Stood-up drop to measure receding contact angles

Abstract

The wetting behavior of drops on natural and industrial surfaces is determined by the advancing and receding contact angles. They are commonly measured by the sessile drop technique, also called goniometry, which doses liquid through a solid needle. Consequently, this method requires substantial drop volumes, long contact times, tends to be user-dependent, and is difficult to automate. Here, we propose the stood-up drop (SUD) technique as an alternative to measure receding contact angles. The method consists of depositing a liquid drop on a surface by a short liquid jet, at which it spreads radially forming a pancake-shaped film. Then the liquid retracts, forming a spherical cap drop shape (stood-up drop). At this quasi-equilibrium state, the contact angle ($\theta_\text{SUD}$) closely resembles the receding contact angle measured by goniometry. Our method is suitable for a wide variety of surfaces from hydrophilic to hydrophobic, overcoming typical complications of goniometry such as needle-induced distortion of the drop shape, and it reduces user dependence. We delineate when the receding contact angle can be obtained by the stood-up method using Volume-of-Fluid (VoF) simulations that systematically vary viscosity, contact angle, and deposited drop volume. Finally, we provide simple scaling criteria to predict when the stood-up drop technique works.