Fog harvesting behavior on alternating vertical hydrophobic-hydrophilic stripes

Abstract

Surfaces with contrasting wettability, such as alternating hydrophobic (Ho) and hydrophilic (Hi) vertical stripes patterns, often exhibit better fog harvesting performance than those with uniform wettability. However, conflicting findings regarding its effectiveness exist in the literature. This article studied how stripe width affects droplet removal and whether certain width combinations can enhance or reduce the water collection rate (WCR) compared to hydrophobic surfaces. We tested three non-patterned and seven patterned samples, which were fabricated using a combination of masking, spray-coating, and thermal treatment. The hydrophilic stripes determine the droplet transport effectiveness and drainage efficiency. We observed that the Hi stripe with a 0.4 mm width showed poor droplet transport. On the contrary, wider widths have better droplet transport at the cost of reduced drainage efficiency. The hydrophobic stripes influenced the droplet transfer mechanism, where, for wide stripes, droplets can slightly move downward before merging with the fluidic channel. Interestingly, when the Ho stripe is 1 mm, the maximum droplet radius can be decreased, potentially enhancing the droplet transfer rate. Furthermore, the combination of Ho-Hi stripes with higher droplet transfer edges generally provides higher WCRs. The greatest enhancements were seen from Ho2-Hi1, with WCR improvements of 1.37× and 1.56× over untreated and oxidized copper, respectively. These values exceeded the uniformly hydrophobic surfaces, which showed smaller improvements of 1.24× over untreated and 1.41× over oxidized copper. In contrast, poor combinations like Ho3-Hi3 only resulted in a slight WCR enhancement of 1.05× and 1.19× compared to untreated and oxidized copper.

Keywords

• Surface wettability contrast
• Fog harvesting
• Droplet dynamics
• Vertical stripes-patterned