Harvesting of aerial humidity with natural hygroscopic salt excretions

Abstract

Plants and animals that thrive in arid regions utilize the diurnal changes in environmental temperature and humidity to optimize their water budget by combining water-harvesting mechanisms and morphophysiological traits. The Athel tamarisk ( Tamarix aphylla ) is a halophytic desert shrub that survives in arid, hypersaline conditions by excreting concentrated solutions of ions as droplets on its surface that crystallize into salt crystals and fall off the branches. Here, we describe the crystallization on the surface of the plant and explore the effects of external conditions such as diurnal changes in humidity and temperature. The salt mixtures contain at least ten common minerals, with NaCl and CaSO 4 ·2H 2 O being the major products, SiO 2 and CaCO 3 main sand contaminants, and Li 2 SO 4 , CaSO 4 , KCl, K 2 Ca(SO 4 ) 2 ·H 2 O, CaMg(CO 3 ) 2 and AlNaSi 3 O 8 present in smaller amounts. In natural conditions, the hanging or sitting droplets remain firmly attached to the surface, with an average adhesion force of 275 ± 3.5 µN measured for pure water. Rather than using morphological features of the surface, the droplets adhere by chemical interactions, predominantly by hydrogen bonding. Increasing ion concentration slightly increases the contact angle on the hydrophobic cuticle, thereby lowering surface wettability. Small amounts of lithium sulfate and possibly other hygroscopic salts result in strong hygroscopicity and propensity for deliquescence of the salt mixture overnight. Within a broader context, this natural mechanism for humidity harvesting that uses environmentally benign salts as moisture adsorbents could provide a bioinspired approach that complements the currently available water collection or cloud-seeding technologies.