Precursor-Film-Mediated Thermocapillary Motion of Low-Surface-Tension Microdroplets

Abstract

In\ncontrast to microdroplet condensation with high contact angles,\nthe one with low contact angles remains unclear. In this study, we\ninvestigated dynamics of microdroplet condensation of low-surface-tension\nliquids on two flat substrate surfaces by using reflection interference\nconfocal microscopy. Spontaneous migration toward relatively larger\ndroplets was first observed for the microdroplets nucleated on the\nhydrophilic quartz surface. The moving microdroplets showed a contact\nangle hysteresis of ∼0.5°, which is much lower than the\nvalues observed on typical flat substrates and is within the range\nobserved on slippery lubricant-infused porous surfaces. Because the\nmicrodroplets on the hydrophobic polydimethylsiloxane surface did\nnot move, we concluded that the ultrathin precursor film is formed\nonly on the hydrophilic surface, which reduces a resistive force to\nmigration. Also, reduced size of droplets promotes the thermocapillary\nmotion, which is induced by a gradient in local temperature inside\na small microdroplet arising due to the difference in size of adjacent\ndroplets.