The rolling and slipping of droplets on superhydrophobic surfaces

TL;DR

This paper models the motion of droplets on superhydrophobic surfaces, analyzing the role of slip and dissipation mechanisms, and finds that contact line dissipation dominates in ideal conditions, while viscous shear is more relevant on real leaves.

Contribution

It introduces a model incorporating slip into droplet motion on superhydrophobic surfaces and identifies dominant dissipation mechanisms under different conditions.

Findings

Contact line dissipation dominates in ideal surfaces with large slip lengths.

Viscous shear is the main dissipation mechanism on real leaves.

Slip effects can often be neglected in natural droplet motion.

Abstract

The leaves of many plants are superhydrophobic, a property that may have evolved to clean the leaves by encouraging water droplets to bead up and roll off. Superhydrophobic surfaces can also exhibit reduced friction and liquids flowing over such surfaces have been found to slip in apparent violations of the classical no-slip boundary condition. Here we introduce slip into a model for rolling droplets on superhydrophobic surfaces and investigate under what conditions slip might be important for the steady state motion. In particular, we examine three limiting cases where dissipation in the rolling droplet is dominated by viscous dissipation, surface friction, or contact line friction. We find that in molecular dynamics simulations of droplets on ideal superhydrophobic surfaces with large effective slip lengths, contact line dissipation dominates droplet motion. However, on real leaves, droplet motion is likely to be dominated by viscous shear, and slip, for the most part, can be neglected.