Capillary Properties of Natural Fibre Assembly for Evaporative Purposes

Natural fibres are well known for their high efficiency in evaporation and humidification applications, making them a promising solution for cooling processes during heat waves. Mimicking the evapotranspiration mechanisms of trees is essential for designing optimal fibre wicks in canopy-like devices. Such devices are designed by their capillary rising height and water storage capacity. This study examines the water retention properties and capillary rise height of various yarns: retention tests for surface properties and tomographies for topographical aspects are combined to explain observed water rising height in samples. Tomographic analyses document pore sizes, fibre diameters, and water distribution within the wicks. Water retention properties among the samples range from less than 10% to over 20% of dry mass, reflecting significant wettability differences. However, the ultimate height of water ascension primarily depends on the structural yarn properties. Repeated tomographic scans during the rise reveal that water initially ascends in fibre dense regions. The competition between capillary and tensile forces causes displacements, redistributing fibre densities. In plant fibres, swelling amplifies these displacements and propagates them along the wick axis. Water envelops individual fibres or bundles, penetrating into the lumens. The size of wet lumens increases due to internal capillary pressure, which is influenced by the convexity of the internal walls. This pressure drives a creeping progression of the water front along the fibre axis. The study finds that a high number strings with intermediate porosities, which accommodate swelling, enhances the capillary rise height. However, further research is needed to fully understand the role of lumens. While they provide additional water storage capacity and facilitate capillary action through small diameters, they may also reduce the flow rate of water ascent.