Evaporation Dynamics of Deionized Water Droplets on Rough Substrates: The Coupling of Stick-Jump Motion and Evaporation

Abstract

Substrate roughness can greatly affect the evaporation of sessile droplets, thus determining the efficiency of applications, such as ink-jet printing and coating. Here, we conduct experiments on the evaporation of de-ionized water droplets on glass substrates with roughness in the range 0.1–0.2 μm to investigate its effect on the dynamics of the contact angle and radius, as well as the heat and mass transfer during evaporation. We discover a “stick-jump” phenomenon as part of a five-stage process that is determined by the evolution characteristics of the contact angle and radius and includes the volume expansion, first stick, second stick, jump and final stages. Moreover, we find that the evaporation mode of the droplets is not affected by the increase of substrate roughness, whereas the heat and mass transfer processes intensify with the increase of substrate roughness in the presence of nonuniform evaporation effects. Also, the pinning–depinning mechanism of the “stick-jump” phenomenon during evaporation is carefully analyzed in terms of the Gibbs free energy, thus establishing a relation among Gibbs and excess Gibbs free energies and substrate roughness, which predicts the evaporation dynamics of the droplet. We anticipate that this study unravels key aspects of the droplet evaporation mechanisms on rough substates toward optimizing and advancing relevant technology applications.