Surface declination governed asymmetric sessile droplet evaporation

Abstract

This article reports droplet evaporation kinetics on inclined substrates. Comprehensive experimental and theoretical analyses of the droplet evaporation behavior for different substrate declinations, wettability, and temperatures have been presented. Sessile droplets with substrate declination exhibit a distorted shape and evaporate at different rates compared to droplets on the same horizontal substrate, and exhibit more frequent changes in regimes of evaporation. The slip-stick and jump-stick modes are prominent during evaporation. For droplets on inclined substrates, the evaporative flux is also asymmetric and governed by the initial contact angle dissimilarity. Due to a smaller contact angle at the rear contact line, it is the zone of a higher evaporative flux. Particle image velocimetry shows increased internal circulation velocity within the inclined droplets. Asymmetry in the evaporative flux leads to higher temperature gradients, which ultimately enhances the thermal Marangoni circulation near the rear of the droplet where the evaporative flux is highest. A model is adopted to predict the thermal Marangoni advection velocity, and good match is obtained. The declination angle and imposed thermal conditions compete and lead to morphed evaporation kinetics than those of droplets on horizontal heated surfaces. Even weak movements of the contact line alter the evaporation dynamics significantly, by changing the shape of the droplet from an ideally elliptical to an almost spherical cap, which ultimately reduces the evaporative flux. The lifetime of the droplet is modeled by modifying available models for a non-heated substrate, to account for the shape asymmetry. The present observations may find strong implications toward microscale thermo-hydrodynamics.