Investigation of the confinement effect on the evaporation behavior of a droplet pinned on a micropillar structure.

Evaporation of sessile droplet suffers from reduced evaporation rate due to the confinement of vapor diffusion imposed by the bottom substrate. However, it is possible to change the evaporation behavior of a droplet by suspending it from the bottom substrate, in particular, supporting the droplet on a micropillar. This is expected to enable diffusion transport in the downward direction that will subsequently enhance evaporative transport. In this study, we investigate the diffusion confinement effect imposed by the bottom substrate and the side wall of the micropillar through numerical simulations and experimental investigation. The approximate solutions for total evaporation rate and local evaporative flux were subsequently derived from the total evaporation rate predicted by the simulation results. The simulation results, agreeing within 5% with the experimental measurements, show that increasing the micropillar height enhances the total evaporation rate from the suspended hemispherical droplet. This enhancement is due to a dramatic improvement of the local evaporation rate near the contact line region as micropillar heights increase. The micropillar heights examined for maximum evaporation rates were observed under substrate temperatures from 60-98 °C. The increasing pillar height leads to smaller vapor diffusion resistance but greater conduction resistance.