3D Numerical Study of the Transport Characteristics of an Evaporating Water Droplet Sessile on Heated Superhydrophobic Substrates

Evaporation of sessile droplets on superhydrophobic substrates is an important fundamental problem. Classic diffusion-based model only considers vapor diffusion and assumes an isothermal profile along the droplet interface. The diffusion based model extremely overestimates the evaporation rate for droplets evaporating on heated superhydrophobic substrates, and results in a deviation of evaporation lifetime up to 52.5%. The present 3D numerical model considers various effects including vapor diffusion, buoyancy-driven flow and evaporative cooling, etc., with conjugate heat and mass transfer solved throughout the computational domain. Evaporation of a sessile water droplet with an initial volume of 3 mu L is investigated on superhydrophobic substrates (contact angle: 160 deg) with heating temperature ranging from 40 degrees C to 60 degrees C. The deviation of evaporation lifetime is less than 2% for 40 degrees C and 50 degrees C substrates. A single-roll asymmetric vortex is produced inside the droplet rather than the symmetric recirculation flow predicted by 2D axisymmetric simulation. The evaporative cooling along the droplet interface is observed, but the coolest point appears on the one side of the droplet instead of the droplet top owing to the asymmetrical rolling flow inside the droplet. It is seen that the buoyancy-driven convection significantly speeds up the evaporation as the substrate temperature increases. Influence of relative humidity is also discussed and indicates a stronger impact for low substrate temperature. The present model not only precisely predicts the instantaneous evaporation rate and the total evaporation time, but also reveals the important underlying transport characteristics, which provides new insights into evaporation of water droplets resting on heated superhydrophobic substrates.