Dynamic Contact Model Based on Meniscus Adhesion For Wet Bio-Adhesive Pads: Simulation Experiments

Many insects, such as flies, ants, and bugs, and some animals, such as tree frogs, have proven able to run up and down on smooth vertical surfaces using their specialized pads via a wet adhesive mechanism. Although many meniscus adhesion models were proposed to reveal the origin of the interfacial force, few studies have investigated the dynamic contact between this kind of bio-adhesive pad and smooth surfaces, which involves not only the initial approaching and adhesion process but more importantly the rapidly releasing process. By a simulation experiment, we found that a micrometer-sized seta of a pad can provide a mNs adhesive force when a similar to nm thick liquid film is confined in the contact area. Furthermore, based on the observations on the liquid film confined in the contact zone, unstable spreading and shrinking behaviors of the liquid film are found when the volume of confined liquids falls into a critical range and the contact stiffness is very low. The unstable shrinking of the liquid film corresponds to a sharp reduction of adhesive force between the seta and substrate, which are considered as the desired releasing force for a seta to detach from substrate. Experimental observations and analysis allow the conclusion that, to move quickly 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 seta stiffness or decrease the confined liquid volume.