Droplet orthogonal impact on nonuniform wettability surfaces

The vast majority of prior studies on droplet impact have focused on collisions of liquid droplets with spatially homogeneous (i.e., uniform-wettability) surfaces. But in recent years, there has been growing interest on droplet impact on nonuniform wettability surfaces, which are more relevant in practice. This paper presents first an experimental study of axisymmetric droplet impact on wettability-patterned surfaces. The experiments feature millimeter-sized water droplets impacting centrally with $We<100$ on a flat surface that has a circular region of wettability ${\theta }_1$ (Area 1) surrounded by a region of wettability ${\theta }_2$ (Area 2), where ${\theta }_1<{\theta }_2$ (i.e., outer domain is less wettable than the inner one). Depending upon the droplet momentum at impact, the experiments reveal the existence of three possible regimes of axisymmetric spreading, namely (I) interior (only within Area 1) spreading, (II) contact-line entrapment at the periphery of Area 1, and (III) exterior (extending into Area 2) spreading. We present an analysis based on energetic principles for ${\theta }_1<{\theta }_2$, and further extend it for cases where ${\theta }_1>{\theta }_2$ (i.e., the outer domain is more wettable than the inner one). The experimental observations are consistent with the scaling and predictions of the analytical model, thus outlining a strategy for predicting droplet impact behavior for more complex wettability patterns.