Morphology evolution and dynamics of droplet coalescence on superhydrophobic surfaces

Abstract

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. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2913–2921, 2018

Critical and Optimal Wall Conditions for Coalescence-Induced Droplet Jumping on Textured Superhydrophobic Surfaces

“…The viscous-capillary regime during the very early stage of coalescence is neglected due to its insignificant effect on the long-time bridge growth. According to the experiments of Wang et al, 41 the prefactor C b decreases from 1.18 ± 0.06 to 0.83 ± 0.05 due to the change in the azimuthal curvature of the liquid bridge from negative to positive, as illustrated in Figure 8a. In our simulations, C b decreases from 1.12 to 0.88 for Oh = 0.02 and from 1.03 to 0.88 for Oh = 0.12 (see Figure S1), both of which agree well with the experiment (the lower C b value for Oh = 0.12 is caused by the much higher viscous effect compared to the experimental condition).…”

Enhancement and Guidance of Coalescence-Induced Jumping of Droplets on Superhydrophobic Surfaces with a U-Groove

“…When two or more droplets coalesce on a superhydrophobic surface, the merged droplet can jump spontaneously from the surface without requiring any external energy. , This phenomenon is defined as “coalescence-induced droplet jumping,” which has received significant attention owing to its potential in a variety of applications, including anti-icing, − self-cleaning, − condensation heat transfer, − energy harvesting, , thermal diodes, electronics cooling, and atmospheric corrosion protection; , in these applications, a higher jumping velocity or height is always expected and favorable. When two droplets coalesce, the released excess surface energy is partly converted into kinetic energy, resulting in translational motion, but the energy conversion efficiency is inefficient. − The jumping velocity V j for two equal size droplets coalesced on a flat superhydrophobic surface follows the capillary-inertial scaling, , V j ∼ u ic = , where u ic is the capillary-inertial velocity; γ, ρ, and r are the surface tension, density, and initial radius of the droplets, respectively. Previous studies have demonstrated that there is a jumping velocity limit of V j ≤ 0.23 u ic for microscale droplets, , with a maximum energy conversion efficiency η < 6%. ,,− It has been demonstrated that the impact between the liquid bridge and superhydrophobic surface is responsible for the jumping. ,, Recently, the use of the impact between the liquid bridge and micro/macrotextures has provided a strategy to improve coalescence-induced droplet jumping velocity and energy conversion efficiency.…”

Coalescence-Induced Droplet Jumping

“…Nevertheless, the methodology of the droplets’ dividing process for coalescence-jumping behavior is similar. The stages are usually divided according to the changes in the shape of the droplets and the contact angles between the droplets and the substrate surface (with a variation of three to five stages). − …”