Geckos have attracted attention for their sticky-but-dry toe-pads, capable of clinging to smooth surfaces in the air or under water. But who would have guessed that land-dwelling beetles are also remarkably good at sticking underwater? Terrestrial beetles' feet are a model of excellent wet adhesion, using a combination of tiny projections, called setae, and oily secretions to adhere to dry surfaces via capillary action. Naoe Hosoda of Japan's National Institute for Materials Science, and Stanislav Gorb of the University of Kiel, Germany, were curious as to how land-dwelling beetles might also be able to adhere to submerged surfaces, such as leaves after a heavy rain.
One of the main challenges to oil-based underwater adhesion is that water itself is sticky and once a structure has got wet, there is no air to generate surface tension at the oil–surface interface. Hosoda and Gorb thought that leaf beetles' setae might trap air bubbles around their feet, effectively keeping them dry and maintaining the ability of oily setae to stick to a submerged surface.
To see just how well beetle feet could adhere when underwater, the authors tethered specimens of Gastrophysa viridula, the green dock beetle, to small force transducers and submerged them in a tank. As the beetles crawled underwater, the authors observed both the force with which the beetles could pull and the appearance of bubbles beneath the beetles' feet.
Microscopic observation revealed structured air bubbles trapped around the beetles' setae and the smooth substrate. These bubbles moved with the foot while the beetle walked underwater. Hosoda and Gorb then added surfactant to the water to prevent bubbles from forming. With no bubble to hold them down, the leaf beetles floated to the surface, demonstrating that the bubble was essential for underwater adhesion.
Inspired by the beetles' adhesion, Hosoda and Gorb built a underwater adhesive surface, inspired by the beetle's sticky feet, covered with pillar-like outgrowths resembling the beetles' setae. Using the manufactured surface, the authors demonstrated that outgrowths are necessary to trap the air that supports bubble adhesion. Furthermore, the structured material required a substantial pull-off force to be separated from the submerged surface. In fact, the bio-inspired polymer's attachment to submerged hydrophobic surfaces was as good as its adhesion to the same surfaces in air. Though the size of this adhesive force changed depending on the contact angle of the water meeting the foot, and the properties of the substrate, the beetle-mimicking structure generated at least some adhesion under many different conditions.
The authors are hopeful that the beetle's approach to underwater adhesion may lead to new bubble-based bio-inspired adhesives. Indeed, the authors' own biomimetic adhesive shows promise for engineering applications. Terrestrial leaf beetles may have a lot more to teach us about being sticky when wet.