Droplet spreading on rough surfaces: tackling the contact line boundary condition

TL;DR

This paper introduces a novel continuum-level model for droplet spreading on rough surfaces, explicitly deriving contact line dynamics from micro-scale interactions, and successfully validating it against experimental data.

Contribution

The model uniquely incorporates liquid/solid micro-scale interactions via disjoining pressure, overcoming previous boundary condition limitations in droplet spreading simulations.

Findings

The model accurately predicts early spreading dynamics.

Contact radius follows a universal power law.

Validation against experimental impact data.

Abstract

The complicated dynamics of the contact line of a moving droplet on a solid substrate often hamper the efficient modeling of microfluidic systems. In particular, the selection of the effective boundary conditions, specifying the contact line motion, is a controversial issue since the microscopic physics that gives rise to this displacement is still unknown. Here, a sharp interface, continuum-level, novel modeling approach, accounting for liquid/solid micro-scale interactions assembled in a disjoining pressure term, is presented. By following a unified conception (the model applies both to the liquid/solid and the liquid/ambient interfaces), the friction forces at the contact line, as well as the dynamic contact angle are derived implicitly as a result of the disjoining pressure and viscous effects interplay in the vicinity of the substrate's intrinsic roughness. Previous hydrodynamic model limitations, of imposing the contact line boundary condition to an unknown number and reconfigurable contact lines, when modeling the spreading dynamics on textured substrates, are now overcome. The validity of our approach is tested against experimental data of a droplet impacting on a horizontal solid surface. The study of the early spreading stage on hierarchically structured and chemically patterned solid substrates reveal an inertial regime where the contact radius grows according to a universal power law, perfectly agreeing with recently published experimental findings.