Dynamic Spreading of Droplets on Lyophilic Micropillar-Arrayed Surfaces

Abstract

The wetting kinetics of droplets on lyophilic pillar-arrayed substrates is the driving mechanism of several natural phenomena (e.g., insect capturing by Nepenthes) and many industrial technologies (e.g., gas-liquid separation). For a lyophilic pillar-arrayed surface, a fringe film is formed ahead of the contact line, resulting in distinct wetting kinetics, which needs further investigation. In this study, Si(100) substrates with square micropillars were used to investigate the early spreading of droplets on lyophilic pillar-arrayed surfaces through the droplet-spreading method. A fringe film was observed ahead of the contact line for micropillar-arrayed surfaces. The spreading radius was enhanced by micropillars and mainly caused by liquid penetration into the pillar forest, resulting in alteration of the dissipation mechanism. The early spreading of droplets on lyophilic micropillar-arrayed surface was affected only by the solid fraction and independent of the pillar height. A semitheoretical model without adjustable parameters was established on the basis of the global energetic equation, considering the local dissipation, viscous dissipation, and the dissipation in the precursor film. The prediction of the model agrees with the experimental results. Our semitheoretical model may aid in predicting the wetting kinetics on lyophilic pillar-arrayed substrates and assist the design of pillar-arrayed surfaces in practical applications.