Abstract
We review recent developments in nature-inspired superhydrophobic and omniphobic surfaces. Water droplets beading on a surface at significantly high static contact angles and low contact-angle hystereses characterize superhydrophobicity. Microscopically, rough hydrophobic surfaces could entrap air in their pores resulting in a portion of a submerged surface with air–water interface, which is responsible for the slip effect. Suberhydrophobicity enhances the mobility of droplets on lotus leaves for self-cleaning purposes, so-called lotus effect. Amongst other applications, superhydrophobicity could be used to design slippery surfaces with minimal skin-friction drag for energy conservation. Another kind of slippery coatings is the recently invented slippery liquid-infused porous surfaces (SLIPS), which are one type of omniphobic surfaces. Certain plants such as the carnivorous Nepenthes pitcher inspired SLIPS. Their interior surfaces have microstructural roughness, which can lock in place an infused lubricating liquid. The lubricant is then utilized as a repellent surface for other liquids such as water, blood, crude oil, and alcohol. In this review, we discuss the concepts of both lotus effect and Nepenthes slippery mechanism. We then present a review of recent advances in manufacturing polymeric and non-polymeric slippery surfaces with ordered and disordered micro/nanostructures. Furthermore, we discuss the performance and longevity of such surfaces. Techniques used to characterize the surfaces are also detailed. We conclude the article with an overview of the latest advances in characterizing and using slippery surfaces for different applications.
Highlights
Bioinspired slippery surfaces received significant attention in the last two decades
When a superhydrophobic surface is submersed in water, it generates slip flow, which reduces the skin-friction drag exerted on the surface
The results indicated that both drag reduction and static contact angle decreased with increasing salt concentration
Summary
Bioinspired slippery surfaces received significant attention in the last two decades. According to Lafuma and Quéré [16], superhydrophobicity could be explained using the models independently developed by Wenzel [17] and Cassie [18] The former is more applicable to slightly hydrophobic materials Oleophobic coatings provide a different kind of slippery surfaces that could resist wetting of water as well as other lower surface tension liquids such as hexadecane, dodecane, octane, and ethylene glycol [22,23,24,25,26,27,28,29,30]. The coatings could be utilized in a wide range of applications including resisting biofouling and self-cleaning in water polluted oil slicks Another example of manmade omniphobic coatings is called “slippery liquid-infused porous surfaces (SLIPS)” [31].
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