(387c) Droplet Shape Evolution upon Impingement on Randomly Rough Surfaces

Paint-based surface coatings are conventionally manufactured through two consecutive processes, i.e., spray and curing.  It has been found experimentally that the initial morphology of the substrate does affect the final coating morphology, since some textures of coatings on a rough surface do not appear in the coatings on a smooth substrate such as glasses.  Identifying the effect of the substrate morphology on the coating morphology is a challenging task but of great importance.  In this work, we aim to quantitatively explore this effect by starting from the investigation of droplet deposition, which is the first phase of a coating formation process.

Although various efforts have been devoted to the modeling of droplet deposition on smooth or patterned surfaces with ordered structures, we can hardly identify any systematic work focusing on randomly rough surfaces, which are the most common bare substrate surfaces. A phase field modeling method together with a unique analysis approach have been developed by us to characterize droplet deposition on randomly rough surfaces. It is interesting to observe that the droplet shape evolves quite differently on a randomly rough surface, as compared to a patterned surface, even if two surfaces have the same root mean squared roughness and Wenzel roughness parameter.  Thorough parametric studies have been carried out to explore how the surface roughness and other impingement variables (related to the operating conditions) can affect shape evolution.  Furthermore, quantitative correlations between droplet shape metrics and substrate morphology metrics have been established.