Proposal of a Water-repellency Model of Water Strider and Its Verification by Considering Directly Measured Strider Leg-rowing Force

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Abstract
Nano- and micro-structures can be realized by using photofabrication, crystal growth, chemical deposition, etching and other methods. This variety of fabrication methods encourages the development of water-repellent surfaces from the viewpoint of biomimetics. Water striders have attracted interest in biomimetics because their water-repellency property makes it possible for them to live on water surfaces. Micro-hairs, which are present on the surface of the water strider’s legs, can maintain an air layer between the water surface and the legs, providing the repellency property. While there have been various studies which have considered a physical model of the water strider’s water-repellency property, no model has taken directly measured water strider leg-rowing force into account. Therefore, in this study, we proposed a physical model of a water strider using Laplace pressure, and then we considered directly measured leg-rowing force to verify the model. First, we considered the relationship between water pressure around the micro-hairs and the intersection of the surface of the micro-hairs and the water surface using our proposed model. We found that when the micro-hairs were in contact with the water surface and the air layer was maintained, the maximum Laplace pressure Pmax was 35.2 kPa. This meant that if Laplace pressure exceeded 35.2 kPa, the water pressure caused by the rowing motion of the strider legs pushed water into the space between micro-hairs. Additionally, we calculated maximum water pressure PL which was loaded around the surface of the water strider's leg by rowing of the leg. PL was 546 Pa, and this value was significantly smaller than Pmax. This meant that the water pressure did not push water into the space between the leg micro-hairs and the water strider’s legs maintained their water repellency when moving.

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