Liquid actuation based on humidity gradients: Hygrotaxis

We live in a world made of water, and more specifically, water that is constantly melting, freezing, condensing and evaporating. Many important aspects of our lives are determined by how water changes between ice, liquid and vapour. Evaporation is a seemingly simple process which occurs when a droplet is in contact with a gas that is not saturated with vapour molecules. This is why damp clothes take longer to dry on a rainy day. Droplet evaporation occurs in a wide range of important situations, from the drying of droplets from the surface of a car to the minute workings of cooling systems used in microelectronics. Moving droplets about is a very appealing solution for problems involving the transfer of mass and heat, and in recent years there have been advances in identifying mechanisms that lead to the self propulsion of droplets. For example, it is possible to use ratchet shaped surfaces or surfaces of changing rigidity to guide the motion of droplets, and by adding surfactants (such as food coloring) it is possible to make droplets chase each other. However, identifying a mechanism for droplet self-propulsion on solid surfaces that does not rely on ratcheting or external agents has remained elusive. Recently, experimental evidence has emerged indicating that droplets evaporating on solid surfaces in the presence of a non-uniform ambient humidity generate directed flows and even 'bend' towards each other. One may naturally think whether such behaviour can be transformed into the net motion of the droplets. Such a new means for droplet self propulsion is very appealing, as it could lead to advances in heat and mass transfer applications. In this proposal I will explore the concept of "hygrotaxis". This is a new self-propulsion mechanism for liquids that exploits non-uniform ambient humidities to create fluid flow. Using a combination of computational fluid dynamics simulations and proof-of-principle experiments, I aim to investigate the mechanism of hygrotactic motion for i) single droplets, ii) pairs of droplets, and iii) droplets subject to confinement. The results of this research will set the foundations of hygrotaxis as a new concept in Soft Matter Physics. In collaboration with two industrial partners, I will identify opportunities for further research that can inform technologies which harness hygrotaxis, such heat management systems for microelectronics and self-cleaning surfaces for the automobile industry.