Abstract
In the report we demonstrate how, using laser light, effectively trap gas bubbles and transport them through a liquid phase to a desired destination by shifting the laser beam position. The physics underlying the effect is complex but quite general as it comes from the limited to two-dimension, well-known, Marangoni effect. The experimental microscope-based system consists of a thin layer of liquid placed between two glass plates containing a dye dissolved in a solvent and a laser light beam that is strongly absorbed by the dye. This point-like heat source locally changes surface tension of nearby liquid-air interface. Because of temperature gradients a photo-triggered Marangoni flows are induced leading to self-amplification of the effect and formation of large-scale whirls. The interface is bending toward beam position allowing formation of a gas bubble upon suitable beam steering. Using various techniques (employing luminescent particles or liquid crystals), we visualize liquid flows propelled by the tangential to interface forces. This helped us to understand the physics of the phenomenon and analyze accompanying effects leading to gas bubble trapping. The manipulation of sessile droplets moving on the glass surface induced via controlled with laser light interface bending (i.e. “droplet catapult”) is demonstrated as well.
Highlights
The layer of a liquid is placed between two glass plates separated by distance d with the help of spacers so the thin film of liquid is formed in the (x, y) plane
The phenomena demonstrated in this work are quite general as they describe light-matter interaction via process of light absorption
The described phenomena are very complex, as their description requires solving of coupled differential equations of heat transport, mass transport, hydrodynamics, surface phenomena and light absorption
Summary
From the temperature distribution at the interface (cf Fig. 1a) it follows that at the closest point to a heat source (a beam spot) the gradient is zero so there is no net force contributing to flow generation. Laser beam positioned at a distance of 0.5 mm from the gas-liquid interface inside the absorbing liquid promotes the interface bending only if the temperature increase at this border is large enough to substantially decrease the surface tension coefficient.
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