Biological surfaces provide endless inspiration for design and fabrication of smart materials. It has recently been revealed to have become a hot research area in the materials science world.
Biological surfaces that collect water in nature have evolved unique mechanisms, such as the capture silk of the cribellate spider Uloborus walckenaerius, which collects water through a combination of multiple gradients in a periodic spindle-knot structure after rebuilding.[1] This structure drives tiny water droplets (under 100 µm in diameter) directionally toward the spindle-knots for highly efficient water collection. Inspired by the roles of micro- and nanostructures (MNs) in the water collecting ability of spider silk, a series of bioinspired fibers has been designed by integrating fabrication methods and technologies[1-5] such as dip-coating, Rayleigh instability break-up droplets, phase separation, strategies of combining electrospinning and electrospraying, and fluid-coating. Through such fabrications, “spindle-knot/joint” structures can be tailored to demonstrate the mechanism of multiple gradients in driving tiny water drops. The rough and smooth fiber surfaces, together with the gradient of chemistry components, can generate movement and control of water droplets. In the case of larger water droplets, the geometrically-engineered thin fibers have a much higher water capturing ability than previously thought, due to the increased stability of the three-phase contact line (TCL) by the combination of “slope” and “curvature” effects on spindle-knots, providing sufficient capillary adhesion to retain the hanging drops.[2] To extend this functionality, bead-on-string heterostructured fibers (BSHFs) have been fabricated in a one-step electrohydrodynamic method combining electrospinning and electrospraying strategies, thus BSHFs are capable of water collection via intelligently responding to environmental change in humidity.[3] The above research provides insights into designing functional fibers with unique wettability,[4] either by creating special structures on the fiber surface or by modifying the surface with responsive molecules.[5] The ongoing designs of these bioinspired structures may find applications in many fields, such as for water collection, smart catalysis, filtration, and sensing. By designing asymmetric structures or a wettability gradient on the spindle-knot, unidirectional motion of water droplets may be achieved. We can modify the fiber surface with molecules that respond to thermal stimuli, making it possible to control the direction of motion of water droplets. Water collection efficiency can be increased through the directional collection of small water droplets. Smart catalysis may be realized by quickly transporting reactant droplets toward the spindle-knot created through catalysis, and taking the product droplets away from the spindle-knot. A small amount of airborne materials (such as particles) can be captured first by tiny liquid droplets, and further concentrated or filtered on a functional fiber by directionally collecting the liquid droplets toward the spindle-knot.
Except for the effect of water collection on biological/artificial surfaces, biological surfaces such as plant leaves and butterfly wings with gradient structure features display the effect of water repellency. The dynamic suspension of microdroplets appears on a fresh lotus leaf due to a gradient of wettable MNs along the exterior surface of papillae surrounding nanohairs, thus propel water condensation and the directional movement of microdroplets out of the valleys and to the top of papillae. A taper-ratchet array on ryegrass leaf integrates a gradient of retention at solid–liquid interfaces to reversibly generate the release or retention of solid–liquid contact lines in contrasting directions. Iridescent blue Morpho aega butterflies have directional water repellency to be achieved through a directional adhesion mechanism with a flexibly oriented MN. As the butterfly wing is tipped upwards, the direction of the scales prevents the droplet from moving towards the butterfly’s body. To gain a better understanding of these biological multi-structure effects, smart bioinspired surfaces can be fabricated by combining machining, electrospinning, soft lithography, and other techniques to achieve selective control via processes such as gravity and vibration.[6] The strong MN effect may enhance water repellency.[6-7] For instance, it has been shown that the multi-level MNs of iridescent blue Morpho nestira butterflies have low-temperature superhydrophobic properties. This finding offers insight into the characteristics of MNs on butterfly wings in general.[7] Moreover, a MN-surface inspired by the MN feature on butterfly wings has been designed by integrating machine processing and crystal growth. The MN-surface composed of microratchets combined with nanohairs on a metal substrate has a robust icephobic/anti-icing property and outperforms nanostructured surfaces, and is far better than microstructured surfaces and smooth surfaces without any structure.[8] These water repellent properties, in conjunction with a wetting-controlling mechanism based on micro-/nanostructures, would be promising applications such as icephobic/anti-icing surfaces, light- and electricity-modulated liquid transport, anisotropic wettability for selective particle transport, anti-fogging/self-cleaning at low temperatures, revisable rolling of a drop by two superhydrophobic Wenzel and Cassie states, and high-temperature-induced or vibration-induced movement of a drop on oriented taper-ratchet arrays.
Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of the Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing, China. * Corresponding Author: Yongmei Zheng (zhengym@buaa.edu.cn)
References
1. Y. Zheng, et al., Nature 463, 640 (2010).
2. X. Tian, Y. Chen, Y. Zheng, H. Bai, L. Jiang. Adv. Mater. 23, 5486 (2011).
3. X. Tian, H. Bai, Y. Zheng, L. Jiang. Adv. Funct. Mater. 21, 1398 (2011).
4. H. Bai, J. Ju, Y. Zheng, L. Jiang. Adv. Mater. 24, 2786 (2012).
5. Y. Hou, et al., Chem. Commun.49, 5253 (2013).
6. P. Guo, Y. Zheng, C. Liu, L. Jiang. Soft Matter 8, 1770 (2012).
7. H. Mei, et al., Soft Matter 7, 10569 (2011).
8. P. Guo, et al., Adv. Mater. 24, 2642 (2012).