Exploring the Physics of Superhydrophobic Surfaces

A superhydrophobic (SH) surfaces is one which repels water (or some other liquid) very strongly. Such surfaces have a myriad of exciting technological applications such as for fabricating self-cleaning surfaces, for low friction flow of water past obstacles, and for desalination of sea water. From a physical standpoint, the degree of hydrophobicity of a surface is quantified in thermodynamic terms by the contact angle that a water drop makes with the surface. The weaker the attraction of the surface for the liquid drop, the larger the contact angle becomes. In the limit where the contact angle approaches 180 degrees, the drop rolls up into a sphere. This corresponds to a type of phase transition known as “drying” whereby a macroscopic film of vapor intrudes between the wall and the bulk liquid; this is the analogue of the well known wetting transition that occurs for strongly attractive surfaces when the contact angle goes to zero degrees and the liquid spreads out over the surface. As well as its technological relevance, drying is interesting from a fundamental perspective because it has recently been discovered [1,2] that it is an example of a critical surface phase transition. A hallmark of such a phase transition is that the liquid density close to the wall exhibits strong fluctuations, the length-scale of which diverges as the transition is approached. However, the nature of this physics is not yet well understood.

Experiments [3] show that the roughness of a surface is key in controlling the degree of hydrophobicity. In general, a rough surface will exhibit a larger contact angle than a smooth one made of the same material. However, there is little theoretical understanding of why this should be the case in terms of the critical properties of the drying transition. Gaining such understanding is important in order to gain a complete picture of the nature of superhydrophobicity, which will likely be crucial in the quest for designing new types of durable surfaces with higher contact angles.

The aim of this project will be use simulation and theoretical methods to try to understand how the drying transition is affected by surface roughness. You will perform Monte Carlo simulations of a model for water interacting with a rough substrate described by a periodic potential. The simulations will be performed using the Bath High Performance Computer and you should have some experience of simulation methods and coding in a high level language such as C. You will vary the scale and periodicity of the roughness and quantify the effects on the drying transition. Finite-size scaling methods will be used to probe the critical properties and you will compare with the results of mean field theory which you will perform using the methods of classical density functional theory. This latter aspect of the work will be performed in collaboration with physicists from Bristol University. The results of the project should provide a basis for the rational design of the next generation of superhydrophobic surfaces.

You will join a vibrant group of academics, postdocs and PhD students working in a number of areas of theoretical and computational physics. You will present your work in oral and poster form at group meetings as well as at international meetings and conferences. By the end of the PhD you will have acquired a diverse range of skills in computational and simulational techniques, as well as the ability to undertake independent research and solve challenging problems. This will serve you in good stead for following a variety of postgraduate career paths: previous graduates from the group have landed high level jobs in software engineering, banking, data science and academia.