Understanding the role of the substrate texture on superhydrophobicity. (2018)
Type of ContentElectronic Thesis or Dissertation
Thesis DisciplineMechanical Engineering
Degree NameDoctor of Philosophy
PublisherUniversity of Canterbury
AuthorsRashidian, Hosseinshow all
Synthetic super-hydrophobic surfaces have varied important applications ranging from the improved design of heat-exchangers in power plants and the development of high-performance wetsuits to the formulation of pesticides which best adhere to natural surfaces. When using micro-patterns to design these functional surfaces, it is crucial to understand how the droplet wets the topographical features of the substrate. To achieve a greater understanding and control of the wetting of complex surfaces, this thesis explores the interaction between an impacting droplet and a limited number of occlusions using the multiphase lattice Boltzmann method.
Firstly, the impact of a droplet onto a surface with a single ridge was simulated. Depending on two control parameters, namely the dimensionless distance from the impact point to the ridge and the Weber number, different wetting outcomes were observed including pinning, wetting and splashing. A simple model based on energy conservation was proposed to analyse the pinning-wetting and wetting-splashing transitions. Secondly, the stability of the thin liquid lamella which is formed after impact when it interacts with a small occlusion was investigated. The critical thickness below which the lamella punctures was predicted analytically. The numerical results confirmed this analytical model and demonstrated that an increase in the diameter of the occlusion, the impact velocity, or the hydrophobicity of the surface promote the formation of a hole in the lamella.
Next, simulations were performed to study the wetting outcomes for a droplet which impacts on a surface with a pair of pillars. In addition to the classical Wenzel, Cassie-Baxter, and break-up states, another wetting regime was revealed, namely the engulfed state, as the droplet wets both sides of pillars but an air pocket appears directly under the droplet in the gap between the two pillars. Furthermore, the influences of the geometrical parameters, Weber number, and wettability of the surface on the potential outcomes for such impacts were investigated.
Finally, the wetting of an occlusion array during oblique drop impact was modelled. To the best of the applicant’s knowledge no one has attempted detailed numerical simulation of such drop impacts onto sizable textured substrates. Based on the simulations by varying the impact angle and the Weber number, four various outcomes were observed: asymmetric spreading, bilateral splashing including a prompt splash and a corona splash, one-sided coronal splashing and asymmetric break-up. The numerical results revealed that the substrate texture parameters such as the post height, the space between posts, and wettability of the substrate play an important role on the occurrence of splashing.