Study of superhydrophobicity according to surface structure of knitted fabrics

This study seeks to analyze the effect of geometric structures of weft-knitted fabrics on superhydrophobicity and the dynamic behavior of water droplets. A flat knitting machine with different stitch patterns was used to prepare 100% polyester knitted fabrics. For the superhydrophobic surface, nano-roughness through alkaline treatment and a hydrophobic coating were introduced on prepared knitted fabrics. To analyze micro-roughness, pore size, cover factor, surface roughness, and air permeability were measured. Surface wettability was evaluated by contact and shedding angle measurements, and the dynamic behavior of droplets. Micro-roughness was greater in the order of tuck, purl, and plain jersey stitch patterns with a small cover factor and large pore size. In addition, tuck and purl stitches showed differences in surface roughness according to the wale and course directions. Nano-roughness was discernible as the alkaline treatment time increased. Following an evaluation of the wettability, the purl stitches exhibited a contact angle of 150° or more with only the hydrophobic coating. After imparting nano-roughness by alkaline treatment, the contact angle was more than 150° in all the samples. In the case of shedding angle, the tuck and purl stitches showed differences according to the course and wale directions. The shedding angle was lower when the roughness was high and the ridge and the droplet sliding directions were parallel. This difference decreased as the nano-roughness increased according to the alkaline treatment time. An evaluation of the dynamic behavior of water droplets on the superhydrophobic knitted fabric showed that rebound behavior appeared in all the samples on the horizontal surface, when the water droplet was small. However, with large droplets, the rebound behavior appeared only in purl stitches. Meanwhile, on the surface inclined at 15°, rebound behavior was observed in the tuck and purl stitches, with the tuck stitches rebounding faster in the wale direction and the purl stitches in the course direction regardless of the droplet volume. The plain jersey stitches showed pinning behavior after water droplets fell on the surface. Therefore, it is important not only to introduce nano-roughness but also properly to form geometrical micro-roughness of knitted fabric with pores and loops to induce rebound behavior of water droplets.