Effects of Engineered Wettability on the Efficiency of Dew Collection

Surface wettability plays an important role in dew collection. Nucleation is faster on hydrophilic surfaces, while droplets slide more readily on hydrophobic surfaces. Plants and animals in coastal desert environments appear to overcome this trade-off through biphilic surfaces with patterned wettability. In this study, we investigate the effects of millimeter-scale wettability patterns, mimicking those of the Stenocara beetle, on the rate of water collection from humid air. The rate of water collection per unit area is measured as a function of subcooling (Delta T = 1, 7, and 27 degrees C) and angle of inclination (from 10 to 90). It is then compared for superbiphilic, hydrophilic, hydrophobic, and surperhydrophobic surfaces. For large subcooling, neither wettability nor tilt angle has a significant effect because the rate of condensation is so great. For 1 degrees C subcooling and large angles, hydrophilic surfaces perform best because condensation is the rate-limiting step. For low angles of inclination, superhydrophobic samples are best because droplet sliding is the rate-limiting step. Superbiphilic surfaces, in contrast to their superior fog collecting capabilities, generally collected dew at the slowest rate due to their inherent contact angle hysteresis. Theoretical considerations suggest that this finding may apply more generally to surfaces with patterned wettability.