(334i) Nanodroplet Behavior on Soft Substrates

Sessile liquid droplet deforming soft substrates is of specific interest to surface science and soft technologies. Here, we investigate the surface topography of condensed nano-scale droplets on soft impermeable substrates using AFM and report sigmoidal profiles of convex drops on concave substrates. Interestingly we observed a smoothly varying surface with little discontinuity in slope at the contact line. Further, the height of capillary ridge extends greater than what the apparent droplet volume would indicate. To this, we propose a continuum model based on long wave hydrodynamics, coupled with disjoining pressure and Winkler’s foundation as a model for substrate mechanics. Results of a parametric study, corroborated by a collapse of experimental data against model predictions, suggests a significant proportion of the water droplet reside below the visible surface, forming a deep crater. For dynamic drop growth, we deduced a physical regime where the coupled time scales results in stick-slip behavior, in agreement with macroscopic observations. Our presents work extends to the use of meso-scale computation to characterize polymeric gels as substrates whether specific interactions are of interest.