Unravelling the mechanisms controlling droplet growth dynamics during condensation on micro-patterned surfaces

Vapour condensation is relevant for several applications with high environmental impact such as water desalination systems, enhanced water harvesting and efficient thermal management. The formation of liquid droplets on a cold surface is a complex process depending on the surface temperature, surrounding humidity and material surface properties. Surfaces are usually made hydrophobic or even superhydrophobic with the aim of improving the longevity of the dropwise condensation regime. This project investigates the behaviour of droplets forming on a condensing surface, both experimentally and numerically, comparing their behaviour on smooth and pillar-like structured surfaces. A thermodynamic-based model has been implemented, where droplets grow following thermodynamic laws without any ad-hoc information about the condensation rate. In smooth surfaces with wettability gradient, droplets grow asymmetrically while moving over the surface. A group of droplets growing on a cooled surface show dynamics different from an isolated growing droplet. Understanding how droplets grow, coalesce and and interact with neighbouring droplets will help to improve the efficiency of liquid collection in the application areas mentioned above, such as water harvesting systems and refrigerant condensation in thermal management systems.

Vapour condensation is relevant for several applications with high environmental impact such as water desalination systems, enhanced water harvesting and efficient thermal management. Here, dropwise condensation (condensation in the form of droplets) is preferred due to its high heat transfer capabilities. Nano-fabrication allows developing micro-patterned superhydrophobic surfaces to enhance dropwise condensation. However, design of stable dropwise condensation surfaces still remains a challenge since surfaces become flooded very fast at high humidity conditions. The problem lies on the lack of understanding of the physics controlling this loss of superhydrophobicity. To close this gap in knowledge, the goal of this project will be to identify the main mechanisms determining droplet growth dynamics on patterned surfaces. While most studies of dropwise condensation have been experimentally based, numerical simulations in this area have not been given enough attention and they should be exploited further. I will use numerical simulations as a high-resolution laboratory for studying small-scale phenomena impossible to obtain from experiments. The novelty of this approach will be in the consideration of both heat transfer and flow dynamics aspects, a coupling that has been mostly overlooked. Thermodynamic based models will be used to describe phase change, contrary to the usually used empirical models. In parallel, experimental measurements of local heat flux will be obtained for the first time by using an ultra-small RTD sensor fabricated on the condensing surface. The project will result in new theory and fundamental understanding of the phenomenon, providing the basis for development of more efficient condensation surfaces. This will have a strong impact also for development of superhydrophobic surfaces for related applications such as anti-icing/anti-fog surfaces, condensation of CO2, petrochemical industry (liquefied natural gas) and biotechnological devices.