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. The droplet on hollowed micropillars is completely suspended above the vapor but inherently sticky. Hence, this counterintuitive phenomenon is termed as the sticky superhydrophobic state in contrast to the conventional superhydrophobic state with low adhesion. A model building upon the dynamics of the contact line and liquid-vapor interface, which successfully predicts the droplet depinning force on filled and hollowed pillars, is introduced.