Contact angle variation on single floating spheres and its impact on the stability analysis of floating particles

The contact angle (CA) as a measure of surface hydrophobicity is of scientific and technological importance for many wetting and dewetting processes. CA hysteresis (i.e., the difference between the advancing and receding CAs) due to roughness on a flat surface has been recognized but is not considered in the stability analysis of floating particles. In this study, the CA of single floating spheres at the air-water interface was determined directly from the deformed meniscus by applying a new method recently developed by our team (Feng, D.-x. and Nguyen, A. V., 2016. A novel quantitative analysis of the local deformation of the air-water surface by a floating sphere. Colloids and Surfaces A: Physicochemical and Engineering Aspects, Vol. 504, pp. 407-413). The experiments for each of the single spheres were repeated to estimate the variation in the CA. The experimental results show a normal distribution of the CA. The CA hysteresis occurs randomly because of the dispersion of small-scale surface roughness. This random hysteresis substantially affects the stability analysis of floating spheres. The analysis also shows that the CA determined by the force balance method (valid for ideally smooth surfaces) yields higher values than those determined on the basis of the deformed meniscus around the particle. This difference is attributed to the effect of surface roughness on the CA hysteresis, which should be considered in analyses of the stability and detachment of floating particles. (C) 2017 Elsevier B.V. All rights reserved.