Joint effect of air flow and substrate wettability on evaporation of a sessile droplet on a solid surface

Abstract

We study the evaporation of microliter droplets of water on hydrophilic, hydrophobic, and superhydrophobic substrates subjected to airflow around them. While there is a large body of literature on diffusion-limited evaporation, evaporation in the presence of external convection is not fully-explored. We use side-view imaging and a custom-built flow channel to record the evaporating droplets from the side and develop an analytical model for evaporation in the presence of convection. Based on the droplet’s inlet flow velocity and wetted diameter, the maximum Reynolds number is 203. We quantify the time-variation of droplet volume, wetted diameter, and dynamic contact angle as a function of substrate wettability and airflow velocity. We found that the droplet lifetime decreases with the airflow, which is explained by the larger contribution of the convective mass loss rate compared to the diffusive mass flow rate computed by the model. Interestingly, the droplet lifetime is longer on a hydrophobic substrate due to the smallest area of liquid-gas interface among the three substrates for a given airflow. In general, evaporation occurs at constant contact radius (CCR) mode for hydrophilic surface in case of without flow and for all cases of airflow velocity. The CCR followed by constant contact angle (CCA) mode is observed for hydrophobic surface, and CCA followed by mixed mode is reported for superhydrophobic surface. The theoretical model reasonably predicts the measurements of different cases of airflow velocity and substrate wettability.