Mechanisms of reversible underwater adhesion in ladybug beetles
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Abstract
Many animals are known to possess the remarkable ability to stick, walk or climb on any surface. While some of the bigger animals such as monkeys or cats use their limbs or claws to mechanically grip to available protrusions on a vertical surface to climb up, some smaller animals, such as lizards and insects, can also climb on smooth surfaces where such a gripping mechanism should not be possible. On closer inspection, one would find that these animals possess specialised organs at the bottom of their feet, known as adhesive pads. The adhesive pads have naturally evolved into various forms depending on the animal: some have a dense array of hair-like structures, some are smooth and flexible, while some also secrete an adhesive fluid at the bottom. A curious property of these pads, which is not fully understood, is their ability to control adhesion instantly and achieve fast attachment and detachment as per necessity. Some animals such as ladybugs and geckos, which possess ‘hairy’ pads, can also surprisingly walk on underwater surfaces. Underwater adhesion is usually difficult to achieve due to the presence of interfacial water and thus, it is not entirely clear how these animals are able to accomplish this. In the present work, I attempt to resolve these mysteries by using a combination of experiments on live insects and numerical simulations. I show that, the surface tension forces due to the foot's secretions is the the primary driver of the ladybug beetle's adhesion to surfaces. Here, the balance of the different interfacial energies made by the secretion fluid with the surrounding medium is found to create the right wetting conditions for the fluid to show strong capillary forces, even when the insect's foot is submerged underwater. In order to characterise these secretions, which are of femtolitre scale volume, I develop a general method to perform surface tension measurements on microscopic liquid droplets with the help of Atomic Force Microscopy. I further show, based on a simple theoretical model, that a ‘hairy’ pad design of the insect's foot is not only useful to improve contact with most surfaces, but can also help control its adhesion by simply tilting the foot relative to the surface. My findings here highlight some new strategies through which underwater adhesion can be achieved and controlled, which could potentially inspire the design of artificial reversible adhesives.