Wetting hysteresis induced by nanodefects

TL;DR

This paper investigates how nanometric surface defects cause wetting hysteresis by pinning the liquid front, using a novel combination of density functional theory and the string method to quantify energy barriers and pinning forces.

Contribution

It introduces an innovative approach combining microscopic density functional theory and the string method to analyze wetting hysteresis caused by nanodefects, revealing the role of metastable pinning.

Findings

Nanodefects induce metastable pinning of the liquid front.

Energy barriers for defect crossing are quantified.

Weak nanoscale defects can cause significant hysteresis phenomena.

Abstract

Wetting of actual surfaces involves diverse hysteretic phenomena stemming from ever-present imperfections. Here we clarify the origin of wetting hysteresis for a liquid front advancing or receding across an isolated defect of nanometric size. Various kinds of chemical and topographical nanodefects are investigated which represent salient features of actual heterogeneous surfaces. The most probable wetting path across surface heterogeneities is identified by combining, within an innovative approach, microscopic classical density functional theory and the string method devised for the study of rare events. The computed rugged free energy landscape demonstrates that hysteresis emerges as a consequence of metastable pinning of the liquid front at the defects; the barriers for thermally activated defect crossing, the pinning force, and hysteresis are quantified and related to the geometry and chemistry of the defects allowing for the occurrence of nanoscopic effects. The main result of our calculations is that even weak nanoscale defects, which are difficult to characterize in generic microfluidic experiments, can be the source of a plethora of hysteretical phenomena, including the pinning of nanobubbles.