A reduction-to-absurdity approach using absolutely smooth solid surfaces to unveil the origins of wetting

TL;DR

This study uses ultra-large-scale molecular dynamics and helium ion microscopy to demonstrate that contact angle hysteresis and dynamic angles originate from frictional effects, challenging theories that ignore surface roughness.

Contribution

It provides the first direct evidence that friction, even at atomic roughness levels, causes contact angle deviations and hysteresis, using both simulations and advanced microscopy.

Findings

Hysteresis and dynamic angles vanish on absolutely smooth surfaces in simulations.

Nanoscopic distortions along the contact line are observed on atomically smooth surfaces.

Results challenge disjoining pressure theory by showing roughness is essential for hysteresis.

Abstract

Contact angle hysteresis and generation of dynamics angle are two fundamental phenomena about the contact angle deviation from the equilibrium state. Roughness on the solid surface, disjoining pressure in the thin film, and liquid-solid adhesion have all been considered as the origins of the phenomena. This work for the first time made a reduction to absurdity by employing absolutely smooth solid surfaces in ultra-large-scale molecular dynamic simulation. The results showed that the equilibrium angles were well established on the absolutely smooth surface just as regular, while the hysteresis and the dynamic angle vanished. The critical structure of the convex nanobending for advancing contact lines vanished as well. In contrast, the solids that made of atoms, even at the minimum roughness, would bring significant angle deviation and convex nanobending. A 3D observation was further made using state-of-the-art helium ion microscopy for the first time revealing the ubiquitous nanoscopic distortion along the contact line on atomically smooth surfaces. The results answer the question of the origin of the angle deviation, that the hysteresis and dynamic angle can be unified to originate from the friction, either static or dynamic; and at the same time resolve the puzzle of the hysteresis existence on reported smooth surfaces, showing that even the minimum i.e. atomic roughness is great enough to be effective on the contact line. The results are against the disjoining pressure theory which ignored the roughness and predicted the occurrence of the hysteresis on absolutely smooth solids.