Adaptive adhesion by a beetle: manipulation of liquid bridges and their breaking limits

A drop brought into contact with a nearby substrate can wet and spread against the substrate, forming a liquid bridge that exerts a capillary force. This force due to surface tension can be used to "grab" the substrate, pulling it toward the drop. "Wet" adhesion results from the parallel action of an array of small liquid bridges. The Florida palm beetle, Hemisphaerota cyanea, uses wet adhesion to defend itself against attacking predators by adhering to the palm leaf using an array of about 120,000 μm-sized liquid bridges. The beetle's survival depends on the strength of adhesion which, in turn, depends on how liquid bridges break. Individual bridges break when they go unstable, according to their response curves. However, the ultimate strength of an individual bridge depends on the class of disturbances to which it is subjected, and it has been speculated that the beetle may have some control over this class. The authors experimentally study families of liquid bridge equilibria for their breaking limits when subjected to constant-length (L) and constant-force (F) disturbances. While to control constant-L disturbances is straightforward, to apply and control constant-F disturbances on a liquid bridge requires more ingenuity. The authors introduce an apparatus with a lever-arm and a ball-bearing slide. The authors then compare our experimentally measured bridge response curves to the force trace from experiments on the beetle (prior literature) to infer the mode of beetle detachment. Under normal loads, the beetle detaches as a constant-L instability for smaller loads and as a constant-F instability for larger loads. The beetle's ability to adjust the type and magnitude of loading in real time is not only crucial to its survival but has implications for the design of various engineering devices.