Novel Water Harvesting Fibrous Membranes with Directional Water Transport Capability

Water is among the most critical prerequisites for life on earth, in fact, astronomers seek for water in exoplanets before they classify them as habitable or not. Harvesting water directly from ubiquitous atmospheric moisture has been a handy method to relieving the scarcity of clean water that severely jeopardizes the survival of human beings and other creatures. At present, following inspiration from nature and consequent bio-mimicry, various researchers have developed artificial water harvesters, majority of which are based on either a 2D surface or a 1D filament.

In a recent publication, researchers have uncovered that porous membranes with wettability gradient or wettability contrast in thickness direction can have a “smart” directional wicking effect, which enables water to transfer automatically across the membrane just in one direction. However, limited attention has been devoted to using them for water harvesting.

To this note, Australian researchers at Deakin University namely: Dr. Hua Zhou, Dr. Hongxia Wang and led by Professor Tong Lin together with Dr. Jing Wu at Beijing Institute of Fashion Technology Beijing developed new techniques concerning harvesting water from air using hydrophobic/super-hydrophilic directional wicking nanofibrous membranes. They developed a wicking fibrous membrane of much higher water harvesting capacity in comparison to those previously reported. Their work is currently published in the research journal, Advanced Materials Interfaces.

In brief, their approach employed a two-step electrospinning method to prepare directional wicking fibrous membranes using polyacrylonitrile (PAN) as a starting material. Stepwise, Hydrophobic PAN membranes were prepared followed by Hydrophobic PAN membranes. Eventually, the dual layer membrane shows Directional Wicking property. The resulting samples were characterized using various techniques, such as: scanning electron microscopy and Fourier-transform infrared spectroscopy.

The authors reported that directional wicking membranes exhibited a much higher water harvesting capacity than those with homogeneous hydrophobicity or superhydrophilicity on the two sides. In fact, both porous structure and pore dimension were seen to contribute to water harvesting. In addition, the researchers recorded that the larger pores in the hydrophobic layer and smaller pores in the super-hydrophilic layer in the directional wicking membranes helped provide stronger forces to draw water from the hydrophobic to the super-hydrophilic surfaces, a process that enhanced water harvesting.

In summary, the study demonstrated that fibrous membranes with directional water wicking function have great potential for harvesting tiny water drops from air. Remarkably, it was highlighted that the variation in pore dimension between the hydrophobic and the super-hydrophilic layers could lead to 1.7 times difference in water harvesting rate. In addition, a novel technique to measure the adhesion force between water droplet and nanofibrous membrane during water transport was developed and could be useful for the development of advanced water harvesters for various applications.