Beyond wrinkles: Multimodal surface instabilities for multifunctional patterning
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Introduction Biological surfaces are rarely flat. Surface patterning has long been found to occur concurrently with growth, development, and aging of biological systems. These surface patterns not only manifest the beauty of nature,1,2 they can also be critical to the survival and well-being of living organisms (Figure 1a). For example, surface patterns help organs grow extended surface areas to facilitate mass exchange in blood cells3 and intestinal villi,4–7 and to enhance intellectual capacity of the brain.8,9 Furrows form on animal skins10–12 or plant surfaces13–15 to adapt to the aging process; biofilms form channels within structures to aid the transport of nutrients and water critical for biofilm growth;16,17 and epithelial cells delaminate to avoid tissue overcrowding.18,19 Although these surface patterns are believed to be the result of complex interactions between genetic, biochemical, and biological processes, accumulating evidence has shown that mechanical forces play a critical role in controlling their formation.20–24 In engineering systems, surface instability patterns have been traditionally regarded as indicators of failure. For example, buckling of stiff skins on soft cores is regarded to be a failure mechanism of core–shell composites.25–27 Similarly, the delamination of thermal barrier coatings has been found to compromise the overall barrier performance and may further lead to exfoliation.28–31 Conversely, researchers have recently begun to realize that surface instabilities,
rather than being viewed only as a detrimental failure, can also be harnessed to give tunable topographic features that are useful in many technological applications (Figure 1b). For example, wrinkling instabilities on stressed layered structures, manifested as sinusoidally undulating surfaces, have been used in tunable adhesion, optics, hydrophobicity, microfluidics, property metrology, and flexible electronics.32–37 Beyond wrinkling, a number of new modes of surface instabilities that yield topographic characteristics distinctly different from sinusoidal undulation have been recently observed in experiments and simulations, including creasing, delaminated buckling, folding, period doubling, and ridging. These new modes of instabilities not only require new physical and mechanical explanations, they also open new avenues to exploit diverse surface patterns across multiple length scales for new applications.20,38 Despite intensive research, prior studies have mostly focused on theory, experiments, or applications of individual modes of instability such as wrinkling. A systematic understanding of various instability patterns and quantitative prediction of their topographic characteristics will not only significantly contribute to the knowledge of fundamental physics of surface instabilities, but also provide rational guidance to design new patterns for novel applications. Given the complexity and importance of this emerging field, this article is aimed at providing a systematic summary
Qiming Wang, University of Southern Californi
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