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Optical levitation and transport of microdroplets: Proof of concept
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Image of FIG. 1.
FIG. 1.

Thermocapillary-driven permanent nonwetting system. (a) Silicone-oil droplet heated by the attached 3-mm-diameter pedestal and squeezed against a cooled glass plate. The liquid droplet is prevented from wetting the normally wetting solid substrate due to the presence of a thin film of air within the apparent contact region. The reflection of the droplet can be seen on the glass surface above it. (b) Cartoon showing thermocapillary-driven flows within the apparent contact region. The temperature near the apex of the droplet is cooler than the rest of the droplet, thus lowering surface tension in the region owing to the thermocapillarity effect. The surface of the droplet is pulled from the pedestal to this region, resulting in bulk thermocapillary convection within the adjacent liquid and gas phases as indicated by the arrows in the sketch. The lubricating pressure within the gas film sufficiently separates the two surfaces of concern by a minimum distance of .

Image of FIG. 2.
FIG. 2.

Droplet levitated above a solid substrate. A 1–1.5-mm-diameter silicone-oil droplet is heated from above by absorbing incident energy from a laser. The glass surface is cooled from below. Due to thermocapillary convection described in Fig. 1, a lubricating film is dragged into the apparent contact region between the droplet and substrate. The pressure distribution within this region supports the weight of the droplet. Flow visualization employing sheet illumination combined with smoke shows a toroidal structure within the air surrounding the droplet driven by the motion of the free surface. The pressure field around a statically levitated droplet and within the lubrication region is axisymmetric, implying a zero coefficient of static friction for such a levitated system, analogous to a statically levitated air-hockey puck (enhanced online). [URL: http://dx.doi.org/10.1063/1.3005394.1]10.1063/1.3005394.1

Image of FIG. 3.
FIG. 3.

Linear translation of a levitated droplet of 1–1.5 mm diameter. [(a)–(d)] The motion of the droplet follows that of the heat source, which is controlled by a fast-tracking mirror. Once the incident beam is displaced from the center of the droplet, the symmetry of the temperature distribution on the free surface is broken, as well as within the flow fields inside and outside the droplet. This results in a net flux of gas momentum away from the cold side of the droplet, resulting in propulsion toward the heat source. The time interval between the images in this sequence is 5 s (enhanced online). [URL: http://dx.doi.org/10.1063/1.3005394.2]10.1063/1.3005394.2

Image of FIG. 4.
FIG. 4.

Encapsulated water droplet inside of nonwetting media. (a) Water droplet injected inside a nonwetting silicone-oil droplet heated by a 3-mm-diameter pedestal. (b) The same compound-droplet system is shown to withstand a relatively high compression without wetting. (c) Demonstration of compound-droplet levitation composed of silicone oil (outer layer) and water (inner drop) achieved by spraying the levitated oil droplet with a water mist. This inner droplet is observed to undergo vigorous motion, driven by thermocapillary convection within the carrier liquid. The diameters of the droplets in this image are roughly 1.6 and 0.2 mm for the silicone-oil and water droplets, respectively.



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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Optical levitation and transport of microdroplets: Proof of concept