Water is the source of life. However, with the boost of the global population and the sharp increase of the industrial water consumption, water resource is getting into crisis in the world wide. Therefore, the problem of water crisis has to be tackled urgently. The traditional desalination and secondary recovery of waste water both require complex equipments and are high cost. Fog collection, a method to translate the latent water (water drops with diameters ranging from 5 to 40 micrometers) into the dominant, available water, is simple and low cost and is expected to solve the water crisis.
Researchers from the Institute of Chemistry and Beihang Univ. got a breakthrough in the continuous and efficient fog collection after discovering the directional water collection ability of spider silk for the first time in the world.
They studied the drought-tolerant cactus Opuntia microdays originating from the Chihuahua desert systematically and found that the cactus had evolved unique structural features: the plump and succulent stems of it are covered evenly with clusters of spines and trichomes. A single conical spine can be divided into three parts with different subtle structures: the tip with oriented conical barbs; the middle with gradient grooves; the base with belt structured trichomes.
The integration of the structures gives rise to the integration of the multiple functions. Specifically, when the fog flows to the spine, tiny water drops deposited on the tip of it are collected through frequent coalescence, and then they are transported via the grooves with width gradient in the middle part, finally absorbed as soon as they contact with the trichomes at the base. Meanwhile, the new born fresh surface starts next cycle for water drop deposition-collection-transportation-absorption. The quick transport and depart of the water drops collected grantees the continuous fog collection.
Further analysis of the mechanism underlying shows that the gradient of Laplace pressure arising from the conical shape of the spines and barbs and the gradient of surface free energy arising from the gradient of the grooves along the spines cooperatively drive the water drops to move towards base of the spine with high speed. Investigation on the structure-function relationship provides new ideas for designing and fabricating continuous and efficient fog collectors. It also offers a new avenue to relieve the global water crisis and benefits to the development of the global agriculture, industry as well as military.