Evaporation of sessile droplets on nanoscale periodic cubic-pillar surface: Experimental investigation

Abstract

To enhance liquid–vapor phase-change heat transfer, the evaporation of droplets on nanostructured surfaces has been experimentally and numerically investigated. Nevertheless, understanding of how the nanostructured surfaces influence evaporation remains insufficient. In particular, physical models have been unable to predict evaporation characteristics. The present study employs periodic nanostructured surfaces. Electron beam lithography is used to fabricate nanoscale cubic pillars in a periodic configuration on silicon specimens. The pillars are nominally 100 nm in width, height, and gap opening. Water droplets can spread on the nanostructured surface, which shows better wettability than the silicon flat surface. In addition, the contact line of the droplet is effectively pinned on the nanostructured surface. We confirm that the nanostructured surfaces can reduce the lifetime of an evaporating droplet by approximately one-half at maximum. In addition, the nanostructured surfaces induce a unique intermediate evaporation mode between the constant contact radius (CCR) and constant contact angle (CCA) modes, herein referred to as the transition mode. In the transition mode, the contact line slowly recedes, together with the transient change in contact angle, during the evaporation process and the dynamics differ from those in the CCR and CCA modes. A novel phenomenological predictive model is proposed for the nanostructured surface.