1 Department of Micro- and Nanotechnology, Technical University of Denmark2 Theoretical Microsystems Optimization, Department of Micro- and Nanotechnology, Technical University of Denmark3 Nanointegration, Department of Micro- and Nanotechnology, Technical University of Denmark4 Polymer Micro & Nano Engineering, Department of Micro- and Nanotechnology, Technical University of Denmark5 Center for Nanostructured Graphene, Center, Technical University of Denmark
The present thesis deals with the wetting of micro-structured surfaces by various fluids, and its goal is to elucidate different aspects of this complex interaction. In this work we address some of the most relevant topics in this field such as superhydrophobicity, oleophobicity, unidirectional liquid spreading and spontaneous drop removal on superhydrophobic surfaces. We do this by applying different numerical techniques, suited for the specific topic. We first consider superhydrophobicity, a condition of extreme water repellency associated with very large static contact angles and low roll-off angles. Such behaviour arises when drops are suspended on a micron or submicron texture, so that their contact with the substrate is minute. This suspended state (known as Cassie-Baxter state) is however prone to failure if the liquid-air interface is perturbed, a common situation in real life circumstances. We apply the numerical method of Topology Optimization to this problem, in order to find the optimal texture to support the superhydrophobic configuration. Our optimization provides designs which are consistent with strategies employed by Nature to achieve the same effect. Furthermore, our control over the length scale and resolution of the design allow us to obtain patterns which are not only optimal but also suitable for microfabrication. We next consider oleophobicity, which is the ability to repel low surface tension liquids through a combination of surface patterning and chemical properties. Our analysis considers a simple geometry already described in literature. We however characterize it in a novel way, trying to account simultaneously for both the wetting and mechanics properties of the texture. Such analysis is of high relevance for technical applications of these micro-patterns, and suggests that there is a balance between optimal wetting properties and mechanical robustness of the microposts. We subsequently analyse liquid spreading on surfaces patterned with slanted microposts. Such a geometry induces unidirectional liquid spreading, as observed in several recent experiments. Our numerical analysis shows how such spreading can be tuned and controlled in terms of lattice properties of the texture and wetting properties of the materials. We conclude by analysing the phenomenon of self-propelled ejection of coalescing droplets on superhydrophobic surfaces. This remarkable phenomenon is due to a transformation of surface energy to kinetic energy, and could have several technical applications in the fields of heat exchange and enhanced condensation. We discuss different dissipation mechanisms in the process as well as how the drop properties (size, shape) affect the phenomenon. Although the modelling and simulation of these wetting interactions plays a major role in the thesis, throughout the research activity we focus on two further aspects. First, we tune the relevant physical parameters to be as close as possible to experimental data. We also had different opportunities to collaborate with colleagues at DTU Nanotech and at other research institutes to experimentally test the wetting properties of selected surface patterns. Second, we apply an optimization approach to our analysis, i.e. we try to enhance specific wetting properties through changes in the texture geometry. A successful optimization is the natural consequence of an in-depth understanding of the wetting process, since a meaningful choice of design variables and optimization functions is fundamental to achieve an improved performance.
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Bøggild, Peter, Taboryski, Rafael J., Okkels, Fridolin