Droplet capture and water transport on thin fibers for water harvesting

Abstract

Water harvesting is a potential solution to the challenge of water scarcity. Among all harvesting techniques, fog harvesting is a promising option, which uses permeable collectors to capture droplets from fog streams. The structure of the collector is the key to achieving high water collection efficiency. The fast-growing demand for efficient collectors requires research into the structural design of fog collectors. Fiber-based collectors have received considerable attention among all the structures of fog collectors. Generally, fibers can be woven into flexible grids or meshes, which are naturally permeable to serve as fog collectors. In addition, thin fibers that benefit droplet capture can be easily fabricated via multiple mature spinning techniques. Furthermore, functional structures can be created on fibers, enhancing water transport for efficient fog harvesting. However, gaps exist in the design of fiber-based collectors in terms of the effects of grid structure and waterdrop clogging on water collection efficiency. In addition, existing fiber-based collectors with water-transport ability rely on the creation of complex fiber morphologies, which hinders the large-scale application due to difficult fabrication. This thesis study aims to fill the gaps in fiber-based collector design by obtaining knowledge in terms of droplet capture and water transport on thin fibers. The thesis starts with developing a multi-scale numerical model for fog harvesting to understand the effect of fiber grid structure on water collection efficiency. The numerical model can simulate fog harvesting at two extreme length scales that are comparable to collector scale at the large end and fiber scale at the small end. The results confirm two important effects of fiber grid geometries on water collection efficiency. First, dense thin-fiber grids negatively influence the collection efficiency because of the wall effect caused by viscous boundary layers. Second, the sparse thin-fiber grids can benefit from isolated clogging waterdrops and maintain relatively high efficiency when clogging blocks multiple grid openings. The two identified effects are then included to develop a new performance map for fog collectors, thereby shaping new design rubrics for fog harvesting. Then, the experimental study of droplet capture on microfiber grids is carried out to understand the positive clogging effect. Microfiber grids are fabricated by NFES with the structural design guided by the obtained performance map. The results show that waterdrops clog the grid openings with a pattern that small waterdrops satellite large ones. Due to the small fiber diameter, the waterdrops are "visible" to incoming airflow and strongly affect droplet capture. The large waterdrops deflect incoming fog flow towards the small ones, and the small waterdrops efficiently capture the fog droplets. Consequently, the fog collectors based on microfiber grids demonstrated an exceptional water collection efficiency of up to 21.4%. The micro-fiber grids require minimal material usage and no special surface treatment, highlighting a potential in fog harvesting. Last, this thesis study discovers the water transport on ribbon-like fibers due to the long-wave Plateau-Rayleigh instability. The experimental study reveals that the deposited fog water is aggregated on the broad side of the fiber, where the low surface curvature triggers Plateau-Rayleigh instability with long wavelengths. The resulting drops are connected by a flowing film, which continuously transports water over centimeter-scale distances without the presence of external driving forces. A particle-image velocimetry analysis reveals that a pair of opposing flow exists in the film and forms organized vortices within the shear layer, which are explained by capillary effects on film-wise flow. Based on the long-wave Plateau-Rayleigh instability, a rivulets-on-fiber structure is developed using liquid bridges as artificial drops to continuously transport liquid over a 10 square centimeter fiber grid. The unique characteristics of water transport on the ribbon-like fibers and fiber grids provide new prospects for efficient collector design with simple fabrication methods.