But now, the Caltech team has combined both of these functions into one device. The key is a new hydrogel membrane with a very specific nanoscale pattern etched into it. The surface contains an array of tiny structures modeled after the spines of cacti, made of a hydrophilic material that attracts water.
“Cacti are uniquely adapted to survive dry climates,” says Ye Shi, co-author of the study. “In our case, these spines, which we call ‘micro-trees,’ attract microscopic droplets of water that are suspended in the air, allowing them to slide down the base of the spine and coalesce with other droplets into relatively heavy drops that eventually converge into a reservoir of water that can be utilized.”
When a membrane of this hydrogel is placed in a box, it can get to work harvesting drinking water. During the day, it absorbs heat from sunlight, which heats up dirty water underneath the membrane. The steam then collects on a transparent cover and runs into a reservoir. At night, this cover can be removed to expose the membrane to fog from outside.
The team tested the system using samples of the material ranging between 55 and 125 cm2 (8.5 and 19.4 in2). They found that during the day the material could collect about 125 ml (4.2 oz) of water from solar steam, and about 35 ml (1.2 oz) from fog overnight. That doesn’t sound like much, but the researchers reckon the eventual daily yield could be up to 34 L (9 gallons) per m2 (10.8 ft2) of the material.
The research was published in the journal Nature Communications.