Effect of Surface Tension on Thermocapillary Convection-Driven Droplet Transport

The transport of microliter-scale droplets on solid surfaces is critical for various applications, including microfluidics and microengines. Recently, droplet manipulation strategy using thermocapillary convection has received attention due to its precise and remote controllability. The mobility of liquid droplets in this method depends on several parameters, such as laser power and the light absorption coefficient. Additionally, surface tension significantly influences droplet movement although its underlying mechanism remains unclear. In this study, we investigate the effect of surface tension on droplet movement via thermocapillary convection. Aqueous dispersions of polypyrrole (PPy) nanoparticles (NPs), which absorb near-infrared (NIR) light and convert it into heat, are employed as droplets. Upon NIR laser irradiation, the PPy droplets generate localized heat, resulting in thermocapillary convection. The lubricated surface (LuS) is used as a substrate. Due to the mobile lubricant layer, droplets are easy to move with low friction. Surface tension is modified by adding a surfactant, and the droplet movement speed increases with decreasing surface tension. Here, this phenomenon is investigating the parameters acting to Marangoni force: contact line length and surface tension gradient. We confirm that the Marangoni force, which propels the droplet, is induced more effectively by low surface tension liquids. This study provides fundamental insights into droplet behavior governed by wettability differences, advancing droplet manipulation techniques for diverse fluidic systems.