Quasi-static and dynamic breakdown of superhydrophobicity

The breakdown of superhydrophobicity caused by liquid penetration into microstructures leads to the loss of various surface functionalities but is not fully understood yet. We conjecture that unified criteria for superhydrophobicity breakdown can be established if microscopic liquid penetration processes and the applied macroscopic pressure can be measured simultaneously.

Using direct visualization of microscopic liquid penetration dynamics through the bottom of transparent substrates, the critical hydrostatic pressure inducing quasi-static liquid penetration and the critical droplet impact speed causing dynamic liquid penetration on surfaces with pillars and hollowed pillars are measured.

The capillary pressure resisting liquid penetration is determined by the force balance between the vertical capillary force along the pillar outer perimeter and the pressure force acting on the voids between pillars. Based on a sudden deceleration of liquid from the impact speed within a time interval governed by compression shockwaves traveling between adjacent micropillars, a water-hammer-type pressure is proposed to explain the additional pressure contributing to dynamic liquid penetration. A predictive model for the critical droplet impact speed accounting only for surface geometrical dimensions is proposed and verified by experiments using water droplets. The amount of liquid experiencing sudden deceleration on hollowed pillars is less than that on regular pillars, which explains why hollowed pillars require a larger droplet impact speed for liquid penetration.