The materials for fog harvesting have been paid much more attention as water scarcity is facing mankind. Interestingly, biological surfaces create the enigmatical reality that can be contributed to learning of human beings. Such biological surfaces with multi-gradient micro- and nanostructures display unique wetting functions in nature via evolvement, e.g., water collection or repellency, which have inspired researchers to design originality of materials for promising future applications.
In nature, a combination of multiple gradients in a periodic spindle-knot structure take on surface of spider silk after wet-rebuilding process in mist. This structure drives tiny water droplets directionally toward the spindle-knots for highly efficient water collection. Inspired by gradient MN of wet spider silk, artificial bioinspired fibers with water collecting properties can be fabricated to integrate the gradients of curvature and roughness by means of designing humps or spindle-knots, with features of multiple gradients (e.g., roughness, smooth, temperature-respond, photo-triggering, etc.,) via improved techniques such as dip-coating, fluid-coating, tilt-angle coating, electrospun and self-assembly, to combine the Rayleigh instability theory. The geometrically-engineered thin fibers display a strong water capturing ability than previously thought. The bead-on-string heterostructured fibers are capable of intelligently responding to environmental changes in humidity. Also a long-range gradient-step spindle-knotted fiber can be driven droplet directionally in a long range. An electrospun fiber at micro-level can be fabricated by the self-assembly wet-rebuilt process, thus the fiber displays strong hanging-droplet ability. The temperature or photo or roughness-responsive fibers can achieve a controlling on droplet driving in directions, which contribute to water collection in efficiency. In addition, a conical spine can be designed to combine with periodic roughness gradient, which can be used to integrate an array for high efficiency of fog harvesting. Inspired by desert beetles, strategies have been explored to fabricate hydrophobic−hydrophilic patterned surfaces for high-efficient fog collection. The surfaces with star-shape hydrophilic patterns are designed to raise the efficiency of water collection.
These bioinspired as-designed materials are significant to develop novel functional materials for fog harvesting tasks.
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