Fabrication of Flexible and Re-entrant Liquid-Superrepellent Surface Using Proximity and PNIPAM-Assisted Soft Lithography

The nature-inspired flexible and re-entrant liquid-superrepellent surface has attracted significant attention due to its excellent superomniphobic performance against low-surface-tension liquids. Although conventional photolithography and molding methods offer the advantage of large-area manufacturing, they often involve multiple double-sided alignment and exposure steps, resulting in complex procedures with long processing cycles. In this study, we proposed a straightforward single-exposure ultraviolet proximity lithography method for re-entrant liquid-superrepellent surface fabrication using a photomask with a coaxial circular aperture and ring. A theoretical calculation model for the three-dimensional light intensity distribution in proximity lithography was developed for the prediction of feature sizes for both singly and doubly re-entrant microstructures. Soft lithography techniques, which rely on surface modification and the modulation of the transfer material's flexibility, efficiently optimized the fabrication of flexible re-entrant molds and patterns. By incorporating nanoclay-modified poly(<i>N</i>-isopropylacrylamide) (PNIPAM) into the molding process, we fabricated a three-layer hierarchical structure featuring micrometer-scale wrinkles, re-entrant microstructures, and nanoscale fluorinated silica particles, significantly enhancing the surface's robustness and pressure resistance. The resulting large-area flexible and re-entrant liquid-superrepellent surface demonstrated excellent superomniphobic self-cleaning performance and satisfactory optical transparency, as evidenced by reflection and transmission experiments, showcasing its potential applications in self-cleaning, membrane distillation, and digital microfluidics.