Removal of contaminants by jumping droplets from superhydrophobic surfaces – a numerical study

Doktorsavhandling, 2024

Superhydrophobic surfaces are advantageous in numerous applications due to their anti-icing, water-repellent and enhanced heat-transfer abilities. A great number of studies have been performed in order to improve those properties, with their self-cleaning ability standing out as their most desirable feature. This thesis focuses on passive self-cleaning mechanisms and uses advanced numerical simulations to characterize the properties of superhydrophobic surfaces and a series of fundamental phenomena that facilitate liquids and contaminant removal. A detailed numerical framework has been developed in discrete steps, tackling the increased complexity that each separate study introduced. Initially, a volume of fluid (VOF) model is used to simulate droplets jumping from superhydrophobic surfaces, a phenomenon initiated when two or more droplets coalesce on a superhydrophobic surface. This part of the work was within the capillary-inertial dominated spectrum, where the droplets pertain the strongest capillarity. Simulations of coalescence and jumping of droplets of various sizes were performed, providing insights into the interaction of hydrodynamic forces and surface tension. The next step was the understanding and implementation of effective contact angle hysteresis and explore its influence on the jumping efficiency. Hysteresis, introduced either by static or dynamic modeling of the contact angle, increased the energy dissipation of a surface and showcased re-attachment events for the merged droplet. During the course of the project, another study reported the removal of a particle by a single droplet. This self-cleaning mechanism occurs when a droplet initiates spreading on a hydrophilic particle, capturing it in the process. Due to its oscillations, the droplet interacts with the superhydrophobic surface and the particle-droplet system jumps from the surface. To describe this phenomenon, a numerical study was performed using a combined VOF-immersed boundary method, with a special focus on formulating and implementing the capillary force attracting the particle. The latter formulation was validated benefiting from an experimental study, and a parameter study followed with regards to the properties of the involved phases and the wettability of the solids. In the final part, pillar-shaped structures were introduced on the superhydrophobic surface to explore the effect of having structured surfaces on the particle-droplet coalescence and jumping. A comparison was performed between jumping from a smooth superhydrophobic surface and those from a pillared surface, showcasing the ability of the latter to achieve jumping and to undergo all the stages of the process. Finally, different configurations of pillars were tested, revealing that narrower and taller pillars with a sufficient spacing promote self-cleaning with the least dissipated energy.