Biphilic Jumping-Droplet Condensation

Abstract

Jumping droplet condensation on superhydrophobic surfaces exhibit increased heat transfer rate compared to dropwise condensation on hydrophobic surfaces However, the performance of superhydrophobic surfaces is limited by the low individual droplet growth rates associated with their extreme apparent advancing contact angles (θ_app → 180°). Our detailed condensation heat transfer modeling coupled with numerical simulations of binary (N = 2) and coordinated (N > 2) droplet coalescence, show that biphilic surfaces with smooth, low surface energy spots on a superhydrophobic background exhibit an unprecedented 10X higher jumping droplet condensation heat transfer coefficient when compared to homogenous superhydrophobic surfaces. By promoting faster droplet growth rates on these spots and controlling spot adhesion, we engineer the droplet distribution density on the surface while promoting droplet departure efficiency beyond what is currently available. Model predicted design optimization of the biphilic surface is validated against condensation experiments. Our findings clarify the role of droplet jumping dynamics and distribution densities by revealing optimum design guidelines for biphilic surface development for maximum condensation heat flux. Contrary to current understanding, we observe that spot wettability should not be optimized towards minimizing droplet nucleation energy barrier, rather it should minimize droplet adhesion while maximizing individual droplet growth rate.

Keywords: droplet, heat transfer, energy, superhydrophobic, adhesion, structured surfaces, biphilic, jumping