Heat Transfer and Thermocapillary Flow of a Double-Emulsion Droplet Heated Using an Infrared Laser by the Photothermal Effect: a Numerical Study

Using a laser to heat microfluid has the advantages of non-contact local operation, high accuracy, and good adjustability. In this study, a focused infrared laser with a 1550-nm wavelength was applied to heat an oil–water-oil double-emulsion droplet in a microchannel. The Finite Volume Method was used to numerically study the thermocapillary flow and heat transfer of this laser-heating process. In the simulation, the laser energy distribution was modeled using a volumetric Gaussian heat source. The attention was focused on the heat transfer and thermocapillary flow of the double-emulsion droplet. The influences of laser parameters (power and beam diameter) and the temperature coefficient of interfacial tension were studied. We found that the intensity of the thermocapillary flow and the temperature linearly increased with input power; they first decreased and then increased as the size of the input beam increased because of the combined effect of absorbing energy and energy concentration. Moreover, there were four and two thermocapillary vortices inside the middle water phase when the sign of the temperature coefficient of interfacial tension in the double interfaces was the same and different, respectively. In all cases, the uneven temperature coefficient of the inner droplet was lower than that of the middle water phase, but the average temperatures of both regions were extremely close. These results can prove useful in the future operation of double-emulsion droplet-based microfluidics using a laser as a precise and sensitive heating source for drug discovery and delivery, cell analyses, and micro/nanoparticle synthesis.