Controllable Water Adhesion and Anisotropic ... - ACS Publications

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Controllable Water Adhesion and Anisotropic Sliding on Patterned Superhydrophobic Surface for Droplet Manipulation Xiaolong Yang,† Xin Liu,† Yao Lu,‡ Jinlong Song,*,† Shuai Huang,† Shining Zhou,† Zhuji Jin,† and Wenji Xu† †

Key Laboratory for Precision and Non-Traditional Machining Technology of the Ministry of Education, Dalian, University of Technology, Dalian 116023, People’s Republic of China ‡ Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom S Supporting Information *

ABSTRACT: Superhydrophobic surfaces with hydrophilic patterns have wide applications in biomedical and chemical analysis domains. In this work, a rapid, simple, and top-down micromilling method was proposed for fabricating hydrophilic patterns such as micro dots, line, and circle grooves on superhydrophobic surfaces. This method could be extended to construct various linear patterns on diverse substrates on account of its mechanical material-removal mechanism. Hydrophilic micro dots, line, and circle grooves were milled on the superhydrophobic Al alloy surface. The milled micro dots demonstrate great adhesion toward water droplets without changing the contact angles, whereas the prewetted line grooves exhibit strong anisotropic water adhesion; that is, the resistance force that restricts the droplet from detaching in directions parallel and perpendicular to the grooves is significantly different due to the different stress state of the droplet on the grooves. The adhesion force perpendicular to the groove, together with the sliding resistance that is generated by the milled dot, was investigated through experiments. The results show that the relationship between the adhesion force and the droplet−pattern interfacial widths could be well described using the classical Furmidge equation. This research could possibly be employed in such biomedical and chemical analysis domains as water harvesting, droplet storage, and droplet transport.

1. INTRODUCTION Surfaces with contact angles (CAs) higher than 150° are demonstrated as ideal water-repellent surface and have been investigated intensively on account of the wide applications, including self-cleaning,1,2 drag reduction,3 anti-icing,4 and oil/ water separation.5−7 On the contrary, inspired by the rose petal, a kind of sticky superhydrophobic surface has been reported.8 On the surface, CAs of water droplets usually exceed 150°, while the droplets would be pinned and would not detach even if the substrate was turned upside down. Such high-adhesion superhydrophobic surfaces provide important enlightenment on the design of functional surfaces for lab-on-chip applications.9,10 Recently, biomimetic superhydrophobic surfaces with controllable high-adhesion patterns have drawn more and more attention due to their potential biomedical and industrial applications in water harvesting,11 droplet manipulation,12−20 and other fluidic devices.21−23 In nature, rice leaves have the capacity of directionally controlling the droplets motion on account of the different sliding angle (SA) values along two directions, namely, anisotropic sliding.24−26 This anisotropic sliding property is attributed to the long-term evolution and allows the leaves shed water droplets from the veins to the roots. The anisotropic sliding effect on superhydrophobic surfaces can find various applications in liquid manipulations for lab-on-chip devices.24 © 2016 American Chemical Society

So far, superhydrophobic surfaces with controllable adhesion or anisotropic sliding properties are usually realized by patterning of hydrophobic or hydrophilic regions at specific areas.16,27 Various methods including soft lithography technology,28 laser irradiation,15,29,30 plasma treatments,31−34 lithography,19,33,35−37 drop-casting,23 desktop printing technology,38−40 and ink patterning27,41 have been reported to obtain such patterning surface. For instance, Lee et al.19 prepared a reproducible superhydrophobic surface with hydrophilic patterns by dip-coating of dodecyltrichlorosilane (DTS) and UV-enhanced photodecomposition of the DTS, which could make water droplets roll exactly along the designed tracks with a high speed. Hess and Breedveld et al.38−40 reported a lab-onpaper device by printing the high adhesion wax or black phase ink on superhydrophobic papers and systematically investigated the droplet adhesion versus the printed pattern shape and dimensions. Lai et al.27,41 presented a patterned superhydrophobic surface with high adhesion and contrast wettability by a site-selective alcohol-based or oil-based ink patterning method. The created wettability patterns on the superhydrophobic surface can be easily removed and Received: February 28, 2016 Revised: March 23, 2016 Published: March 23, 2016 7233

DOI: 10.1021/acs.jpcc.6b02067 J. Phys. Chem. C 2016, 120, 7233−7240

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The Journal of Physical Chemistry C regenerated. Franssila et al.33 proposed a method combining lithography and plasma oxygen treatment to successfully fabricate precision completely wetting areas on the deep reactive ion-etched silicon superhydrophobic surfaces. Hwang et al.30 achieved patterns with different wettability using an industrial laser system. Wang et al.17 fabricated superhydrophobic films from carbon nanotube (CNT)/polybenzoxazine coatings, and a groove scraped on the surface with a blade exhibits interesting anisotropic wettability. These methods can achieve excellent hydrophilic or superhydrophilic patterns on diverse superhydrophobic substrates and have provided enlightenments on the future research; however, to the best of our knowledge, the direct, rapid, and easy-to-implement traditional machining technology has been scarcely reported to prepare hydrophilic patterns on superhydrophobic surfaces. Milling, a commonly used traditional machining technique, could fabricate smooth structures on various materials. Herein, micromilling was proposed for the first time to construct smooth patterns such as micro dots and grooves on the superhydrophobic Al alloy surfaces. The milled regions are smooth and have a very clear boundary with the surrounding rough superhydrophobic areas. Because of the absence of the essential micronano structures for superhydrophobicity, the fabricated patterns show specific adhesion and wettability. For instance, the sliding resistance of droplets on the micro dot patterns is much larger than that on the superhydrophobic surfaces, and the prewetted groove patterns exhibit anisotropic adhesion toward water droplets in directions parallel and perpendicular to the groove. The underlying mechanism of this adhesion phenomena was analyzed systematically through experiments.

(ZYGO, America). Photographs and videos of the sliding behaviors were captured using an SLR camera equipped with an EFS 18-135 mm lens (Canon 700d, Japan). CCD images of water droplets on the samples were taken using a MVVD030SC electron microscope (Microvision, China), and contact angles were measured in a mapping program based on the definition of CA. The sliding angles (SAs) were measured by a precision whirler. CA and SA values are the average of five measurements at five different locations.

3. RESULTS AND DISCUSSION Morphology. Figure 1a shows the SEM images of the electrochemically etched Al alloy surface. Numerous step-like

Figure 1. SEM images of the superhydrophobic surface and the micro patterns on the surfaces: (a) Hierarchical structure of the superhydrophobic Al alloy surface. The insert is the magnified image of the rough structure. (b) Micro dot, (c) line groove, and (d) arc groove were fabricated on the superhydrophobic Al alloy surface using micromilling technique. The inserts in panels b−d are the magnified images of the pattern bottoms. Scale bars: (a) 10 μm, (b−d) 300 μm, and (b−d insets) 10 μm.

2. EXPERIMENTAL METHODS Materials. 6061 aluminum alloys sheets (3 mm thick) were purchased from the Suzhou Metal Material Manufacturer (China). Fluoroalkylsilane [FAS, C8F13H4Si(OCH2CH3)3] was purchased from the Deguassa Co. (Germany). All chemicals are analytically pure and were used as just received. Fabrication of Superhydrophobic Surfaces. The Al alloy plates (60 × 60 mm) were polished using 800# and 1500# abrasive papers and ultrasonically cleaned in deionized water to remove oxide layers and impurities. The pretreated plate was then electrochemically etched at 600 mA·cm−2 for 8 min in the 0.1 mol·L−1 NaCl aqueous solutions.42 After the etching process, the sample was ultrasonically cleaned in deionized water, dried, and subsequently immersed in the 1 wt % ethanol solution with fluoroalkylsilane [FAS, C8F13H4Si(OCH2CH3)3] for 90 min to lower the surface energy. Fabrication of Micro Patterns on the Superhydrophobic Surface. A milling system was set up by mounting a motorized spindle on the Z axis of a three-axis motion platform. The relative motion of the motorized spindle and the workpiece could be accommodated by the control system of the three-axis motion platform. Utilizing the milling system, micro dots and groove patterns were milled on the superhydrophobic surface. The spindle speed of 20 000 r·min−1 and the scanning rate of 10 mm·min−1 were held constant during this process. Characterization. The morphology of the prepared surface and the milled patterns was characterized by a JSM-6360LV scanning electron microscopy (SEM, JEOL, Japan). The 3D model, surface profile, and the roughness of the samples were investigated on a NewView 5022 3D surface profilometer

embossments and cavities with sizes of 1−5 μm were homogeneously distributed on the surface, as demonstrated in Figure 1a and the inset of Figure 1a. Neighboring embossments hold together and therefore form microscale plateaus and holes. These hierarchical structures have the same function as those micro/nanostructures of the lotus,43 which were essential for superhydrophobicity. The average CA of 5 μL droplets on the as-prepared surface was 163.8°, and the average SA was 6.8°, indicating excellent dynamic superhydrophobicity. A micro dot with a diameter of 0.5 mm, a line groove with a width of ∼0.47 mm, and an arc groove with a width of ∼0.26 mm were milled on the prepared superhydrophobic surface (Figure 1b−d). Bottoms of the patterns are smooth (insets of Figure 1b−d), and boundaries between the patterns and the surrounding superhydrophobic areas are distinct (Figure 1b−d). The 3D models and surface profiles (Figure 2) of the milled patterns were investigated by a 3D surface profilometer. The 3D models (Figure 2a,b) show that the milled dot and groove were well patterned and have legible outlines, which is consistent with the SEM images in Figure 1b−d. The cross-sectional profiles of the micro dot and groove in Figure 2c,d indicate that heights of the patterns are ∼40 μm. In addition, it can be seen from Figure 2e,f that the roughness of the milled pattern surface is much smaller than that of the superhydrophobic surface. 7234


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Figure 2. 3D models of (a) the milled micro dot and (b) groove pattern. Cross-sectional profiles of (c) the micro dot and (d) groove pattern; the patterns have smooth internal surfaces and distinct boundaries. (e) Surface profile of the bottom surface in the patterns. (f) Surface profile of the superhydrophobic area.

Figure 3. (a) Photographs of 15 μL water droplets laid on the superhydrophobic area (left) and sliding off the surface (right). (b) Photograph of 15 μL water droplets laid on the milled micro dot (left) and sliding off the dot (right). (c) Schematic illustration of a droplet sliding off a milled dot on the superhydrophobic surface. (d) Residual water stain left in dot after water droplets sliding off the dot. Scale bars: (a,b) 1 mm and (d) = 200 μm.

Controllable Adhesion. Controllable water adhesion on patterned superhydrophobic surfaces has many potential applications in water harvesting11 and biotechnological devices.20 Figure 3a,b shows CAs and SAs of 15 μL water droplets on the superhydrophobic area and the milled micro dot pattern (diameter of 0.5 mm), respectively. The SAs of droplets on the superhydrophobic area (the right image in Figure 3a) and on the dot (the right image in Figure 3b) were 3 and 30°, respectively, showing that the micro dot has great

additional adhesion toward water droplets (see Supporting Information Movie S1); however, the CAs on the superhydrophobic area and on the dot both exceed 150° (the left images in Figure 3a,b), which are not significantly different due to the surrounding superhydrophobic contact areas. Schematic of the specific adhesion phenomenon was illustrated in Figure 3c. When contacted with the water droplet, the milled micro dot surface was readily wetted and exhibited hydrophilic due to the lack of essential micro/nanostructures for superhydropho7235


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The Journal of Physical Chemistry C bicity. Therefore, the water droplet laid on the dot has to overcome the additional adhesion force Fad‑adh, which is ascribed to the fluid tension and is much higher than that adhesion force Fs‑adh on the superhydrophobic surface. The residual water stain left on the dot after water droplets sliding off indicates that the milled micro dot was hydrophilic (Figure 3d). By modifying diameters of the dots, controllable water adhesion on the surface is feasible. Dots with various diameters were obtained using different sized milling cutters, and SAs of different-volume water droplets on the dots were measured. On the basis of research in the midtwentieth century,44−46 the adhesion force (Fadh) that restricts the droplet from sliding off inclined surfaces is proportional to the width (Wdroplet) of the droplet−solid interfaces perpendicular to the sliding direction and can be expressed as the following semiempirical Furmidge equations45 ρVg sin α Fadh = = γLV(cos θR − cos θA) Wdroplet Wdroplet

Figure 4. Experimental and predicted sliding angles versus the droplet volumes on milled dots with different diameters.

(1)

where ρ is the liquid density, V is the droplet volume, g is the acceleration of gravity, γLV is the liquid surface tension, and θR and θA are the receding and advancing contact angles of droplets on the surface. Using the θR and θA of 166.2 and 149.9° as experimentally measured with 5 μL droplets, the acceleration of gravity g of 9.8 m/s2, the liquid surface tension γLV of 72.0 μN/mm, and the measured droplet−substrate interfacial width of 0.878 mm, the sliding angle of 5 μL droplets on the electrochemically etched superhydrophobic surface could be predicted to be 7.9°, which is consistent with the experimental value of 6.8 ± 1.4°. When the droplet was laid on the hydrophilic micro dot, the droplet−substrate interface turned into composite; that is, besides the Fs‑adh between the droplet and superhydrophobic areas, there would be an additional contribution (Fad‑adh between the droplet and hydrophilic dot) to the overall adhesion force Foverall. Assuming these two adhesion forces are independent, the predicted adhesion force Foverall that prevents the droplet from sliding off a hydrophilic dot can thereby be calculated as the following equations38

water droplets on the prewetted 0.47 mm wide groove were 76 and 159°, respectively (the left images in Figure 5a,b), which have significant anisotropy because of the droplet easily spreading out in the direction parallel to the groove pattern due to the capillary (Figure 5c). In addition, the groove pattern demonstrates directionally controlled SAs. The parallel SA (SA//) is ∼3.6° (the right images in Figure 5a), showing little resistance along the groove, while the perpendicular SA (SA⊥) is over 90° (the right images in Figure 5b) on the contrary, displaying strong sliding resistance to the droplet (see Supporting Information Movie S2). The interface of the droplet on the groove is composed of two parts: the droplet− groove interface Adg and the droplet−substrate interface, which corresponds to the surrounding superhydrophobic areas contacted with the droplet. To slide along the groove, the droplet should overcome the overall viscous force Fvis of water in the region Adg and the negligible interfacial force Fdsu between droplets and the superhydrophobic areas (Figure 5c,d). On the basis of Newton’s Viscosity Law,47 viscous force Fvis is a function of contact areas Adg, dynamic viscous coefficient μ, and the velocity gradient du/dy

Foverall = Fad ‐ adh + Fs ‐ adh = [WdotγLV(cos θRHD − cos θASU)] + [(Wdroplet − Wdot)γLV(cos θRSU − cos θASU)]

(2)

Fvis = Adg μ

where Wdot is the diameter of the milled hydrophilic dot and θRHD is the receding contact angle of the hydrophilic dot, which can be calculated to be 42.5° using the experimental values. θRSU and θASU are the receding and advancing contact angles on the electrochemically etched superhydrophobic surface, and Wdroplet is the overall droplet-substrate interfacial width, which can be obtained by experimental measurements. On the basis of eq 2, it can be inferred that different SAs of the same droplet could be realized by regulating the diameters of the milled dots (Wdot). Figure 4 shows the measured SAs of different-volume droplets on the milled dots with different diameters, which indicate a wide range of water adhesion force could be acquired. The dotted lines in Figure 4 correspond to the predicted SA values calculated using eq 2, which demonstrates a good agreement with the experimental values. Anisotropic Sliding. The groove patterns milled on the superhydrophobic surfaces were hydrophilic and exhibited directionally controlled droplet shapes and SAs (Figure 5). The parallel CA (CA//) and perpendicular CA (CA⊥) of 15 μL

du dy

(3)

The velocity gradient approaches 0 at the droplet−groove interface due to the low sliding velocity; therefore, the viscous force Fvis can be ignored. However, when sliding in the direction perpendicular to the groove, the droplet would mostly need to conquer the liquid tension Fp‑adh (Figure 5e), which is much higher than the combination of the viscous force Fvis and the interfacial force Fdsu. On account of the difference of the adhesion mechanism in two directions, droplets sliding parallel to the groove are much easier than that perpendicular to the groove, displaying strong sliding anisotropy. By adjusting the groove widths or droplet volumes, the different degree of the SA anisotropy could be realized (Figure 6). The SAs in two directions both decreased with increasing volume of the droplets due to the larger gravity than the sliding resistance. With constant droplet volume, the SA⊥ together with the effect of anisotropic contrast increased upon increasing the groove widths, while SA// were almost constant maintaining at very low level. 7236


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Figure 5. Photographs of static contact angle (left) and sliding angle (right) (a) parallel and (b) perpendicular to the groove pattern. (c) Top-view illustration of a droplet sliding off along the milled groove; the inset is a CCD image of a 15 μL droplet on the groove pattern, which is consistent with panel c. Schematic cross-sectional illustration of a droplet sliding off the groove pattern in directions (d) parallel and (e) perpendicular to the groove. Scale bars: (a,b) 1 mm.

Figure 6. Parallel and perpendicular SAs of various volume droplets on groove patterns with different widths.

Controllable Adhesion Force. The sliding anisotropy was influenced mainly by the SAs⊥ (Figure 6), which is determined by the adhesion force Fp‑adh. According to the previous discussion, the adhesion force that restricts droplets from sliding off patterns was the function of the droplet−pattern interfacial width, the droplet−substrate interfacial width, and the CA hysteresis (the receding and advancing CAs on the pattern and on the substrate). Therefore, the droplet−groove interfacial widths Wcon‑gro were first investigated. Figure 7 shows the CCD images of water droplets contacting on the milled line grooves with widths of 0.28 and 0.47 mm. CAs⊥ varied slightly with the increasing droplet volume, while CAs// increased greatly. The droplet−groove interfacial widths of differentvolume droplets on grooves with different widths were obtained by measuring the droplet−groove contact lengths parallel to the groove (Figure 8a). For constant droplet volumes, the interfacial widths increased with the increasing groove widths, showing that droplets have the trend to cover on the grooves with larger widths. For each groove pattern, the interfacial widths rose with the increase in droplet volumes. For a droplet to slide perpendicular to the milled groove, it needs to deform so that the receding and advancing CAs can both reach the critical values. The deformation makes the front end of the droplet contacted with the superhydrophobic area,

Figure 7. Images of different-volume droplets contacting on the groove patterns with widths of 0.28 and 0.47 mm: (a) Parallel and (b) perpendicular droplet contacting images on the 0.28 mm wide groove pattern; (c) parallel and (d) perpendicular droplet contacting images on the 0.47 mm wide groove pattern. From left to right, the droplet volumes were 1, 5, 10, 20, and 30 μL. Scale bars = 1 mm.

while the trailing end is still pinned on the milled grooves. In this case, the receding CA is the value of the prewetted milled groove (defined as θWR), and the advancing CA is set by the electrochemically etched superhydrophobic surface. The perpendicular adhesion force Fp‑adh could therefore be modeled as the following equation Fp ‐ adh = Wcon ‐ groγLV(cos θ WR − cos θASU)

(4)

Using eq 4 to fit the experimental values of Fp‑adh (Figure 8b), the results are in great coincidence with the experimental values having a minimum relative error of 0.05%, and most of them are under 15% except a maximum of 15.9%. Equation 4 could be applied for predicting the sliding resistance of droplets on hydrophilic grooves and providing a reference for the design of lab-on-chip devices. 7237


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Figure 8. (a) Droplet-groove interfacial width of various-volume droplets on groove patterns with different widths. (b) Adhesion force versus the droplet-groove interfacial width; the straight line in panel b corresponds to the results fitted using eq 4.

Figure 9. (a) Sequence of images displaying a 5 μL droplet guided by a circle track on the superhydrophobic substrate with a tilted angle of 25°. (b) Droplet mergence process on the track. (c) Droplet transport on the face-down substrate with a tilted angle of 25°. The circle track has a diameter of 20 mm and a groove width of 0.38 mm. Scale bars: 10 mm.

Droplet Transport. Hydrophilic grooves on superhydrophobic surface have strong anisotropic adhesion toward water droplets (Figure 5), which can be applied to directionally

transport the liquid. A circle groove with diameter of 20 mm and groove wide of 0.38 mm was milled on the superhydrophobic Al alloy surface. The patterned substrate was tilted 7238


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The Journal of Physical Chemistry C at ∼25°, and a syringe needle was set above at the highest point of the circle groove. A 5 μL droplet released from the syringe needle slid precisely along the route, crossed the lowest point at 267 ms, keeping moving upward along the circle due to the kinetic energy, and finally stabilized at the bottom after two oscillations at 717 ms (Figure 9a). More droplets were continued to be released then transported and got merged at the bottom until it broke away from the groove after eight droplets (Figure 9b, see Supporting Information Movie S3). In light of the previous analysis, the droplet would break away from the groove when the component of gravity in sliding direction was larger than the adhesion force Fp‑adh. 5 μL droplets could stay firmly on the groove even the substrate was turned upside down (the far left in Figure 9c) because of the strong adhesion produced by the groove pattern. The droplet began to slide when the substrate was tilted at ∼25° (Figure 9c), and similar to the process in Figure 9a it was transported and swung three oscillations before stopping at the bottom (see Supporting Information Movie S3). The droplet crossed the lowest point at 100 ms, indicating a higher sliding speed than that on the face-up substrates. That is because when the substrate faced down the droplet would detach from the surrounding superhydrophobic areas due to gravity, there would be only Fvis but no Fdsu. Meanwhile, the adhesion force Fp‑adh could hold the droplet and prevent the perpendicular sliding off the groove. Hence, to realize the water transport on a face-down substrate, the adhesion force Fp‑adh should be larger than the gravity of the manipulated droplet, as illustrated by eq 5 Fp ‐ adh = Wcon ‐ groγLV(cos θ WR − cos θASU) > ρgV

adhesion-control method as well as the anisotropic adhesion mechanism would both have enlightening effects on the design of the lab-on-chip devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b02067. Movie S1: Video of droplets sliding off superhydrophobic area and the milled dot with a diameter of 0.5 mm. (AVI) Movie S2: Video of water droplets sliding off the milled 0.47 mm wide groove in directions parallel and perpendicular to the groove. (AVI) Movie S3: Scene 1: Droplet transport and mergence implemented by a milled circle track with a diameter of 20 mm and groove width of 0.38 mm. Scene 2: Droplet transport on a face-down substrate with a tilted angle of 25°. (AVI) Movie S1−S3 captions. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: 86-411-84708422. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

(5)

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to National Natural Science Foundation of China (NSFC, Grant Nos. 51275072 and 51305060) and the Fundamental Research Funds for the Central Universities (DUT15RC(3)066).

4. CONCLUSIONS In summary, hydrophilic smooth structures including micro dots, straight, and circle grooves on the superhydrophobic Al alloy surface were constructed for the first time by micromilling technology, which is a direct, environmentally friendly, easy-tooperate, and universal method on account of its mechanical material-removal mechanism. By regulating the size of the milled patterns, the adhesion force toward droplets can be controlled. The milled dots exhibit great sliding resistance because of the high water adhesion. Experimental SAs are in good agreement with the theoretical values predicted using the Furmidge equation. When a droplet was placed on prewetted milled grooves, adhesion force that restricts the droplet from sliding in directions parallel and perpendicular to the grooves showed significant difference. The perpendicular adhesion force was much larger than the parallel ones and was similar to the sliding resistance of droplets laid on the dot. The perpendicular adhesion force calculated using the Furmidge equation fits well with experimentally measured results. Finally, a circle groove pattern was milled on the electrochemically etched superhydrophobic substrate and was applied to transport droplets. Because of the strong anisotropic adhesion, droplets could slide easily and precisely along the route even if the substrate was flipped over, and according to the analysis, the terms for droplet transport on a face-down substrate were that the adhesion force perpendicular to route is larger than the droplet gravity. This simple patterning method can be extended to diverse substrates and is valuable for applications such as water harvesting, droplet transport, and storage. The discussed

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REFERENCES

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