angle of 668. The data for a flat lotus wax film on a flat replica show that the lotus wax by itself is hydrophobic. However, it has been stated that the wax on the lotus leaf surface, by itself, is weakly hydrophilic (Tuteja et al. 2008). We cannot confirm this. The data presented here demonstrate that the native, flat wax of lotus leaves, with a static contact angle of 1198, is hydrophobic and can turn superhydrophobic (1678) by increasing the surface roughness after self-assembly into threedimensional wax tubules. The static contact angle of the lotus wax film is 1198, which is higher than that of the T. majus wax film of 1128. However, films made of n-hexatriacontane showed static contact angles of only 918. SEM investigations, made directly after contact angle measurements, revealed no morphological differences between these films. Based on the chemical composition, it should be assumed that the non-polar n-hexatriacontane molecules are more hydrophobic than the plant waxes, which contain large amounts of oxygen atoms. At this point, these differences cannot be explained by structural or chemical differences of the films, but they will be of interest for further studies. Phil. Trans. R. Soc. A (2009)


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In order to identify propensity for air pocket formation for the various surfaces, the roughness factor (R f) and fractional liquid–air interface (fLA) are needed. The R f value for the nano- and microstructures in lotus leaf replica was calculated using the AFM maps. Since the entire microstructure cannot be imaged using an AFM because of the large height differences, the top and bottom scans were taken. The R f value of the top and bottom scans were then calculated and summed together to obtain the R f of the entire surface. The calculated results were reproducible within G5%. The R f value for the micropatterned Si replica was calculated using the method described earlier. The roughness factor for the hierarchical structure of the lotus replica and micropatterned Si replica is the sum of (R f)micro and (R f)nano. The values calculated for various surfaces are summarized in table 6. The fLA value for the micropatterned Si replica and hierarchical structure on micropatterned Si replica was calculated using the methods described earlier. The fractional geometric area of the top surface for the nanostructure, microstructured lotus replica and hierarchical structure on lotus replica was calculated from an SEM image with top view (08 tilt angle). The SEM image was converted to a high-contrast black and white image to eliminate the darker (smaller) structures that were visible in the original SEM image. The values calculated for various surfaces are summarized in table 6. The roughness factor and fractional liquid–air interface of hierarchical structure are higher than those of micro- and nanostructures. Adhesive force measured using a 15 mm radius borosilicate tip in an AFM also shows a similar trend as the wetting properties for the artificial surfaces (figure 26; Koch et al. in press). The adhesion force of the hierarchical surface structure was lower than that of the micro-, nanostructured and flat surfaces because the contact between the tip and the surface was lower as a result of the contact area being reduced. However, for the fresh lotus leaf, there is moisture within the plant material that causes softening of the leaf, and so when the tip comes into contact with the leaf sample, the sample deforms, and a larger area of contact between the tip and the sample causes an increase in the adhesive force (Bhushan 1999, 2002). (d ) Self-cleaning efficiency of hierarchical structured surfaces For deposition of contamination on artificial surfaces, various structures were placed in a contamination glass chamber (Bhushan et al. 2009b). Silicon carbide (SiC; Guilleaume, Germany) particles in two different size ranges of 1–10 and 10– 15 mm were used as the contaminant. SiC particles have been chosen because of their similarity in shape, size and hydrophilicity to natural dirt contaminations. The number of particles per area was determined by counting them from a 280!210 mm image taken by an optical microscope with a camera before and after water cleaning. For the cleaning test, specimens with the contaminants were subjected to water droplets of approximately 2 mm diameter, using two microsyringes (Bhushan et al. 2009b). In order to obtain a relative measure of the self-cleaning ability of hierarchical structures, which exhibit the lowest contact angle hysteresis and tilt angle compared with other structures (flat, nano- and microstructures), the tilt angle chosen for cleaning tests was slightly above the tilt angle for the hierarchical structure. Thus, experiments were performed with 38 for surfaces with lotus wax. The water cleaning test was carried out for 2 min Phil. Trans. R. Soc. A (2009)


Micro-, nano- and hierarchical structures epoxy resin flat with thin wax layer nanostructure 100

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Figure 27. Bar chart showing the remaining particles after self-cleaning experiments applying droplets with nearly zero kinetic energy on various structures fabricated from lotus wax using 1–10 and 10–15 mm SiC particles. The experiments on the surfaces with lotus wax were carried out on stages tilted at 38. The error bars represent G1 s.d. (Bhushan et al. 2009b).

(water quantity, 10 ml) with nearly zero kinetic energy of droplets. For watering with nearly zero kinetic energy, the distance between the microsyringes and the surface was set to 0.005 m (nearly zero impact velocity). The chosen impact velocity represents a low value compared with a natural rain shower, where a water droplet of 2 mm diameter can reach an impact velocity of 6 m sK1 (measured under controlled conditions; van Dijk et al. 2002). Figure 27 shows that none of the investigated surfaces was fully cleaned by water rinsing (Bhushan et al. 2009b). The data represent the average of five different investigated areas for each experiment. Most particles (70–80%) remained on smooth surfaces, and 50–70% of particles were found on microstructured surfaces. Most particles were removed from the hierarchical structured surfaces, but approximately 30 per cent of particles remained. A clear difference in particle removal, independent of particle sizes, was only found in flat and nanostructured surfaces, where larger particles were removed with higher efficiency. Observations of droplet behaviour during movement on the surfaces showed that droplets rolled only on the hierarchical structured surfaces. On flat, micro- and nanostructured surfaces, the droplets first applied did not move, but the continuous application of water droplets increased the droplet volumes and led to sliding of these large droplets. During this, some of the particles were removed from the surfaces. However, rolling droplets on hierarchical structures did not collect the dirt particles trapped in the cavities of the microstructures. The data clearly show that hierarchical structures have superior cleaning efficiency. Phil. Trans. R. Soc. A (2009)


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Figure 28. Schematic showing wetting of the four different surfaces fabricated. The largest contact area between the droplet and the surface is given in (a) flat and (c) microstructured surfaces, but is reduced in (b) nanostructured surfaces and is minimized in (d ) hierarchical structured surfaces.

6. Conclusion In this paper, the theoretical basis of superhydrophobicity and self-cleaning and the characterization of artificial superhydrophobic surfaces are presented. The effect of micro- and nanopatterned polymers on hydrophobicity was studied by analysing the roughness factor and static contact angle. It was found that increasing roughness on a hydrophilic micro- or nanostructured surface decreases the contact angle, whereas increasing roughness on a hydrophobic micro- or nanostructured surface increases the contact angle. Also, silicon surfaces patterned with pillars and deposited with a hydrophobic coating were studied to demonstrate how the effects of pitch value, droplet size and impact velocity influence the transition. For a droplet of fixed volume, the experimental observations showed that there is good agreement between the experimental data and the theoretically predicted values on patterned surfaces with varying pitch values. Surfaces with micro-, nano- and hierarchical structures were produced by replication of a micropatterned silicon surface and of the microstructure of a lotus leaf and by self-assembly of wax platelets (n-hexatriacontane) and tubules (T. majus and lotus) to create nanostructures. This two-step process provides flexibility in the fabrication of a variety of hierarchical structures. The influence of micro-, nano- and hierarchical structures on static contact angle, contact angle hysteresis, tilt angle, air pocket formation, adhesive force as well as efficiency of self-cleaning was studied. It was shown that, for micro-, nano- and hierarchical structures, the introduction of roughness increased the hydrophobicity of the surfaces. Micro-, nano- and hierarchical structures led to a high static contact angle, e.g. for lotus wax of the order of 160, 167 and 1738, respectively. Contact angle hysteresis for the hierarchical structure was the lowest. Structured surfaces provide air pocket formation. Air pockets inside the grooves underneath the liquid reduce the contact area between liquid and surface (figure 28), resulting in reduction in contact angle hysteresis, tilt angle and adhesive force. Based on low contact angle hysteresis, only the hierarchical structure provides the self-cleaning property even at low tilt angle. Structural parameters for superhydrophobicity and low static contact angle hysteresis, superior to those of lotus leaves, have been identified and provide a useful guide for the development of biomimetic superhydrophobic surfaces. The flexible and low-cost technique demonstrates that hierarchical surfaces can be produced in the laboratory for further investigations of the properties of hierarchical structured materials. Phil. Trans. R. Soc. A (2009)



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References Adamson, A. V. 1990 Physical chemistry of surfaces. New York, NY: Wiley. Barbieri, L., Wagner, E. & Hoffmann, P. 2007 Water wetting transition parameters of perfluorinated substrates with periodically distributed flat-top microscale obstacles. Langmuir 23, 1723–1734. (doi:10.1021/la0617964) Barthlott, W. & Neinhuis, C. 1997 Purity of the sacred lotus, or escape from contamination in biological surfaces. Planta 202, 1–8. (doi:10.1007/s004250050096) Bhushan, B. 1999 Principles and applications of tribology. New York, NY: Wiley. Bhushan, B. 2002 Introduction to tribology. New York, NY: Wiley. Bhushan, B. 2003 Adhesion and stiction: mechanisms, measurement techniques and methods for reduction. J. Vac. Sci. Technol. B 21, 2262–2296. (doi:10.1116/1.1627336) Bhushan, B. 2008 Nanotribology and nanomechanics—an introduction, 2nd edn. Heidelberg, Germany: Springer. Bhushan, B. & Jung, Y. C. 2006 Micro and nanoscale characterization of hydrophobic and hydrophilic leaf surface. Nanotechnology 17, 2758–2772. (doi:10.1088/0957-4484/17/11/008) Bhushan, B. & Jung, Y. C. 2007 Wetting study of patterned surfaces for superhydrophobicity. Ultramicroscopy 107, 1033–1041. (doi:10.1016/j.ultramic.2007.05.002) Bhushan, B. & Jung, Y. C. 2008 Wetting, adhesion and friction of superhydrophobic and hydrophilic leaves and fabricated micro/nanopatterned surfaces. J. Phys. Condens. Matter 20, 225010. (doi:10.1088/0953-8984/20/22/225010) Bhushan, B., Hansford, D. & Lee, K. K. 2006 Surface modification of silicon and polydimethylsiloxane surfaces with vapor-phase-deposited ultrathin fluorosilane films for biomedical nanodevices. J. Vac. Sci. Technol. A 24, 1197–1202. (doi:10.1116/1.2167077) Bhushan, B., Nosonovsky, M. & Jung, Y. C. 2007 Towards optimization of patterned superhydrophobic surfaces. J. R. Soc. Interface 4, 643–648. (doi:10.1098/rsif.2006.0211) Bhushan, B., Koch, K. & Jung, Y. C. 2008a Nanostructures for superhydrophobicity and low adhesion. Soft Matter 4, 1799–1804. (doi:10.1039/b808146h) Bhushan, B., Koch, K. & Jung, Y. C. 2008b Biomimetic hierarchical structure for self-cleaning. Appl. Phys. Lett. 93, 093101. (doi:10.1063/1.2976635) Bhushan, B., Jung, Y. C., Niemietz, A. & Koch, K. 2009a Lotus-like biomimetic hierarchical structures developed by the self-assembly of tubular plant waxes. Langmuir 25, 1659–1666. (doi:10.1021/la80249k) Bhushan, B., Jung, Y. C. & Koch, K. 2009b Self-cleaning efficiency of artificial superhydrophobic surfaces. Langmuir 25, 3240–3248. (doi:10.1021/la803860d) Bhushan, B., Koch, K. & Jung, Y. C. In press. Fabrication and characterization of the hierarchical structure for superhydrophobicity. Ultramicroscopy. Bico, J., Thiele, U. & Que´re´, D. 2002 Wetting of textured surfaces. Colloids Surf. A 206, 41–46. (doi:10.1016/S0927-7757(02)00061-4) Bunshah, R. F. 1994 Handbook of deposition technologies for films and coatings: science, technology and applications. Westwood, NJ: Applied Science Publishers. Burton, Z. & Bhushan, B. 2005 Hydrophobicity, adhesion, and friction properties of nanopatterned polymers and scale dependence for micro- and nanoelectromechanical systems. Nano Lett. 5, 1607–1613. (doi:10.1021/nl050861b) Burton, Z. & Bhushan, B. 2006 Surface characterization and adhesion and friction properties of hydrophobic leaf surfaces. Ultramicroscopy 106, 709–719. (doi:10.1016/j.ultramic.2005. 10.007) Cassie, A. & Baxter, S. 1944 Wettability of porous surfaces. Trans. Faraday Soc. 40, 546–551. (doi:10.1039/tf9444000546) Choi, S. E., Yoo, P. J., Baek, S. J., Kim, T. W. & Lee, H. H. 2004 An ultraviolet-curable mold for sub-100-nm lithography. J. Am. Chem. Soc. 126, 7744–7745. (doi:10.1021/ ja048972k) Phil. Trans. R. Soc. A (2009)


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