Simulation of self-propelled droplet jumping on superhydrophobic surfaces

• Simulation von Tropfenkoaleszenz auf superhydrophoben Oberflächen

Figueiredo Coronado, Patric Nelson; Kneer, Reinhold (Thesis advisor); Scheid, Benoit (Thesis advisor)

Aachen (2021)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2021

Abstract

Nanostructured coatings offer the potential to create substrates with specific wetting properties. On these low-adhesion surfaces, coalescing droplets can spontaneously jump off, known as coalescence-induced droplet jumping. The fluid dynamics of coalescing droplets on superhydrophobic surfaces is governed by a subtle balance of surface forces, kinetic energy and viscous dissipation. This phenomenon is observed on a variety of synthetic and natural superhydrophobic surfaces, and gives rise to a range of applications. In condensers, self-propelled droplet jumping hinders the formation of an insulating condensate layer, which may significantly improve heat transfer. Under freezing conditions, it hinders the formation of an ice layer if droplets jump off the superhydrophobic substrate before solidifying. From an application point of view, it is important to predict condensation rates, removal rates and heat transfer on superhydrophobic surfaces on the macroscopic scale by an appropriate wall model. Through three dimensional simulations, this thesis studies the fluid dynamics of droplet jumping. Parametric studies show the influence of droplet mismatch, viscosity, gravity and contact angle on jumping velocities, where liftoff regimes are defined on the basis of Ohnesorge number, contact angle, Bond number and droplet size ratio. Results show that droplet jumping is a very inefficient process, as only a small fraction of the released surface energy leads to useful translatory kinetic energy. Moreover, droplet jumping is impaired with rising Ohnesorge and Bond numbers. Additionally, surfaces exhibiting higher contact angles also impair droplet detachment.