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.

Sticky Superhydrophobic State.

It is common sense that the droplet is stickier to substrates with larger solid-liquid contact areas. Here, we report that this intuitive trend reverses for hollowed micropillars, where a decrease in solid-liquid contact area caused by an increase in the pore size of a pillar top leads to an increase in the droplet depinning force. As compared to relief of liquid-vapor interface distortion caused by the sliding of the contact line on filled pillars, the pore hinders the contact line sliding, hence leading to enhanced interface distortion and droplet adhesion.