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.

Industrial Pathways

The scope of the research activities in hygrotaxis falls within the general concept of liquid transport and discharge from solid surfaces over timescales shorter than the timescale of evaporation of the liquid. This principle can be translated into a wide range of applications within industrial Research and Development (R&D).

Heat Transfer Sector - Heat pipes are devices that combine temperature differences with a liquid-gas phase change to mediate the transfer of heat between solid interfaces. The use of heat pipes spans a wide range of applications including cooling systems for microelectronics and spacecraft and as heat-storing elements for energy harvesting. A key feature in heat-exchange technology is the efficiency of heat transfer. More precise management of the liquid-gas phase change, including the spatio-temporal control of liquid structures can lead to optimised devices that increase the net heat transfer between target surfaces. In this proposal I will work with Sustainable Energy Systems (SES), an SME dedicated to R&D in heat exchangers, as an Industrial Partner of the project. The planned activities are focused on identifying surface geometries and operational conditions that exploit hygrotaxis to increase the efficiency of heat transfer devices. The main outcome of this collaboration will be a set of proof-of-principle results (simulations and experiments), which can be developed as a follow-up project focused on heat-exchange applications.

Automotive Sector -- According to the Annual Business Survey of the Office of National Statistics the UK vehicle sector contributed with a £60.6 bn turnover to the UK economy in 2013. Moreover, the Society of Motor Manufacturers and Traders reported that 158,000 jobs were directly related to vehicle manufacturing industry in 2014. A challenge faced by the automotive sector is how to enhance liquid transport and minimise surface debris upon evaporation from automobile coatings. Current technologies focus on aesthetics, protection against corrosion, durability, the ability for production en masse and cost (Akafuah et al., Coatings 6, 2016). In this project, I will work with Jaguar Land Rover, a UK multinational automotive company. The pathways to impact activities of this collaboration will be aimed at identifying combinations of specific surface geometries, surface properties and operational conditions, which lead a better liquid transport upon evaporation. The result of this collaboration will be a first set of proof-of-principle results that underpin a subsequent collaboration project with JLR specifically aimed at developing new self-cleaning surfaces which exploit hygrotaxis. This can help maintain the current niche capability enjoyed by the UK in the automotive sector.

Public Engagement

Evaporation is a seemingly simple process, and it occurs in a multitude of everyday life situations. Evaporation can be used to illustrate a variety of concepts in Physics, for example capillarity, superhydrophobicity, stability, heat transfer and phase transitions. It also satisfies the fascination we have for slow processes as illustrated by time-lapse videography. In this project I will work with Think Physics, a HEFCE-Catalyst project aimed at encouraging young people, with a particular focus on women, to pursue careers in STEM disciplines, to develop an trial an outreach demonstrator based on evaporation. In collaboration with Nature's Raincoats, an outreach project run by Northumbria, Nottingham Trent and Oxford Universities, I will run this demonstrator in Public Outreach events.

People Development

This project will employ a Post Doctoral Research Fellow, whose expertise will add to the UK's academic skills-base in theoretical and computational Soft Matter Physics. The PDRF will also benefit from training in project management, scientific reporting and networking through attendance to conferences, and in public engagement training.