Droplet Microfluidics for Robust Omniphobic Surfaces

Abstract

Liquid-repellent surfaces dislike liquids with the apparent contact angles larger than 90°. Such functional materials may have tremendous potential applications such as self-cleaning, chemical shielding, non-fouling, water-oil separation, etc [1]. Transparency could further expand the range of applications, such as coatings for windows, display screens, and solar panels [1]. Surface chemistry and surface structure are the two primary factors responsible for liquid repellency. Solid materials with lower surface energy tend to support liquid droplets at higher contact angles and surface roughness largely reduces the solid-liquid contact area for better performance in liquid repellency. To obtain well-controlled wetting property, regular surface structure and homogeneous chemical composition are highly appreciated, which imposes demanding requirements in the fabrication method for liquid-repellent materials. Generally, the fabrication techniques are categorized into top-down and bottom-up methods [2]. Top-down processes, such as lithography, enable the fabrication of well-defined surface morphology, but expensive equipment and laborious fabrication processes are usually involved, preventing scale-up of surface manufacturing for real-world applications. In contrast, bottom-up approaches (for example electrodeposition, electrospinning, spin-coating, spray coating, sol-gel synthesis and template-assisted synthesis) are facile, cost-effective, and scalable, but they produce random surface structures, jeopardizing the uniformity in liquid repellency. Therefore, very few methods are facile, cost-effective, scalable, and able to produce well-defined surface structures. Here, we report a bottom-up microfluidic emulsion templating method for scalable fabrication of transparent liquid-repellent surfaces. The stand process involves four stages: emulsion generation, emulsion deposition, solvent evaporation, and template removal. Firstly, uniform o/w emulsions (oil dispersed in an aqueous polymer solution) are produced using droplet microfluidic technique. The emulsion is then deposited onto a substrate. Within several minutes, the dispersed droplets self-assemble into hexagonally packed arrays. After water evaporation in the aqueous solution, the polymer solidifies into a membrane embedded with oil droplets. By removing the droplet templates, the membrane is decorated with hexagonally packed micro-cavities. Finally, the polymer surface is cross-linked to improve water resistance. As a proof of concept, a transparent polymer, PVA (poly(vinyl alcohol)), is used for the fabrication of surface with good transparency. Depending on the dimension of the structure, the optical transmission can be up to 85% at the wavelength of 400-800 nm. The resultant surface repels both water and low-surface-tension oils, attributed to the re-entrant profile [3] on the side-wall of micro-cavities. The surface displays superhydrophobicity (contact angle > 150° and sliding angle < 5°) after coating of fluorinated silica nanoparticles, while the transparency is barely affected. REFERENCES: [1] Liu, M., Wang, S. & Jiang, L. 'Nature-inspired superwettability systems,' Nat. Rev. Mater. 2017, 2, 17036. [2] Chu, Z. & Seeger, S. 'Superamphiphobic surfaces,' Chem. Soc. Rev. 2014, 43, 2784-2798. [3] Tuteja, A., Choi, W., Mabry, J. M., McKinley, G. H. & Cohen, R. E. 'Robust omniphobic surfaces,' Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 18200-18205.