微纳间隙受限液体的界面黏着机理研究

利用球-盘接触黏着试验仪对微量受限液体的界面黏着行为的研究发现：临界体积范围内（皮升到纳升）的受限液滴达到临界厚度后将出现自动铺展和瞬时收缩行为（分别对应球-盘趋进和分离过程），并同时提供一定幅值的法向黏着力及伴随出现力的突变；该界面黏着力与液滴体积、球的直径等相关，并很好地揭示了昆虫或树蛙爪垫与光滑表面间的湿黏着行为.在进一步试验的基础上，利用球—面接触模型的毛细黏着力公式对所观察到的试验现象进行了机理揭示，认为临界体积受限液滴出现的自动铺展和瞬时收缩行为与接触副的刚度相关，最大法向黏着力随液体体积的减小而减小，与接触副之间受限液体的中心区厚度有关.结合生物黏着爪垫进行了受限液体界面黏着控制机理的分析，可以指导仿生黏着爪垫的设计与控制. 关键词： 生物黏着爪垫 毛细黏着力 受限液体

Interfacial adhesive mechanism of confined liquid thin film of micro/nano-meter thickness

Many insects and some animals, such as and crickets, tree frogs, can utilize a thin film of confined liquid between the pad and the smooth surface to facilitate climbing vertical walls and even walking on the ceiling. Our previous experiments based on a ball-disk microtribometer have revealed that a sudden unstable spreading or shrinking of the confined liquid film occurs when the volume reaches a critical value, which respectively corresponds to an unstable increasing or decreasing of interfacial adhesive force. This unstable behavior of the confined liquid film has important implications on the adhesion/releasing mechanism of wet bio-adhesive pad to or from the smooth surface. In this article, we highlight some of the switching mechanisms for capillary adhesion through theoritcal analysis and further experiments. It is found that, the unstable spreading and shrinking of confined liquid film is connected with the instability of the cantilever stiffness, and the maximum interfacial adhesive force decreases with the liquid volume and strongly depends on the film thickness of the central area. Inspired by these results, strategies for a bio-adhesive pad to realize `switchable' adhesive mechanism is analyzed, which will shed light on the design and control of a biomimetic adhesive pad.