Water striders are a type of insect with the remarkable ability to stand effortlessly and walk quickly on water. This article reports the water repellency mechanism of water strider legs. Scanning electron microscope (SEM) observations reveal the uniquely hierarchical structure on the legs, consisting of numerous oriented needle-shaped microsetae with elaborate nanogrooves. The maximal supporting force of a single leg against water surprisingly reaches up to 152 dynes, about 15 times the total body weight of this insect. We theoretically demonstrate that the cooperation of nanogroove structures on the oriented microsetae, in conjunction with the wax on the leg, renders such water repellency. This finding might be helpful in the design of innovative miniature aquatic devices and nonwetting materials.

Surface effects often play a significant role in the physical properties of micro- and nanosized materials and structures. In this letter, the authors presented a theoretical model directed towards investigation of the effects of both surface elasticity and residual surface tension on the natural frequency of microbeams. A thin surface layer was introduced on the upper and lower surfaces to rationalize the near-surface material properties that are different from the bulk material. An explicit solution is derived for the natural frequency of microbeams with surface effects. This study might be helpful for the design of microbeam-based sensors and some related measurement techniques.

The water strider is an insect that lives on the surface of ponds, slow streams, marshes, and other quiet waters. [1][2][3][4] Trying to understand the physical mechanism behind its ability to float on the water surface has become a very interesting area for bio-inspired research. [5,6] Hu et al. have demonstrated that there are two kinds of upward forces to support the water strider's weight: the curvature force and the buoyancy force. Through theoretical analysis, they have concluded that the curvature force produced by the insect's legs is much larger than the buoyancy force. [7,8] Furthermore, Cheng, [9] Andersen, [10,11] and Gao and Jiang [12] have discovered that the legs of water striders have superhydrophobic coatings that bear hierarchical structures of many oriented tiny hairs with fine nanogrooves. The superhydrophobic coatings were believed to be responsible for the water striders' floating. However, since many other kinds of insects, such as mosquitoes, can float on the water surface with only normal hydrophobic coatings, it is not clear why superhydrophobic coatings are required for the water strider's legs. In the work reported here, using gold threads that are modified with normal hydrophobic or superhydrophobic coatings as model systems, we studied to what extent the hydrophobicity contributes to the gold thread's floating and movement on the water surface. In addition, combining experimental results and theoretical force analysis, we report, for the first time, that deformation of the gold thread contributes to the total supporting force of the water on the floating gold thread. For easily deformed floating objects, such as very thin gold threads or legs of water striders, the contribution of deformation is significant compared to the other components of the supporting force. Our results suggest that the superhydrophobic coating of a water strider's legs can not only provide a larger supporting force, but also help to decrease the insect's total density beneath the water surface and allow it to move faster on the water surface. Therefore the superhydrophobic coating is indispensable, although the normal hydrophobic coating provides enough supporting force for the water strider merely to float. We anticipate that our results provide a deeper understanding of the mechanism of aquatic insects' floating, and hope that it may lead to further research on drag-reducing materials and rapid underwater motion.In an attempt to understand why a surperhydrophobic surface is needed by water striders, we modified gold threads with superhydrophobic coatings (contact angle > 150°) or normally hydrophobic coatings (contact angle ≈ 110°).[13] By comparing these two kinds of gold threads, we expected to obtain some information on how the supporting force is enhanced by superhydrophobicity. Several techniques have been developed to prepare superhydrophobic coatings; [14][15][16][17][18] however, here we needed a new technique to fabricate superhydrophobic coatings on curved surfaces such as gold threads. In our prev...

Based on the conventional Euler buckling model, uniaxial compression tests have been utilized recently to measure the mechanical properties of nanowires. However, owing to the increasing ratio of surface area to bulk at nanoscale, the influence of surface energy becomes prominent and should be taken into consideration. In this letter, an analytical relation is given for the critical force of axial buckling of a nanowire by accounting for both the effects of surface elasticity and residual surface tension. This study might be helpful to characterize the mechanical properties of nanowires or design nanobeam-based devices in a wide range of applications.