Numerical analysis of coalescence-induced jumping droplets on superhydrophobic surfaces

Licentiate thesis, 2022

Bio-inspired superhydrophobic surfaces are used in numerous technological applications due to their self-cleaning ability. One of the several mechanisms reported in literature and responsible for self-cleaning is the phenomenon of coalescence-induced jumping of droplets from such surfaces. The phenomenon is observed for scales below the capillary length and when gravity is negligible. Primary applications of this technology are on heat-exchangers or any other that involve surfaces for which anti-icing and water-repellency properties are desired. This thesis comprises two publications that involve high-fidelity numerical investigations on fundamental features of the jumping droplets phenomenon and focuses on two important aspects. The first one is a study on coalescing and jumping of microdroplets (R < 10 µm). The differences in the jumping process (for example, reduction of the merged droplet jumping velocity) are pointed out as a function of the initial size of the droplets. Through an analysis of the energy budget, several degrees of dissipation are found, which is attributed to a competition between viscosity and the strong capillarity on the interface. The second publication focuses on the interaction of the merged droplet with a superhydrophobic surface with hysteresis. It is found that such a case has a reduced jumping velocity as compared to a no-hysteresis one. Using a dynamic contact angle model is beneficial to capture the receding contact angle and provide a more accurate estimation of the overall process. In this work, a combined Immersed Boundary -- Volume-of-fluid method with different contact angle models and a Navier-slip boundary condition is used. The numerical framework has been extensively validated.