Numerical investigation of nature inspired fog harvesting for water collection

Abstract

Fresh water scarcity has become a major problem especially in arid environments. Although water is abundant on earth, only 0.36% freshwater is available for human use that arises from frozen glaciers and polar ice caps. This has triggered the need for search of alternative sources of fresh water. Harvesting fresh water present in the form of fog can be a viable solution to this issue. Traditionally, a simple device, known as fog collector, consisting of mesh net is used to harvest water from air. The mesh net has a lower water collection efficiency though, which can be improved five-folds by optimum tuning of mesh wetting characteristics and topography. Recently, surfaces inspired from Namib dessert beetle's wing topographical and wettability features have shown higher water collection rates. The fog harvesting is essentially the collection of small fog droplets intercepting on the surface, which signifies the need to understand the dynamics of droplet impact on beetle inspired surfaces in order to achieve higher water collection rates. In literature, some studies have also investigated fog collection due to condensation on surfaces at lower temperature. Therefore, the present work aims to study this nature inspired fog harvesting phenomenon, with the focus placed on the two key physical processes, namely droplet impact and condensation on beetle inspired bumps and surfaces. To tackle these complex multiphase problems, a lattice Boltzmann method (LBM) based simulation framework has been developed for this research. Four major issued were addressed in this research. First, droplet impact on beetle inspired hemispherical bumps was investigated. The bumps were nanotextured and superhydrophobic in nature. The effects of several key parameters, including the interpost spacing, post height, bump radius and Weber number, were investigated. The results showed that droplets impacting on bumps with higher posts and larger radius were generally in the Cassie state and hence favorable for water collection, whereas droplets impacting on bumps with higher posts and smaller radius were easy to rebound which are difficult to collect. Secondly, the influence of surface slope on impact of two successive droplets was investigated. The effects of surface inclination, lateral/longitudinal offset, the impact dynamics of the two droplets and subsequent dynamics of the combined droplet were studied. It was observed that oblique impact causes asymmetric droplet spreading, with the downward spreading dominant over the lateral spreading. Furthermore, it was highlighted that the coalescence of the two droplets can result in abrupt changes in the evolution of the back and left/right contact edges, which is attributed to the partial landing of the trailing droplet on the leading droplet. Thirdly, inspired from beetle's bump structure, shedding of condensing droplet from hydrophilic/superhydrophobic bump was studied. Bumps of two different shapes, i.e., cylindrical and hemispherical, were studied. It was found that water droplet condenses on the top hydrophilic area of a bump until it reaches a critical volume and sheds from the bump due to the gravity. The critical volume was found to be strongly dependent on the bump height. Fourth, to study fog condensation on cold surfaces, the impact of droplets at saturation condition on a cold superhydrophobic surface was investigated. The effects of several key parameters, including the Jakob number, the Prandtl number, Weber number and surface slope, were investigated. It was revealed that the maximum spreading factor increases in non-isothermal impact compared to isothermal impact for both level and slanted surfaces. Furthermore, the Jakob number and the Prandtl number have strong influence on motion of the back contact edge in the case of droplet impacting on inclined surfaces. The research presented in this thesis reveals the effects of dessert beetle inspired bump topographical features and surface inclination on droplet impact dynamics and condensing droplet shedding, which are useful in improving the future design and implementation of such bio-inspired water collectors.