15. Evaporation Dynamics of a Sessile Droplet on a Hydrophobic Surface

Contrasting dropwise condensation, evaporation of a water droplet on a hydrophobic but conducting surface under ambient conditions is discussed in the present chapter. Interfacial evaporation rates and evaporative flux are often predicted using the vapor mass diffusion model. It accounts for differences in the humidity ratio between the interface region and the far-field, the humidity flux being determined by Fickian diffusion. Two other factors that are relevant are evaporative cooling of the air-liquid interface and heat conduction in the solid-liquid-gas domain. Thus, thermophysical properties of the substrate become relevant, limiting droplet evaporation rates. In this chapter, an extended vapor diffusion model is built in COMSOL^® that incorporates these additional transport mechanisms and predicts evaporation rates, interfacial evaporative flux, and temperature distribution at the base of the sessile droplet. The model is validated against controlled laboratory experiments for sessile droplet evaporation of water on copper and glass substrates that offer distinct wettability. The study reveals the limitations of the vapor diffusion model and the importance of additional mechanisms relevant to droplet evaporation.