Droplet evaporation dynamics on heterogeneous surfaces: Numerical modeling of the stick-slip motion

Abstract

The interplay between the solid surface properties and transport phenomena during sessile droplet evaporation is extremely important in solid deposition processes. Engineering design requires realistic modeling of evaporation dynamics, on heterogeneous surfaces, which is very challenging. In this study, a continuum level, sharp-interface, ALE (Arbitrary Lagrangian- Eulerian) numerical model is developed. The model can efficiently deal with the problem of the moving three-phase contact line (TPL) during evaporation by incorporating the solid-liquid interactions in a Derjaguin pressure (commonly called disjoining pressure) term. It is found that, depending on the surface roughness and the material wettability, the evaporation dynamics proceeds following the Constant Contact Angle (CCA), Constant Contact Radius (CCR) or a mixed mode (stick-slip behavior). Numerical predictions concerning droplets evaporating on rough, hydrophobic substrates, are experimentally validated showing exceptional agreement. The stick-slip motion is found to appear during evaporation and is particularly affected by specific roughness features of the surface. Our findings are compared to Shanahan's theory. Interesting insights into the mechanisms of the stick-slip motion are presented for a rough hydrophobic substrate with weak pinning effects.