Assessment of Water Droplet Evaporation Mechanisms on Hydrophobic and Superhydrophobic Substrates

Abstract

Evaporation rates are predicted and important transport mechanisms identified for evaporation of water droplets on hydrophobic (contact angle ~110°) and superhydrophobic (contact angle ~160°) substrates. Analytical models for droplet evaporation in the literature are usually simplified to include only vapor diffusion in the gas domain, and the system is assumed to be isothermal. In the comprehensive model developed in this study, evaporative cooling of the interface is accounted for, and vapor concentration is coupled to local temperature at the interface. Conjugate heat and mass transfer are solved in the solid substrate, liquid droplet, and surrounding gas. Buoyancy-driven convective flows in the droplet and vapor domains are also simulated. The influences of evaporative cooling and convection on the evaporation characteristics are determined quantitatively. The liquid-vapor interface temperature drop induced by evaporative cooling suppresses evaporation, while gas-phase natural convection acts to enhance evaporation. While the effects of these competing transport mechanisms are observed to counterbalance for evaporation on a hydrophobic surface, the stronger influence of evaporative cooling on a superhydrophobic surface accounts for an overprediction of experimental evaporation rates by ~20% with vapor diffusion-based models. The local evaporation fluxes along the liquid-vapor interface for both hydrophobic and superhydrophobic substrates are investigated. The highest local evaporation flux occurs at the three-phase contact line region due to proximity to the higher temperature substrate, rather than at the relatively colder droplet top; vapor diffusion-based models predict the opposite. The numerically calculated evaporation rates agree with experimental results to within 2% for superhydrophobic substrates and 3% for hydrophobic substrates. The large deviations between past analytical models and the experimental data are therefore reconciled with the comprehensive model developed here.

Experimental study of water evaporation of sessile droplets on a solid substrate with different thermal conductivities

“…Digital microscope (4) was located on the side of the working area. Temperature and relative humidity of ambient air were measured by thermohygrometer (6). Experimental data were collected using a computer (1).…”

“…Thus the understanding of the evaporation process of sessile droplets is important for such applications. Recently there have been many active studies of evaporation of sessile droplets on the hydrophobic textured surfaces [5,6].…”

Effect of superhydrophobic surface morphology on evaporative deposition patterns

“…In recent studies, [23][24][25] the droplet evaporation rate was reported to be reduced on superhydrophobic surfaces due to increased influence of evaporative cooling at the droplet interface. Three modes of droplet evaporation on superhydrophobic surfaces have been reported 20,26,27 as well: a constant contact radius (CCR) mode, a constant contact angle (CCA) mode, and a mixed mode.…”

Spatiotemporal infrared measurement of interface temperatures during water droplet evaporation on a nonwetting substrate

“…[27][28][29][30] An experimental study by Dash and Garimella 31 revealed a significant discrepancy between vapor-diffusionbased model predictions and the measured rate of droplet evaporation on nonwetting surfaces, which was attributed to a large temperature drop along the droplet height induced by evaporative cooling. High-fidelity numerical modeling by Pan and coworkers 27,32 mapped the competing effects of external natural convection and evaporative cooling as a function of the surface wettability; the relatively tall droplets supported on nonwetting surfaces have a large effective thermal resistance between the substrate and droplet interface, such that evaporative cooling governs the droplet temperature profile and evaporation rate. Gleason and Putnam 33 showed that imposing a nonuniform interface temperature profile as a correction to the vapor-diffusion model more accurately predicted the experimental evaporation data.…”