Simple Model for Estimation of the Influence of Velocity on Advancing Dynamic Contact Angles

TL;DR

This paper introduces simple semi-empirical models to estimate how the velocity of the wetting line influences dynamic contact angles across various geometries and conditions, using extensive data fitting and comparison.

Contribution

The authors develop a new semi-empirical model for dynamic contact angles that is applicable broadly without additional experiments, outperforming existing models especially at low capillary numbers.

Findings

Hydrodynamic model outperforms others statistically.

Models are valid over five decades of capillary numbers.

Empirical model is most accurate at low capillary numbers.

Abstract

Given the importance of dynamic contact angle, its numerous applications and the complexity and difficulty of use of available approaches, here we present a new simple semi-empirical models for estimation of dynamic contact angle's dependence on the wetting line velocity. These models should be applicable to any geometry, a very large range of capillary numbers and static contact angles and all solid-liquid-gas systems without requiring further experiments. Two simple equations are intuitively derived from the most promising theoretical dynamic contact angle models, the hydrodynamic and the molecular-kinetic models. Then the models, along with the basic form of the Hoffman model, are fitted to a large pool of data. The data are extracted from numerous studies and cover over 5 decades of capillary number, include static contact angles up to the superhydrophobic region and comprise of various geometries. The resulting models are compared to each other where the hydrodynamic model's predictions are found to be superior to the other two models by all statistical measures. Then, noting that the molecular forces become dominant at lower capillary numbers we separate the data into low and high capillary regions and repeat the process. There was only a minuscule difference between the results obtained for general models and high capillary models ($\textrm{Ca}>10^{-4}$) but the empirical approach resulted in the most accurate model at low capillary numbers ($\textrm{Ca}<10^{-4}$).