It started when Jolanta Watson put a frozen box-patterned gecko on a glass slide. The lizard’s skin is adorned with beautiful auburn and tan blotches, and Watson wanted to study it under a microscope. But as she reached for a scalpel, she noticed that tiny water droplets had formed on the slide. The longer she looked, the more droplets there were. Where were they coming from?
The microscope revealed the answer. Through its lens, Watson saw that droplets would condense on the gecko’s skin, roll into each other, and jump off under their own power. That’s why the slide was wet. The box-patterned gecko’s skin can actively repel water even if it’s dead and immobile. And when it’s alive, it can use this phenomenon, which Watson calls “geckovescence”, to clean itself with no effort.
High-speed footage slowed down 13x shows dewdrops being propelled off a gecko’s skin, a phenomenon that may help keep bacteria and fungi at bay. Credit: Dr. Gregory Watson
There are some 1,500 species of geckos, which are best known for their sticky feet. Their toes are covered in thousands of microscopic hairs that allow them to cling to seemingly flat surfaces—including the walls of Watson’s Australian home. As she and her husband Gregory watched these lizards, they realised that scientists had largely ignored the rest of the gecko’s body. Their toes were cool, but what about the rest of their skin? In particular, how does it deal with water?
The box-patterned gecko lives in the Australian desert, where rainfall is rare and water is scarce. Still, chilly nights and humid mornings can produce a lot of dew, some of which condenses on the gecko’s skin. That’s a problem: water-logged skin is a breeding ground for microbes and fungi, which could potentially cause diseases.
Fortunately, as the Watsons found, the gecko can automatically dry itself. When they looked at the lizard’s skin under the microscope, they saw that its scales are like rounded domes. Each of these is covered in miniscule hairs, just a few millionths of a metre long, about the size of a small bacterium. They’re densely packed too: thousands of them would fit in the cross-section of a single human hair.
Many natural structures, including springtails, leafhoppers, lotus leaves, and guillemot eggs, use similar microscopic textures to waterproof themselves. The principles are always the same: there are raised sections, like the gecko’s hairs, that trap pockets of air and stop water from seeping into the spaces between them. When droplets form, they sit on top of the raised bits as nigh-perfect spheres, rather than flattening out as they would do on a tabletop or on your skin.
The Watsons saw exactly this when they cooled gecko skin to the point when dew started to condense. Spherical droplets appeared, and grew. When they touched each other, they merged. And when they merged, they would occasionally fly off. Why? Because when two droplets unite, their volume stays the same but their combined surface area—and thus, their surface energy—goes down. They convert some of that surface energy into kinetic energy, and if the trade-off is substantial enough, they can launch themselves into the air.
All of this happens without help from any external forces, but external forces can help. In fog, water droplets in the air collide with those on the gecko’s skin, increasing the odds that they will jump off. Here’s a series of images showing one such jump. Wind helps too; it blows droplets into each other, and carries the airborne drops away from the lizard.
All of this makes for effortless auto-cleaning skin. As the droplets form and merge, they carry dirt, spores, and other foreign material with them. When they leap away, they remove those contaminants from the gecko. Other animals probably use a similar trick, including a type of cicada that the Watsons studied a few years ago.
“There are a number of potential practical applications,” says Watson. “Keeping surfaces clean and free from dew or other small droplets may reduce the growth of bacteria and fungi. We are currently investigating a number of properties on replicated gecko skin architecture.”
Reference: Watson, Schwarzkopf, Cribb, Myhra, Gellender & Watson. 2015. Removal mechanisms of dew via
self-propulsion off the gecko skin. Interface http://dx.doi.org/10.1098/rsif.2014.1396