Morphology evolution and dynamics of droplet coalescence on superhydrophobic surfaces

In this work, the coalescence of two equal-sized water droplets on superhydrophobic surfaces (SHSs) is experimentally investigated. The morphologies of droplet coalescence are observed from side-view and bottom-view using high-speed camera system. The related morphology evolution and dynamics of droplet coalescence are explored. The dynamic behaviors of droplet coalescence on SHSs can be decomposed into liquid bridge growth, contact line evolution, and droplet jumping. The liquid bridge radius is proportional to the square root of time, whereas the dimensionless prefactor is decreased from 1.18 to 0.83 due to the transition of interface curvature. The retraction velocity of the contact line shows limited dependence on initial droplet radii as the retraction dynamics considered here are governed by the capillary-inertial effect. The coalesced droplet finally departs the substrate with a dimensionless jumping velocity of around 0.2. A heuristic argument is made to account for the nearly constant dimensionless jumping velocity. (c) 2018 American Institute of Chemical Engineers AIChE J, 64: 2913-2921, 2018