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Abstract
The construction of stable hydrophobic surfaces has increasingly gained attention owing to its wide range of potential applications. However, these surfaces may become wet and lose their slip effect owing to insufficient hydrophobic stability. Pillars with a mushroom-shaped tip are believed to enhance hydrophobicity stability. This work presents a facile method of manufacturing mushroom-shaped structures, where, compared with the previously used method, the modulation of the cap thickness, cap diameter, and stem height of the structures is more convenient. The effects of the development time on the cap diameter and overhanging angle are investigated and well-defined mushroom-shaped structures are demonstrated. The effect of the microstructure geometry on the contact state of a droplet is predicted by taking an energy minimization approach and is experimentally validated with nonvolatile ultraviolet-curable polymer with a low surface tension by inspecting the profiles of liquid–vapor interface deformation and tracking the trace of the receding contact line after exposure to ultraviolet light. Theoretical and experimental results show that, compared with regular pillar arrays having a vertical sidewall, the mushroom-like structures can effectively enhance hydrophobic stability. The proposed manufacturing method will be useful for fabricating robust hydrophobic surfaces in a cost-effective and convenient manner.
Highlights
Hydrophobic/superhydrophobic surfaces have many attractive practical applications, which include micro/nanofluidics, drag reduction,[1,2] self-cleaning windows,[3] water-proofing cloths and textiles,[4] and antibiofouling surfaces.[5,6] The repellency of hydrophobic surfaces is mainly ascribed to the lubricating gas films generated at the interface between the solid substrate and the liquid, which results in a shear-free liquid-air area and dramatically reduces the interaction at the solid–liquid interface.[7]
When the PDMS film with mushroom-shaped patterns on its surface overlapped the photoresist master, clearly visible interference fringes were seen as shown in Fig. 2(b), which indicates the good uniformity of the fabricated microstructures in a large area
By controlling the development time, the main geometric parameters driving the wettability of the fabricated patterns can be modulated conveniently
Summary
Hydrophobic/superhydrophobic surfaces have many attractive practical applications, which include micro/nanofluidics, drag reduction,[1,2] self-cleaning windows,[3] water-proofing cloths and textiles,[4] and antibiofouling surfaces.[5,6] The repellency of hydrophobic surfaces is mainly ascribed to the lubricating gas films generated at the interface between the solid substrate and the liquid, which results in a shear-free liquid-air area and dramatically reduces the interaction at the solid–liquid interface.[7]. Using positive- and negative-tone photoresist materials, Sameoto et al.[16] fabricated pillar arrays with mushroom-shaped tips employing photolithography and a molding process, where the unexposed negative-tone photoresist was used to generate undercut microholes. The limitation of this process is that it is necessary to precisely control the exposure dose to avoid the underneath negative-tone photoresist being exposed. Wang et al.[18] introduced a simpler method for fabricating mushroom-shaped microstructures, where, by performing masked and unmasked exposures, microholes with a bottom undercut are generated and pillar arrays with mushroom-shaped tips are obtained after molding. The hydrophobicity robustness of the prepared surfaces is evaluated theoretically by taking the energy minimization approach and verified experimentally using UV-curable polymer drops with low surface tension by inspecting the profiles of liquid–vapor interface deformation and tracking the trailing of the receding contact line after curing with UV exposure
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