Determination of the Sliding Angle of Water Drops on Surfaces from Friction Force Measurements

Superhydrophobic surfaces have attracted considerable attention because of their unique water-repellency and their wide range of applications. The conventional method to characterize the surface wetting properties of surfaces, including superhydrophobic surfaces, relies on measuring static and dynamic contact angles, and sliding angles of water drops. However, because of the inhomogeneities inherently present on surfaces (smooth and textured), such optical methods can result in relatively large variability in sliding angle measurements. In this work, by using a force-based technique with +/- 1 mu N sensitivity, the friction force between water drops and various surfaces is measured. The friction force can then be used to accurately predict the sliding angle of water drops of various sizes with improved consistency. We also show that the measured friction force can be used to determine the critical drop size below which a water drop is not expected to slide even at a tilt angle of 90 degrees. The proposed technique to characterize the wetting properties of surfaces has a higher accuracy (between 15% and 65%, depending on the surface) compared to optical methods.

Automated summary

Superhydrophobic surfaces have unique water-repellency and wide applications, but conventional methods cannot accurately characterize their wetting properties. In this study, a new force-based technique was used to measure the friction force between water drops and surfaces, accurately predicting the sliding angle of water drops and determining the critical size of immobile drops.