Focal Plane Shift Imaging for the Analysis of Dynamic Wetting Processes.

Droplet-surface interactions are common to a plethora of natural and industrial processes due to their ability to rapidly exchange energy, mass, and momentum. Droplets are particularly of interest due to their large surface-to-volume ratios and hence enhanced transport properties. For example, coalescence-induced droplet jumping on superhydrophobic surfaces has recently received much attention for its potential to enhance heat transfer, anti-icing, and self-cleaning performance by passively shedding microscale water droplets. To study droplet jumping, researchers typically use a two-camera setup to observe the out-of-plane droplet motion, with limited success due to the inability to resolve the depth dimension using two orthogonal cameras. Here we develop a single-camera technique capable of providing three-dimensional (3D) information through the use of focal plane manipulation, termed "focal plane shift imaging" (FPSI). We used FPSI to study the jumping process on superhydrophobic surfaces having a wide range of structure length scales (10 nm < l < 1 μm) and droplet radii (3 μm < R < 160 μm). We benchmarked the FPSI technique and studied the effects of droplet mismatch, multidroplet coalescence, and multihop coalescence on droplet jumping speed. Furthermore, we were able to resolve the full 3D trajectory of multiple jumping events, to show that, unlike previously theorized, the departure angle during droplet jumping is not a function of droplet mismatch or number of droplets coalescing prior to jumping. Rather, angular deviation arises due to in-plane motion postcoalescence governed by droplet pinning. The outcomes of this work both elucidate key fundamental aspects governing droplet jumping and provide a powerful imaging platform for the study of dynamic droplet processes that result in out-of-plane motion such as sliding, coalescence, or impact.