Thermal Marangoni stress induced droplet mobilization in a microfluidic confinement

Abstract

We have demonstrated for the first time how a transverse thermal gradient can be utilized for mobilizing a trapped droplet through a porous medium. To this end, we have conducted numerical simulations for a droplet approaching a constricted passage under the influence of thermal field. Thermal Marangoni stress generated at the interface results in weakened capillary forces and attracts the interface toward the hotter side, thereby aiding mobilization. The mobilization process has been found to slow down, exhibiting lower droplet tip velocity, as surface tension reduces. As non-intuitive as it might seem, the droplet dynamics depends on which of the forces dominate. It has been deduced that hydrodynamic force prevails upon the thermal Marangoni effect, thereby reducing the droplet tip velocity. Furthermore, the droplet tip transforms into a conical shape owing to elevated Marangoni stress at high MaT. The peak value of pressure buildup inside the droplet Pin has been observed to decrease with surface tension reduction when the droplet escapes. We present a regime map where we clearly show that the application of thermal field, indeed, can successfully transport a droplet.

Topics & Concepts