Manipulation, aggregation, fusion and separation of droplets using laser-induced Marangoni force

Abstract

Micro-scale control of liquid/droplet motion is crucial for chemical and biochemical analysis. There are several methods to manipulate droplet motion such as the Marangoni effect which can be triggered by introducing temperature gradients on suspended droplets in mineral oil.In this study, a focused continuous wave (CW) laser beam was used to induce the Marangoni force to separate, fuse, aggregate, and control droplets. Droplets experience a force due to an interfacial tension gradient near the locally heated area created by the laser beam. The work is done on a chamber and a microchannel platform.In order to fuse water droplets, we used an optical fiber (200-µm core and 220-µm glass cladding, which has a 0.065 numerical aperture (NA)) to direct focused laser beams into the oil medium, where the droplets were first moved to the heat source and then stopped at this point and merged a second time. Also, for the first-time droplet separation for different radii of 47 and 55 in the microchannel and droplet aggregation in the chamber was performed by the Marangoni effect. We have also examined the behavior of the droplets in relation to their size and distance from the hot spot.By controlling the incident laser power, we can manipulate the droplet to either fuse, separate or aggregate.