Dynamic Contact Model of Bio-Adhesive Pads of Animals: Simulation Experiments

Abstract

Many insects such as ants, bugs, and animals such as tree frogs, are proved able to run up and down on smooth vertical surfaces using their specialized pads via a wet adhesive mechanism. Few studies have investigated the dynamic contact between animal pads and smooth surfaces, which involves not only the initial approaching and adhesion process, but more importantly the rapidly releasing process. By a simulation experiment on fiber cells of tree frog pads, we found that a micrometer-sized bio-adhesive cell can provide a ∼mN adhesive force when a ∼nm thick liquid film is confined in the contact area. Unstable spreading and shrinking of the liquid film, which correspond to sharp changes of adhesive force (called the spreading adhesive force and the shrinking releasing force in this article) are observed, which occurs only when the volume of confined liquids falls into a critical range and the contact stiffness is very low. To move fast and effortlessly on a smooth surface, insects may optimize the releasing process by utilizing the unstable shrinking process of the confined liquid thin film. To ‘trigger’ this, the insects must be able to reduce the fiber cell stiffness or to decrease the confined liquid volume.