Controlled-motion of floating macro-objects induced ...

Controlled-motion of floating macro-objects induced by light Daniele E. Lucchetta, Francesco Simoni, Luca Nucara, and Riccardo Castagna Citation: AIP Advances 5, 077147 (2015); doi: 10.1063/1.4927419 View online: http://dx.doi.org/10.1063/1.4927419 View Table of Contents: http://scitation.aip.org/content/aip/journal/adva/5/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in SU-E-J-115: Correlation of Displacement Vector Fields Calculated by Deformable Image Registration Algorithms with Motion Parameters of CT Images with Well-Defined Targets and Controlled-Motion Med. Phys. 42, 3291 (2015); 10.1118/1.4924202 Liquid mixing driven motions of floating macroscopic objects Appl. Phys. Lett. 90, 144102 (2007); 10.1063/1.2719029 Position of objects floating in a glass Phys. Teach. 36, 410 (1998); 10.1119/1.879907 LETTERS: Floating Objects Phys. Teach. 10, 67 (1972); 10.1119/1.2352102 LITTLE THINKERS: A Floating Object Phys. Teach. 2, 290 (1964); 10.1119/1.2350831

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AIP ADVANCES 5, 077147 (2015)

Controlled-motion of floating macro-objects induced by light Daniele E. Lucchetta,1,a Francesco Simoni,1 Luca Nucara,2 and Riccardo Castagna3,b 1

Dipartimento SIMAU, Università Politecnica delle Marche, Via Brecce Bianche, 60131 Ancona, Italy 2 The BioRobotics Institute, Scuola Superiore Sant’Anna, Viale Reginaldo Piaggio 34, 56025 Pontedera (PI), Italy; Center for Micro-BioRobotics @SSSA, Istituto Italiano di Tecnologia, Viale Reginaldo Piaggio 34, 56025 Pontedera (PI), Italy 3 NEST, Istituto Nanoscienze – CNR, Scuola Normale Superiore di Pisa, 56127 Pisa, Italy

(Received 30 April 2015; accepted 14 July 2015; published online 22 July 2015) Photons energy can be conventionally converted to mechanical work through a series of energy-expensive steps such as for example delivery and storage. However, these steps can be bypassed obtaining a straightforward conversion of photons energy to mechanical work. As an example, in literature, high power near infrared light is used to move small objects floating on fluid surfaces, exploiting the Marangoni effect. In this work we use a low power non-collimated visible laser-light to induce thermal surface tension gradients, resulting in the movement of objects floating on fluid surfaces. By real time tracking of the object trajectories, we evaluate the average applied driving force caused by the light irradiation. In addition we show how transparent objects can be moved by light when the supporting fluids are properly doped. C 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4927419]

Conversion of photons energy to mechanical work usually requires intermediate, energyexpensive steps which makes the use of light unfavorable for generating work. In order to bypass these steps, different approaches have been proposed.1–3 In this work we focus our research on light induced-motion phenomena in fluids. In particular, we use visible laser light to induce a local change of the surface tension of different liquids. The surface tension gradient, deriving from this change, is responsible for the motion of small floating objects placed on the fluid surface. These phenomena fall into the general framework of the Marangoni effect.1 The Marangoni effect consists of a mass transfer along an interface between two fluids due to a surface tension gradient. The gradient on the liquid surface can be thermally or chemically generated. Regarding thermal surface tension effects, the fast motion of a composite engineered object on fluid surfaces2 has already been shown. In this case the heating source was a high power IR laser light. Other interesting approaches involve the possibility of chemically moving micro-droplets of oil in micro-channels by light induced-pH gradients3 or through a photo-induced cis-trans isomerization effect.4 In the present work we use a low power visible light source to induce a thermal surface tension gradient on fluid surfaces. The gradient created in this way is exploited to effortlessly move a small object on the surface. In addition we show how even transparent objects can be moved by light when the supporting fluid is properly doped.5 In the first case in order to induce motion we need to asymmetrically heat the object by illuminating half of it in order to maximize the thermal gradient. The resulting motion will be along the r direction connecting the irradiated (warmer) and not irradiated (cooler) regions. In the second case, the laser light impinges on the doped liquid near (not on) the floating

a E-mail: [email protected] b E-mail: [email protected]

2158-3226/2015/5(7)/077147/5

5, 077147-1

© Author(s) 2015

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object which tends to move away from the illuminated area. In other words, the two approaches are symmetric: in the first one we apply heat to the floating object whereas in the second one we apply heat to a doped fluid. In both cases the result is the motion of the object. In order to characterize the motion we performed a series of experiments on different liquids using an Ar++ laser as the light source. The objects used are simply circular cork pieces having a diameter of 5 mm, a thickness of ∼ 2 mm and a mass m = 0.0058 g. A computer controlled Basler camera mod. acA2000-50gc (max resolution of 2046 x 1086 pixels and maximum acquisition speed of 50 fps) connected to a 5x standard microscope objective was used to record the object displacements as a function of time during the irradiation process. All the acquired frames were post-processed and the object position tracked using a proprietary algorithm. We also used a laser wavelength λ = 476.5 nm and a power P = 300 mW (laser spot diameter ∼ 3 mm). Concerning the motion on undoped fluids, a typical result is shown in Fig. 1 where the cork’s displacement on two different liquids, namely ethanol and water, is reported as a function of time. Our experimental approach consists of maintaining the impinging laser-light in the same fixed spatial position, while the object moves away from the irradiated area as shown in the inset of Fig. 1. The asymmetrically irradiated object starts moving due to the light-induced surface tension gradient until it stops due to the action of dissipative viscous forces. Each curve in the figure clearly shows three different stages of irradiation. At the very beginning, the curves are almost flat: the object slowly moves until the fluid temperature reaches a value corresponding to the onset of a suitable thermal gradient. Suddenly, the object accelerates until the heating effects are relevant. When the object is outside of the irradiated area, the thermal gradient becomes quickly ineffective and a deceleration process takes place until the motion stops. The object motion along the r direction is described by the following equation:2 (

( )2 ) D dr d 2r − =0 + 2 m dt m dt

(1)

where is the average force due to light irradiation and D is defined by D = 1/2ρSCD . In the last equation ρ is the density of the solution, S the displaced area and CD the drag coefficient. When the heating effects prevail on the dissipative term, the object velocity increases until a maximum value is reached. At this point the acceleration is zero and there is equilibrium between the two terms. After that, the motion slows down and stops. These two situations are clearly visible in the two curves reported in Fig. 1, where the inflection point is evident. The irradiation effects are important while even a small portion of the cork disk stays in the laser spot region. Outside this area the contribution to the motion due to irradiation becomes negligible. Indeed, in our experimental conditions the direct irradiation of the fluid doesn’t significantly affect its surface tension (for either water or ethanol). Thus, the contribution to the object motion comes only from the heat transmitted from the irradiated cork’s area to the supporting fluid. When this effect becomes negligible, the deceleration process occurs and eq. (1) becomes:

FIG. 1. Object displacements as function of time in ethanol (filled triangles) or water (empty triangles). Inset shows the laser spot and the tracked cork disk at the beginning (a) and at the ending (b) of motion.

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(

) ( )2 D dr d 2r + =0 m dt dt 2

(2)

that is solved to give the following two expressions for velocity and displacement: v (t) =

v0 1+

Dv0 m

(t − t 0)

(3)

and ( ) Dv0 m (t − t 0) x (t) = x 0 + ln 1 + D m

(4)

Equation (4) is successfully used to fit the experimental data concerning the deceleration process. As an example, in figure 1 it is reported as a dashed line the experimental data fit concerning the motion of the cork disk on water made using D = 0.047 g/cm and v0 = 0.59 cm/s. Once D is known, the drag coefficient CD or the density of the solution ρ can be calculated. Moreover, the average value of Fl is evaluated by fitting the first part of the curve using the calculated values for D and the solution of eq. (1):  D m (t − t 0) (5) x = x 0 + log(cosh( D m In our experiments, we obtain the following values for ∼ 0.1 µN for water and ∼ 0.5 µN for ethanol. In order to validate our experimental approach a comparison between the expected values of the temperature difference ∆T, calculated as reported in Ref. 1, and those derived when a direct measurement is performed. The measurement is made by placing a thermal sensor just under the floating cork disk during irradiation. The results for the two different tested fluids are shown in Fig. 2. After the start of the irradiation, the rise of temperature when the supporting fluid is ethanol is about ∆T = 1.5 ◦C. This value is achieved in a very short period of time. In contrast in water the same system takes a much longer time to record a similar temperature change. These results are explained taking into account the different specific heat capacities of the two fluids. In other words, the different physical properties of the liquids affect the floating disk motion resulting in a faster process when ethanol is used. The average value for Fl - obtained from the experimental fit - is connected to the Marangoni number Ma in the following way: < Fl > = Ma ηα = −

dγ L∆T dT

(6)

FIG. 2. Induced temperature change in water (empty circles) and ethanol (filled squares) as function of the irradiation time measured using a PT100 thermocouple placed under the cork disk and near the upper liquid surfaces.

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FIG. 3. Sample displacement as function of time when the cork disk floats on 4, 4′-di-hydroxy-azobenzene doped solution (0.05% w/w). The laser spot is at a starting distance of ∼ 2 mm from the sample.

Where η is the dynamic viscosity and α the thermal diffusivity. Since the temperature dependence of the surface tension for water is ∼ 1.8 µN/cm K - whereas for ethanol it is around ∼ 0.9 µN/cm K and since our disk has an absorbing region whose length is ∼ 0.5 cm, we evaluate ∆T ∼ 0.1 K for water and ∆T ∼ 1.1 K for ethanol. Both values are comparable, as order of magnitude, to the ones measured. Concerning the motion on doped fluids, it is worth noting how by doping the ethanol with an azobenzene-based molecule (4, 4′-di-hydroxy-azobenzene, 0.05 % w/w, synthesized by us according to Ref. 5) able to absorb part of the irradiated energy, the object is slowly moved even if it is not directly irradiated. In this case it is not necessary the use of light absorbing objects because the surface tension gradient is directly generated on the fluid surface. This means that even transparent objects, such as PDMS boats,2 are moved by light. Acting in this way our cork disk is slowly moved on the supporting liquids as shown in Fig. 3. The order of magnitude of the two effects are comparable even if the last approach appears to be less efficient. However, In strongly doped liquids, any object is quickly moved under irradiation, and the motion controlled, as shown in Fig. 4 (Multimedia view) where four video frames concerning the fast motion of a 1.7 mg

FIG. 4. Light-induced fast motion of a graphite slice floating on 4, 4′-di-hydroxy-azobenzene saturated solution in EtOH / H2O (1:9). A portable laser pointer (∼ 10 mW, λ = 405 nm) is used as light source. Frames are taken at t = 0.0 (a),0.3 (b),0.7(c),0.9(d) s. The white bar is 3mm. (Multimedia view) [URL: http://dx.doi.org/10.1063/1.4927419.1]

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floating graphite slice are reported. In the first frame (a), the object is irradiated from below and instantaneously starts moving along the direction indicated by the arrow in frame (b). If we rapidly move the laser spot in order to touch the moving object again in a different point, as reported in frame (c), we can induce a new direction to the motion as indicated by the arrow in frame (d). In conclusion, our findings show the light-induced controlled motion of floating objects by light exploiting the Marangoni/Gibbs effect. The object motion is controlled by laser light. The resulting direction of the motion is along the line r connecting the center of the laser spot to the center of the object. Using this method complex trajectories are designed and tracked (even the stop motion is achieved - see video). Additionally we report the first evidence of light-induced motion of any object including transparent ones (in the case of doped liquids) which are not able to absorb the laser radiation. This point is very important for bio-fluidics applications when biological systems should not be touched by light in order not to induce oxidation. Clearly, this approach remains valid in any optofluidics application involving light-sensitive samples/materials. In short, we showed that each material can be easily optically manipulated with our technique, without using any particular experimental arrangement. Finally, a complete analytical description of the phenomenon is provided, the most relevant physical quantities calculated and the average force acting on the displaced objects derived from the experimental data fittings. 1

C. Marangoni, Il Nuovo Cimento. Ser. 2, 5/6, 239 (1872). D. Okawa, S. J. Pastine, A. Zettl, and J. M. J. Fréchet, J. Am. Chem. Soc. 131, 5396 (2009). 3 L. Florea, K. Wagner, P. Wagner, G. G. Wallace, F. Benito-Lopez, D. L. Officer, and D. Diamond, Adv. Mat. 19, 7339 (2014). 4 A. Diguet, R.-M. Guillermic, N. Magome, A. Saint-Jalmes, Y. Chen, K. Yoshikawa, and D. Baigl, Angew. Chem. Int. Ed. 48, 9281 (2009). 5 J. Garcia-Amorós, A. Sánchez-Ferrer, W. A. Massad, S. Nonell, and D. Velasco, Phys. Chem. Chem. Phys. 12, 13238 (2010). 2

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