Abstract
Water-walking insects such as water striders can skate on the water surface easily with the help of the hierarchical structure on legs. Numerous theoretical and experimental studies show that the hierarchical structure would help water strider in quasi-static case such as load-bearing capacity. However, the advantage of the hierarchical structure in the dynamic stage has not been reported yet. In this paper, the function of super hydrophobicity and the hierarchical structure was investigated by measuring the adhesion force of legs departing from the water surface at different lifting speed by a dynamic force sensor. The results show that the adhesion force decreased with the increase of lifting speed from 0.02 m/s to 0.4 m/s, whose mechanic is investigated by Energy analysis. In addition, it can be found that the needle shape setae on water strider leg can help them depart from water surface easily. Thus, it can serve as a starting point to understand how the hierarchical structure on the legs help water-walking insects to jump upward rapidly to avoid preying by other insects.
Highlights
Many estimates and experiments show that the adhesion force and the energy required to depart a leg from water surface decrease dramatically as the contact angle approaches 180.4 The adhesion force of water strider legs was measured by Pal Jen Wei, which was found to be 2 dyn.[5]
A high-resolution Scanning electron microscope (SEM) image shows that setae are long and softly serrated, and they are about 100 μm in length and less than 5 μm in diameter in the root and 0.2 μm in diameter in the apex
The function of super hydrophobicity and the hierarchical structure was investigated by measuring the adhesion force of legs when departing from the water surface at different speeds
Summary
Super hydrophobicity may make it very easy to detach legs from the surface, as insects need to jump and skate on the surface. Many estimates and experiments show that the adhesion force and the energy required to depart a leg from water surface decrease dramatically as the contact angle approaches 180.4 The adhesion force (pull-off) of water strider legs was measured by Pal Jen Wei, which was found to be 2 dyn.[5] Keh-chih Hwang stated that nano to micro structural hierarchy is crucial for stable super hydrophobic and water-repellent surface and the second level structure results in dramatic reduction in the contact area and minimizing adhesion between water and the solid surface.[6] Keh-chih Hwang established a 2D model to analyze the process of legs that detach from the water surface. The model shows that the super hydrophobicity of the legs’ surface is critically important for reducing the detaching force and detaching energy.[7]
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