Water-Repellency Model of the Water Strider, Aquarius paludum paludum, by the Curved Structure of Leg Micro-Hairs

Abstract

Water-repellent surfaces that mimic various water-repellent surfaces of living things have been developed using photolithography technology. The water-repellency of water striders in particular has gotten a lot of attention. Micro-hairs on their legs are tilted and curved at the tip. These curving micro-hairs contribute to water-repellency properties and various reports have considered the property origins. However, the water-repellency function produced by the curved micro-hairs has not been compared with the dynamic situation (when water striders are rowing their legs), that is when there is a directly measured leg-rowing force. Therefore here, first we discussed water-repellent properties using the relationship between Laplace pressure and water pressure caused by rowing of the insect legs. In order to obtain the Laplace pressure when affected by the curved micro-hairs, a water-repellency model was proposed. To construct this new model, we measured the morphological information of micro-hairs on the tip of the middle leg (tarsus and pretarsus) and fit the curved properties of the micro-hairs to a log function. Then, the water-repellency model was constructed with the fitting curve. As a result, we derived the maximum value of Laplace pressure ΔP as 3.4 kPa. This maximum value was sufficiently larger than the water pressure (546 Pa) caused by the rowing motion of the water strider’s middle leg. Thus, we concluded that the water-repellent function due to the curved micro-hairs worked when the water strider was rowing its legs.