||Abstract Water striders, Aqyarius elongatus, are insects living on the water surface like ponds, slow streams, marshes, and other quiet waters, and their body range typically 1-2 cm in length and 10-50 dynes in weight. Their weights are supported by the surface tension force and they can move over the water surface very quickly by their hydrophobic elongated legs and rarely get wet. The recent investigate inferred that the mechanism of water strider motion on water surface relies on the vortices transferring momentum backward. The assumption describes that the moving hemispherical vortices carry the momentum of walking water strider approximately. In our experiments we change the depth of water from deep to thin to influence the hemispherical vortex shape. However the results show that the average distance for one stroke of water strider doesn’t vary apparently and the vortices become smaller and almost stationary at shallow water as well as. Therefore we think the hydrodynamic propulsion of water strider locomotion does not directly rely on the momentum transfer by the vortices shed. Through theoretical calculation, we show that the shear flow during the water strider stroking on the water surface provides enough viscous shear stress to drive the water strider moving forward. Furthermore the depth of shear flow is just 0.2 mm, and that explains our results, the water striders move unrestrictedly even the water depth decreases to 0.5 mm. After water strider stroking, the shear flow develops into the vortices in deep water, however at shallow water the momentum transfer to the bottom of tank to cause the vortices small and staying behind nearly. This case shows that the viscous force dominate the locomotion of an aquatic creature even the Reynolds number is thousands.|
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