Nature-inspired special wettable materials

Abstract

Mother nature has various types of creatures that have a state-of-art adaptability to their environ-ments based on sophisticatedly developed micro- or nano-structured material interfaces that display special wettability. Specific examples include self-cleaning effect of lotus leaves, oil-repellent skin of springtails, un-derwater-oil-repellent scale of fish, water-transportable capillaries in legs of wharf roaches, and water-harvesting back of desert beetles. These special wettable materials in nature can be generally categorized into four groups: superhydrophobic materials, superhydrophilic materials, superhydrophilic/superhydrophobic-patterned materials, and superoleophobic materials. Superhydrophobic materials exhibit a static water contact angle of over $150^\circ$ and a dynamic water contact angle hysteresis (difference between receding contact angle and advancing contact angle) of below $5^\circ$. Thus, water droplets are strongly repelled by the superhydrophobic materials and move like a basketball rolling and bouncing on the surface. Superhydrophobic materials can be found in various life forms such as feathers of birds, wings of bugs, skin of gecko lizards, leaves of lotus plants, and feet of water striders. Superhydrophilic (or superwetting) materials exhibit complete water spreading on the surface, resulting in an apparent water contact angle of nearly $0^\circ$. Interestingly, when superhydrophilic materials are submersed underwater, they show strong repellence against oil droplets (or suspension), i.e., oil droplets cannot attach on the surface. Superhydrophilic materials can be found in scale of fish and legs of wharf roaches. Superhydrophilic/superhydrophobic-patterned materials have a superhydrophobic background on which superhydrophilic patterns are spatially generated. These materials can be found in rose petals, leaves of clovers and backs of desert beetles. Finally, superoleophobic materials exhibit a static contact angle of over $150^\circ$ and a dynamic contact angle hysteresis of below $5^\circ$ with droplets of organic solvents as well as aqueous liquids (superoleophobicity is also called superamphiphobicity.). Superoleophobic materials can be found in skin of springtails. Inspired by these special wettable interfaces in nature, many researches have been performed to un-derstand the mechanisms of the special wettability and are still on-going to develop valuable technologies such as robust self-cleaning windows, anti-fogging glasses and car mirrors, anti-fouling coating, anti-reflective surfaces, anti-icing coating, two dimensional (2D) bio-assay chips, fog collection, oil/water separation, critical heat flux (CHF)-enhancing coating, and microfluidics. In chapter 1, we demonstrate a one-step, solution-based surface chemistry that modifies superhydro-phobic surfaces. The surface chemistry to enable this innovation is inspired by adhesion mechanisms of ma-rine mussels, and is compatible with the established soft-lithography techniques. By immersing substrates into a solution containing dopamine, a mussel-mimetic adhesive molecule, the superhydrophobic surface is im-mediately transformed into a hydrophilic substrate. By partial exposure of the substrate via a soft-lithographic technique, micromolding in capillaries (MIMIC), the micropatterned surface remain superhydro-phobic but is found to be adhesive to water. Similar to the mechanism of water collection by a desert beetle, Stenocara sp., the modified surface can be used to capture, guide, and collect water droplets. In chapter 2, a new surface-tension-confined droplet microfluidic device, called “polydopamine (pDA) microfluidic system” is demonstrated. Spatially controlled modification of the aluminum anodic oxi-dation (AAO) superhydrophobic surface through mussel-inspired pDA coating generates hydrophilic micro-lines on the surface. On the pD microlines, movement and mixing of droplets are precisely controlled in an energy-efficient manner by gravity. The narrow pDA microlines on the superhydrophobic AAO surfaces result in the minimization of dragging force as well as in no residual traces of solutions. Introduction of a pDA square patch at the intersection of the Y-patterned pDA microlines enables one to control the movement of a droplet, such as stopping and starting movement. This micropatch allows 100 % droplet mixing. The pDA microfluidic device is used to synthesize monodisperse nanoparticles and rapidly induce structural changes of a protein. In chapter 3, methods for general preparation of superhydrophobic surfaces on any type of material surface using mussel-inspired poly(dopamine) (pDA) is demonstrated. The use of pDA presents several ad-vantages over conventional superhydrophobic fabrication methods: development of superhydrophobicity in a material-independent manner, enhancement of mechanical stability, decreases in angle hysteresis, and ap-plicability to 3D objects. The pDA layer plays a critical role in material-independent superhydrophobic mate-rials. Anti-icing properties and mechanical stability are found to be signicantly enhanced in these materials compared to superhydrophobic surfaces without the pDA coating. In chapter 4, We integrate adhesive properties of marine mussels, the lubricating properties of pitcher plants, and the non-fouling properties of diatoms with nanostructured surfaces to develop a device called a micro-omnifluidic ($\mu$-OF) system to solve the existing challenges of conventional microfluidic devices (low portability, disability to be miniaturized, incompatibility with almost all organic solvents, and fouling of bi-omacromolecules). The $\mu$-OF system utilizes gravity to achieve a pump-free device with omniphobic-omniphilic nanostructured patterns that are compatible to most organic solvents and is re-usable by the non-fouling properties of the nature-inspired surfaces. The fabrication strategies for omniphilic microlines, called microchannel-induced slippery liquid-infused porous surface (mi-SLIPS), are inspired by mussel adhesion and pitcher plant lubrication. This $\mu$-OF device is a promising candidate as a new category of fluidic device that is truly portable and energy efficient. In chapter 5, we demonstrate two facile strategies to transfer a transparent superamphiphobic film onto various types of substrates. First, a sacrificial layer on which the superamphiphobic film is grown is etched by etchant solutions, and the floating superamphiphobic film is transferred to other substrates (‘wet transfer’ method). Second, an elastomeric substrate is deposited on the superamphiphobic film, subsequently pressed by a roller, and peeled off. Then, the superamphiphobic film is directly transferred to the elastomeric substrate (‘dry transfer’ method). These transfer methods have a great potent to be utilized in various appli-cations including fabrication of liquid-repelling protective films for electronic display panels and polymeric windows, and anti-contamination by oils.