A unified model for droplet receding contact angles on hydrophobic pillar, pore, and hollowed pillar arrays

Hypothesis: Current models for receding contact angles of Cassie-Baxter state droplets on textured hydrophobic substrates are applicable only to a specific structural type, e.g., pillar (above which a droplet has isolated contact line and continuous liquid-vapor interface) or pore (continuous contact line and isolated liquid-vapor interface), signifying a lack of universality. We conjecture that a unified model for droplet receding contact angles can be established if the fundamental gap between pillar and pore can be bridged.

Experiments: Droplet receding contact angles on hollowed pillar (a pore inside a pillar) arrays, which possess the characteristics of both pillar and pore, are measured and compared to those on pillar and pore arrays. The microscopic dynamics of contact line and liquid-vapor interfaces on those substrates are also visualized.

Findings: Droplets on hollowed pillar arrays exhibit smaller and larger contact angles than those on pillar and pore arrays, respectively. Based on microscopic energies involved in contact line sliding, relaxation of liquid-vapor interface between pillars, and extension of liquid-vapor interface above pores, a model is developed to predict the droplet receding contact angles irrespective of the variations in surface structural types (pillar, pore, and hollowed pillar) or dimensions (size, spacing, and packing density).