Abstract
The problem of contact line pinning on surfaces is pervasive and contributes to problems from ring stains to ice formation. Here we provide a single conceptual framework for interfacial strategies encompassing five strategies for modifying the solid-liquid interface to remove pinning and increase droplet mobility. Three biomimetic strategies are included, (i) reducing the liquid-solid interfacial area inspired by the Lotus effect, (ii) converting the liquid-solid contact to a solid-solid contact by the formation of a liquid marble inspired by how galling aphids remove honeydew, and (iii) converting the liquid-solid interface to a liquid-lubricant contact by the use of a lubricant impregnated surface inspired by the Nepenthes Pitcher plant. Two further strategies are, (iv) converting the liquid-solid contact to a liquid-vapor contact by using the Leidenfrost effect, and (v) converting the contact to a liquid-liquid-like contact using slippery omniphobic covalent attachment of a liquid-like coating (SOCAL). Using these approaches, we explain how surfaces can be designed to have smart functionality whilst retaining the mobility of contact lines and droplets. Furthermore, we show how droplets can evaporate at constant contact angle, be positioned using a Cheerios effect, transported by boundary reconfiguration in an energy invariant manner, and drive the rotation of solid components in a Leidenfrost heat engine. Our conceptual framework enables the rationale design of surfaces which are slippery to liquids and is relevant to a diverse range of applications.
Highlights
Over the last two decades there have been many studies on super-liquid repellent surfaces originating from the original work of Neinhuis & Bartlott[1,2] inspired by the self-cleaning and water shedding properties of the Lotus leaf and by the original work of Onda et al.[3] on fractal surfaces created using a paper-sizing agent
In this report we have interpreted what often appear to be separate experimental approaches within a single conceptual framework for interfacial strategies to create surfaces slippery to liquids. Three of these strategies have been inspired by how nature moves liquids and borrow the examples of the Lotus leaf, the Nepenthes pitcher plant and galling aphids
The engineering approaches motivated by these examples lead to super-liquid repellent surfaces, liquid/lubricant infused/impregnated surfaces and liquid marbles
Summary
Over the last two decades there have been many studies on super-liquid repellent surfaces originating from the original work of Neinhuis & Bartlott[1,2] inspired by the self-cleaning and water shedding properties of the Lotus leaf and by the original work of Onda et al.[3] on fractal surfaces created using a paper-sizing agent. The above inspiration from nature for surfaces capable of shedding liquids appears diverse from minimizing contact with a liquid (i.e. Lotus leaf), to encapsulating a liquid with particles (i.e. galling aphids), and to covering their surface with a liquid (Nepenthes pitcher plan) Each of these approaches use concepts of solid surface shape or texture and surface chemistry to fundamentally change the liquid droplet-solid interfacial interaction. We focus on how the “stick” can be designed back into these surfaces and how droplet motion might be actuated, and droplets transported on such slippery surfaces
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