Anyone who has seen the gecko will probably know they can climb the walls. But these common lizards can also pass through the water almost as fast as they can move to a solid foundation. However, although we know how smooth the surfaces of smooth vertical surfaces use countless tiny hairs on the feet called the setae, as they manage to avoid sinking into the water, there has so far been some mystery. My colleagues and I have recently completed research that explains how geckos uses a combination of techniques to perform this incredible podvig.
The ability to walk on water is recorded in smaller animals such as water strides, which are light enough to support the surface tension of water, the force between the water molecule on the surface. Meanwhile, bigger animals like a crawfish can walk on water because they are strong enough to slap the surface while running. Fast motion compresses the water beneath your feet and creates a pocket of air around it. The upward pull that is created when the pocket is pushed down under the water is what keeps the lizard suspended briefly on the surface.
But geckos are usually sizes that merge between these two categories. They are too weak to be maintained by surface slamming themselves and too heavy to not leave the water surface uninterrupted. However, their relative velocity of water velocity approaches those of the other known lizards, basilicas (or "Jesus lizards") relying on the technique of hitting.
Initial calculations have signaled and confirmed video analysis, as unlike other types that move on the surface of the water, the gecko uses a combination of faster-moving techniques than can swim with it. By analyzing the gecko video that is moving across the water, we discovered that their walk was similar to that of the basilica. Each step involves dragging the leg through the air, squirting the surface and kissing under the water.
However, unlike the basilica, which is not affected by changes in surface tension of water, our experiments have shown that the speed and head height are cut by half when adding detergent to water, reducing surface tension. This suggests that at least some of the forces are used between the water molecules to remain above the surface.
We also found that geckos uses the combination of hydrostatic force (raising water known as bushing) and hydrodynamic force (the lift created by starting over the surface of the water as in a motorboat's surface). Together, these forces create an additional lift for the gecko, a condition known as semi-planning.
Sting in the tail
For all the ingeniousness of this multi-tasking approach, the gecko can only hold the head and torso completely above the water, leaving its tails pulling below. Being able to move almost as fast as on land when almost half of your body is under water and faced with more resistance and pulling force is a big deal – just ask Michaela Phelps.
Geckos manages this with the tail, which has already shown that it helps them maneuver around obstacles, jump and escape the predator. Viewed from above while traveling over the water, the gecko can look like a crocodile, moving the body and the beak by swirling to create a backward balance drive by pulling the water.
Our research shows that for medium-sized animals, rapid motion on the surface of water requires a complex and clever combination of physical mechanisms that previously thought to appear only in larger and smaller animals. But it could also feed on a better design for animal-inspired robots.
Previous gecko research has inspired several such "biomimetic" endeavors, from better glue to the twisted (and pretty well-liked) tail of the robot, called Tailbot. Better understanding how animals are traveling on complex terrain, we hope to bring robots who can use these techniques to move on both the ground and high performance waters seen in geckos.