Open Access
Add to BibSonomy
Abstract
Near-infrared light is commonly used to move small objects floating on water by exploiting the Bénard-Marangoni convection. This is because infrared light is absorbed well by water and the induced thermal gradients are responsible for the objects’ motion. However, visible light was recently used to move macroscopic objects on the free liquid surfaces. In this work, we show the use of visible light to rotate symmetric millimeter-sized objects. Those objects represent light-driven macro motors that are able to work in a continuous or step-by-step mode. We studied light intensity’s effects on our system’s angular velocity and estimated the entire process’s conversion efficiency.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The direct conversion of light into work allows translational motion of objects of any size floating on fluid surfaces. Starting from the pioneering works of Marangoni [1,2], concerning changes of interfacial tension caused by differences in composition or temperature at the fluid-solid interface, the scientific literature on this subject has progressed significantly with the availability of new tools and materials [3–8]. Recently rotational motion has been investigated as well by using I.R. radiation and asymmetric objects [9-12]. In this work we show how to use visible light to rotate symmetric millimeter-sized objects. Those objects represent light driven macro motors able to work in a continuous or step by step mode. We studied the effects of light intensity on the angular velocity of our system and give an estimation of the conversion efficiency of the entire process. The starting point is the laser induced motion of macro objects floating on free surfaces on a desired path by using visible light [7]. Those surfaces can be transparent or properly doped to absorb the incident light. In the first case the object is partially heated (half illuminated) by using a laser light. The absorbed heat is transmitted to the solid-water interface and the surface tension along the object contour decreases resulting in a linear motion. In the second case the doped fluid directly absorbs the heat and the resulting mass transfer once again moves the object away from the illuminated region. The first effect can also be used to rotate symmetrical objects on a fluid surface if they are designed as non-uniform light absorbers.
2. Experimental part
The shape we designed is the one reported on the lower right side of Fig. 1. For simplicity we used a symmetrical four blades star. The star has a symmetric geometrical shape. By using a transparency film for laser printers, we painted each half blade in black while the rest of the star remained transparent. By acting this way, we obtained an optically asymmetric shape. Based on previous results [7] we expect the star to rotate clockwise when uniformly illuminated. An Argon + + CW at λ = 514.5 nm laser was used to irradiate the star floating on the surface of a transparent liquid which in our experiments was ethanol. To avoid linear displacements, we made a small hole in the center of our star and inserted it on a small pillar as shown in Fig. 1. A camera was used to record the star’s rotations. Movies were post-processed and analyzed with a proprietary tracking software.
Fig. 1 Experimental setup to detect star rotations. Due to the presence of a vertical pillar the half-painted star can only rotate around its vertical axis. Each blade measures 4.5 mm.
3. Results
A typical result was reported in Fig. 2 for a light intensity of 80 mW/cm2.
Fig. 2 The dynamics of rotation: the star first rotates clockwise and tends to return to its starting position after the light was switched off.
The star begins rotating by slowly increasing its rotational velocity until a maximum is reached. Once this state was reached the star gradually decreases its rotational motion until it stops. If we switch off the irradiation, an unexpected behavior was observed: after a short time, the star rotates anticlockwise. This behavior can be easily controlled, and it is completely reversible when the right amount of energy is transferred to the liquid through the uniformly illuminated star. Moreover, we studied the dynamical changes of the rotational speed by increasing the light intensity. We used five different values of impinging light intensity: 40mW/cm2, 80mW/cm2, 170mW/cm2, 340 mW/cm2 and 440 mW/cm2 which was the maximum achievable in our experimental conditions. Data related to the first irradiation cycle are reported in Fig. 3 for different light intensities.
Fig. 3 Star revolutions as a function of the used light intensities. Irradiation was stopped once the star stopped its motion. Open triangles: 40mW/cm2; filled triangles: 80 mW/cm2; open circles: 170 mW/cm2; filled circles: 340 mW/cm2; open squares: 440 mW/cm2.
Frictional effects between the pillar and the star give rise to small deviations of the linearity before the occurrence slowing down. Once the rotational speed reached its maximum value, we measured it and plotted it as function of the impinging light intensity I. The results are reported in Fig. 4.
Fig. 4 Rotational speed as function of the light intensity. Measurements are taken once the star rotational velocity reached its maximum value. The red line shows the data linear regression.
As can be seen, for the used power values, the rotational speed increases linearly with increasing the light intensity. In our experiments the maximum observed speed is ω = 1.27 rad/s for an incident power of P0 = 0.5W. The efficiency ε of the light-to-work conversion can be estimated as the ratio between the induced mechanical power and the incident optical power. Calling Γ the rotational viscous drag acting on the gear and PM = Γω2 the mechanical power that would be required to spin the gear at the same speed by an external torque, we have for Ethanol: ε = PM/P0~6·10−10. Surprisingly this is of the same order of magnitude of the conversion efficiency measured for much smaller gears. A remarkable effect observed in our experiments is the possibility to drive the star in a continuous way or in a step by step motion. As expected, once the maximum speed is reached by symmetrical irradiation, a further illumination results in a slow deceleration that brings the rotational motion to an end. If we illuminate the star in a non-symmetrical way (i.e. off-center irradiation) and use the right power level corresponding to the desired amount of rotational speed, the motion will continue indefinitely (see Visualization 1 associated to Fig. 5). At the same time a non-continuous irradiation will result in a step by step motion in a way similar to common electrical driven motors. (see Visualization 2 associated to Fig. 6). Please note videos are acquired at 10 frames/s and shown at 1/3 of the real speed.
Fig. 5 Off center irradiation and continuous motion of our four blades star (See Visualization 1). The center of the laser spot is 3mm away from the star’s center.
Fig. 6 Step motion obtained by switching ON (a) and OFF (b) the irradiation. A ̴ 0.5s irradiation corresponds to ̴ 15 degrees of rotation at I = 440mW/cm2 (See Visualization 2).
Figure 7 finally reports a plot of the measured angle as function of time for the step motion configuration. As can be seen, for the used light intensity, the star begins moving ̴ 0.2 s from after irradiation and continue its motion for a similar amount of time after the end of irradiation.
Fig. 7 Step motion obtained by switching ON and OFF the irradiation. There is an activation/relaxation time of about 0.2 s in each cycle (See Visualization 2).
4. Discussion
The explanation of the observed effects is quite simple. As shown in Fig. 2, after the start of irradiation the black painted regions absorb more energy than the transparent ones. Along the solid-liquid contact interface of these regions, the surface tension decreases more than for the transparent ones. The resulting net force is responsible for the clockwise star rotation. During irradiation the star has an acceleration followed by a deceleration. In the region where the viscous force balances the thermocapillary effect the total torque ΜΤΟΤ = Ια acting on the star is zero and the Marangoni torque MM balances the viscous one MV. If, as a first approximation, we estimate Γ as the viscous drag acting on a spherical object of radius a sitting in the middle of our ethanol-air interface (Γ = 4πηa3 = 2⋅10−10 N × s × m × rad−1 the Marangoni torque results to be to 2 × 10−11 N × m. Once the values of the Marangoni torque are derived after proper calibration, this experimental configuration can be suitably used to measure fluids viscosity. The behavior reported in Fig. 2, with the star reverting its motion from clockwise to anticlockwise each time we stop the illumination, has a more complex explanation. It is worth noting how the phenomenon is always present even for linear displacement of simple objects on liquid surfaces. In this case the explanation is relatively simple because the light-induced convective motion ensures the departure of the object from the illuminated area but, once the irradiation is stopped the amount of convective cold liquid hitting the previously warmer part of the object becomes higher and higher until the object stops its motion and goes slightly backward along the same path. The extension of this idea to our star configuration could explain the observed reverted motion. In any case the linear relationship between angular speed and illumination power, which is typical in those systems [9], is clear. The measured light-to-work conversion values agree with those measured for much smaller systems and confirm Marangoni propulsion as the best candidate to explain the origin of the star rotations [9]. Concerning the possibility of achieving continuous or step by step motion the explanation is straightforward. With a symmetric illumination the star in time reaches a uniform distribution of heat on its surface and the motion naturally stops. If we move the center of irradiation on one side of the star each blade has enough time to dissipate the heat before it gets illumination again. If the initial conditions are restored the absorption can take place again and the motion never stops. Everything depends on the right balance between the incident power and the induced rotational speed because the blades need to have enough time to dissipate the absorbed heat. Finally, step motion can be achieved if we illuminate the star symmetrically for a short time and don’t give the blades enough time to revert their motion. The activation and relaxation times observed in Fig. 7 are a direct consequence of the heating and cooling processes. The star needs enough energy to start its motion and rotates until the thermal gradient exists.
5. Conclusions
In conclusion we have shown the possibility of using visible light to rotate macroscopic objects on the free surface of an optically transparent fluid. Those objects can be seen as light actuated macro motors able to work in a continuous or step-by-step mode. The effects of light intensity on the angular velocity were investigated and the conversion efficiency of the entire process was evaluated confirming our hypothesis of Marangoni propulsion. Applications of such technology in the fast-growing field of optofluidics appear to be very promising.
Acknowledgment
The authors would like to thank Fabrizio Ciciulla for his help in sample preparation and Nina Sheremet for her valuable support during measurements.
References
1. C. Marangoni, “Sul principio della viscosità superficiale dei liquidi stabili,” Nuovo Cimento Ser 2(5/6), 239–273 (1872).
2. L. E. Scriven and C. V. Sternling, “The Marangoni effects,” Nature 187(4733), 186–188 (1960). [CrossRef]
3. A. Goel and V. Vogel, “Harnessing biological motors to engineer systems for nanoscale transport and assembly,” Nat. Nanotechnol. 3(8), 465–475 (2008). [CrossRef] [PubMed]
4. D. Okawa, S. J. Pastine, A. Zettl, and J. M. J. Fréchet, “Surface tension mediated conversion of light to work,” J. Am. Chem. Soc. 131(15), 5396–5398 (2009). [CrossRef] [PubMed]
5. A. Diguet, R.-M. Guillermic, N. Magome, A. Saint-Jalmes, Y. Chen, K. Yoshikawa, and D. Baigl, “Photomanipulation of a droplet by the chromocapillary effect,” Angew. Chem. Int. Ed. Engl. 48(49), 9281–9284 (2009). [CrossRef] [PubMed]
6. E. Lauga and T. R. Powers, “The hydrodynamics of swimming microorganisms,” Rep. Prog. Phys. 72(9), 096601 (2009). [CrossRef]
7. D. E. Lucchetta, F. Simoni, L. Nucara, and R. Castagna, “Controlled-motion of floating macro-objects induced by light,” AIP Adv. 5(7), 77147 (2015). [CrossRef]
8. R. T. Mallea, A. Bolopion, J.-C. Beugnot, P. Lambert, and M. Gauthier, “Laser-induced thermocapillary convective flows: a new approach for non-contact actuation at microscale at the fluid/gas interface,” IEEE/ASME Trans. Mechatron. 22(2), 693–704 (2017). [CrossRef]
9. C. Maggi, F. Saglimbeni, M. Dipalo, F. De Angelis, and R. Di Leonardo, “Micromotors with asymmetric shape that efficiently convert light into work by thermocapillary effects,” Nat. Commun. 6(1), 7855–7860 (2015). [CrossRef] [PubMed]
10. O. Emile and J. Emile, “Rotation of millimeter-sized objects using ordinary light,” Opt. Lett. 41(2), 211–214 (2016). [CrossRef] [PubMed]
11. E. Hendarto, and Y. B. Gianchandani “Thermocapillary actuation of millimeter-scale rotary structures,” J. Microelectromech. Syst. 23, 2 494–499 (2014).
12. J. Ahn, Z. Xu, J. Bang, Y.-H. Deng, T. M. Hoang, Q. Han, R. M. Ma, and T. Li, “Optically levitated nanodumbbell torsion balance and GHz nanomechanical rotor,” Phys. Rev. Lett. 121(3), 033603 (2018). [CrossRef] [PubMed]
