Validity of the "sharp-kink approximation" for water and other fluids

TL;DR

This study evaluates the sharp-kink approximation's accuracy in predicting contact angles of liquids like water on various surfaces, finding it effective near room temperature but requiring further testing across temperature ranges.

Contribution

It introduces and assesses a simple equation based on the sharp-kink approximation for predicting contact angles, extending its potential applicability to different fluids and surfaces.

Findings

The equation reproduces helium contact angles on alkali metals.

It reasonably predicts water contact angles on gold and graphite.

Further temperature-dependent measurements are needed for comprehensive validation.

Abstract

The contact angle of a liquid droplet on a solid surface is a direct measure of fundamental atomic-scale forces acting between liquid molecules and the solid surface. In this work, the validity is assessed of a simple equation, which approximately relates the contact angle of a liquid on a surface to its density, its surface tension, and the effective molecule-surface potential. This equation is derived in the sharp-kink approximation, where the density profile of the liquid is assumed to drop precipitously within one molecular diameter of the substrate. It is found that this equation satisfactorily reproduces the temperature-dependence of the contact angle for helium on alkali metal surfaces. The equation also seems be applicable to liquids such as water on solid surfaces such as gold and graphite, based on a comparison of predicted and measured contact angles near room-temperature. Nevertheless, we conclude that, to fully test the equation's applicability to fluids such as water, it remains necessary to measure the contact angle's temperature-dependence. We hypothesize that the effects of electrostatic forces can increase with temperature, potentially driving the wetting temperature much higher and closer to the critical point, or lower, closer to room temperature, than predicted using current theories.