Patterned Nanobrush Nature Mimics with Unprecedented Water-Harvesting Efficiency (original) (raw)

2018, Advanced Materials Interfaces

This is due to the fact that at any given moment, the earth's atmosphere contains an astounding 37.5 million billion gallons of water as vapor, [2] and an efficient device to capture a fraction of this water vapor, in a cost-effective way would help solve the water crisis. Over the period of human existence, it has become apparent that biomimicking is the most efficient way to tackle such problems. When we look into nature, there are organisms, which in the course of evolution have acquired physical traits that enabled them to capture atmospheric water, even in the most arid corners of our planet. One such example is the Stenocara beetle of Namib Desert which capture water on its hardened forewings. [3] Electron microscopic images of these have revealed unique array of hydrophilic regions distributed on a superhydrophobic background, [4] creating a surface energy gradient, which facilitates efficient condensation and transportation of atmospheric water. Another such example is spider silk, which shows unique periodic spindle-knot structure when wetted and efficiently collects water from atmosphere through the combination of surface energy gradient and Laplace pressure difference. Other inspirations from nature are some cactaceae species which live in arid environments and are extremely drought-tolerant. [5] These species are shown to have structures with spines and trichomes which enable them to condense humidity efficiently from the atmosphere. Grasslands are also examples of natural atmospheric water harvesters. Hence, micro/nanostructuring of the water-collecting surface plays a critical role in determining the efficiency of water capture. With the advancement of nanotechnology, significant efforts have been directed toward fabricating surfaces with similar morphological features and chemical patterning to enable efficient water capture. [6,7] However, these natural and nature-mimicked surfaces collect atmospheric water in the form of dew/fog and require the temperature to drop below the dew point to cause condensation. For building a practical and round-the-clock operating atmospheric water generator, it is important to cool the condensing surfaces and the surrounding air efficiently, with minimum energy input. Heat transfer efficiency of the condensing surface material is as important as the water transfer efficiency for creating a viable radiative condenser. Although many biomimetic, patterned surfaces have been made for fog collection, inspired by active condensing Water scarcity is one of the most alarming problems of the planet. An ambient ion based method is developed to make hydrophilic-hydrophobic patterned silver nanowires (NWs) as humidity harvesters of unprecedented efficiency. Such water harvesters are developed by two-step surface modification of the as-synthesized NWs (known from a report earlier) using electrospray. These patterned NWs of ≈20 µm length and ≈200 nm width grown over a relatively large area (2 × 2 cm 2) exhibit atmospheric water capture (AWC) efficiency of 56.6 L m −2 d −1 , the highest reported so far. The whole fabrication process of the surface is performed under ambient conditions with a home-built nanoelectrospray ion source, without the help of any sophisticated instrumentation. The synthesized material combines and mimics two exciting examples of AWC in nature, which are cactae and Namib Desert beetles, which utilize AWC for their living. It is believed that the combination of the special features of the above two natural species helps to achieve the highest water capture efficiency reported till date. A working prototype using this surface for AWC is also fabricated.