Evaporation-induced receding contact lines in partial-wetting regime on a heated substrate

Abstract

The moving contact line is a nanoscale joint providing critical boundary conditions for the liquid interface in a wetting system. Our understanding about the partial-wetting contact line is highly limited which is especially true when the situation is complicated by phase change that is ubiquitous in nature and technologies. In this work we explored evaporation-induced receding contact lines in partial wetting regime by means of two modes of atomic force microscopy scanning. By means of tapping mode scanning, the intrinsic meniscus profile was found to be linear down to the apparent contact line without bending in profile, which greatly facilitates the modeling about the intrinsic meniscus. The local angle on the contact line systematically varied with the moving speed, which fact is against the widely used constant assumption in hydrodynamic models. Using the tapping mode scanning and also force curve method with either soft or stiff probes, a layer of nano thin film was detected on the substrate beyond the contact line. The nano film thickness slightly decreased with the distance away from the contact line, and finally reached a constant that was the adsorbed i.e. non-evaporating film thickness if the tests were in a vapor chamber. We further investigated advancing contact lines in open air, and still detected the nano thin film in front of the contact line as a precursor. The nano thin film can provide extra evaporation area, which could rationalize the abnormal ultrahigh evaporation flux observed in recent experiments on nanoscale menisci. Guidance is provided for the calculation of partial-wetting contact line evaporation.