Experiment on the evolution of the energy barrier between microgrooves induced by low-temperature condensation.

Abstract

PDMS is a common hydrophobic material, but its stability and durability still limit its engineering applications. Especially, wetting-state transitions on its surface under low-temperature conditions remain a long-standing scientific challenge. In this study, droplet friction microscopy was used to investigate the motion of deionized water and sodium chloride aqueous solution droplets on PDMS microgrooved surfaces. Lateral retention forces were measured over a temperature range from 5 to 25 °C. The dynamic energy barriers associated with the triple-phase line crossing over a microgroove were quantified. By combining droplet dynamic motion with interfacial retention force, the roles of microgroove geometry, liquid surface tension, and temperature in regulating the energy barrier were clarified. At 25 °C, the energy barrier decreases linearly with increasing groove width, agreeing well with theoretical predictions. At 5 °C, the energy barrier increases sharply as the groove width decreases. A dropwise condensation model was further used to analyze the scale-dependent distribution of condensing droplets on the microgrooved PDMS surface at 5 °C. This behavior indicates a wetting transition from the Cassie state toward the Wenzel state within the microgrooves because the nanodroplet condensation in microgrooves at low temperature significantly decreases the energy barrier.