Many scientists have shown that the beetles have microscopic tricks for catching fog. Their backs are covered in little bumps made from water-attracting substances, with flat, water-repelling valleys between them. The bumps attract moisture and allow droplets to form, while the valleys channel the collected water away. These chemical patterns certainly help, but Park wondered if the bumps themselves, absent any water-attracting substances, might be important.
When he fashioned artificial bumps that match those on the beetle’s back, he found that water droplets would quickly grow on top of them, even if they were coated with water-repelling chemicals. And the smaller and more tightly curved the bumps, the faster the droplets grew. That’s fine, but it creates a problem for a would-be water-collecting surface: The droplets quickly max out the capacity of the bumps and stop growing. Park came up with two solutions.
First, he changed the bumps from spheres into rectangular pillars, with flat tops and curved edges. Now, droplets start condensing on the edges but eventually merge in the central plateau, freeing up the edges for yet more condensation.
Second, Park drew inspiration from cacti, whose spines are also excellent water-harvesters. By adding a ramp to his microscopic pillars, which allows the growing droplet to roll off, he was able to once again free up space for more condensation.
Park also coated the ramp with a material inspired by pitcher plants. These carnivorous plants trap insects in their vase-shaped leaves whose rims are exceptionally slippery; bugs that walk over them lose their footing and fall into the pool of digestive fluids below. In earlier work, Aizenberg developed a material based on the microscopic structure of the pitcher’s traps—an ‘omniphobic’ surface that repels water, ice, blood, crude oil, and even bacteria. She called it SLIPS—Slippery Liquid-Infused Porous Surface—and Park used it to line his ramps.
Together, these features produced a material that collects more than ten times more water than other state-of-the-art surfaces. Droplets form more quickly, reach larger sizes, and roll away more rapidly; they’ll even do so against gravity. All three features—the bumps, the ramps, and the SLIPS—are important; lose any one of them, and the material collects noticeably less water.
SLIPS Technologies, a Cambridge-based company that Aizenberg co-founded, is now trying to commercialize these surfaces. Collecting water in deserts is an obvious application. “In arid environments, there’s a lot of evaporation,” explains Park. “If we can’t collect water with fast growth and transport, we’ll lose it.” He adds that the super-condenser surfaces are also useful for power plants, desalination plants, and other operations whose heat exchangers rely on efficient condensation.