Investigation on Hierarchical Micro-structures of Fabric for Superhydrophobicity

Abstract

Fabrication of the superhydrophobic fabric received strong interest from the research community for several reasons such as self-cleaning, separation of water and oil. Traditionally, the most efficient way to render superhydrophobicity was coating fabrics with C-8 perfluorinated materials, but recent environmental and human health hazard issues have raised concerns on these substances. Therefore, non C-8 perfluorinated materials and fluorine-free materials have been considered as substitute coating materials. But these substitute materials generally showed lower hydrophobicity as water contact angle lower than 150°, organizing structures on the fabrics become crucial for improving the hydrophobicity. When the fabrics were structured, increased air gaps between the structures could separate the liquid from the solid surface more efficiently. In this study, methods of applying superhydrophobicity through padding procedure to a generic plain polyester fabric surface and improving the superhydrophobicity by modifying the hierarchical structure of the fabric through changing the fabric density and yarn cross-section, which are the basic weaving condition, are investigated. For circular-yarn fabric, when superhydrophobic coating was applied with commonly used non C-8 perfluorinated coating material, a water drop rolled-off immediately, regardless of the fabric density. And the static contact angle of 5 µL dodecane (DCA) increased from 71° to 90° when the fabric density was increased from low to high. In case of the crucified-yarn fabric, which was coated identically as the circular-yarn fabric, the water drop also rolled-off immediately. And the DCA increased from 76° to 92° when the fabric density was increased from low to high. To find out the cause of the obtained results, the fabric aspect ratio was set as a variable for each fabric density and calculated. It was found out that the fabric aspect ratio increased as the fabric density increased, which implies that each fabric unit cell became bulgier upward, affecting the wettability of the fabric and that the fabric density is equal when DCA of the crucified-yarn fabric was higher than the DCA of the circular-yarn fabrics. It was expected that this result was from the difference between the characteristic of yarn cross-section because crucified feature is the rougher shape than the circular feature. It was consequently proven by comparing upper roughness of each fabric unit cell and that the roughness of the crucified-yarn fabric turned out approximately 1.2 times higher than the circular-yarn fabric, showing the importance of micro-structures.