Meniscus-climbing behavior and its minimum free-energy mechanism.

Some insects can climb up the top of the meniscus surface generated by a hydrophilic wall by fixing their posture without moving their appendages [Baudoin, R. Bull. Biol. Fr. Belg. 1955, 89, 16. Hu, D. L.; Bush, J. W. M. Nature 2005, 437, 733]. To better understand this interesting phenomenon, we did meniscus-climbing experiments of bent copper sheets. It was found that the sheets do not always climb up the top of the meniscus surface but may stop and stably stay at various positions on the meniscus surface, depending upon their curvatures and masses, and that bent copper sheets can self-assemble into an oriented array (or an anisotropic form) through self-rotating on the water surface. The minimum energy mechanism of meniscus-climbing and self-rotating was then numerically studied. It was further shown that the meniscus-climbing and the rotating behavior is not only a general phenomenon for floating objects with hydrophilic surfaces, even those with fairly large sizes and weights (e.g., a metal bottle cap), but is also conditionally realizable for floating objects with hydrophobic surfaces.