微间隙受限液体行为与昆虫爪垫在光滑壁面的粘着机理

Impact of confined liquid thin film upon bioadhesive force between insect pads and smooth solid surface

自然界中许多昆虫通过分泌一层油性液体薄膜实现其爪垫表皮和光滑壁面之间粘附和解粘附,从而实现在光滑壁面上的快速爬行.为了揭示昆虫爪垫与光滑壁面间微量液体薄膜对生物粘着的意义,基于自行研制的粘着接触实验仪,采用微量的[emim][Tf2N]离子液体和聚α烯烃油,观测其受限在纳米级光滑钢球表面与玻璃表面之间的接触行为以及法向粘着力.实验发现,临界体积(10-12—10-9L)范围内的受限液滴达到临界厚度(小于2μm)后会出现自动铺展和瞬时收缩行为,并同时提供幅值稳定且数值相当大的法向粘着力.这一现象与液桥弯月面的毛细作用有本质区别,受限液体薄膜表现出一定的类固化特征.基于这一现象,昆虫通过控制爪垫与壁面间受限液体的膜厚或液体量,就可以控制爪垫与光滑壁面粘着应力的大小(由0.45mPa减小到0mPa左右),从而实现稳固的粘附和快速的解粘附.微间隙受限液体的粘着行为研究可以为昆虫爪垫粘着机理及相关的仿生技术提供物理基础.

The outstanding climbing skills on smooth solid surface of some insects like beetle, bee and cockroach or some small reptiles like gecko have been under investigation for a long time. When some kinds of insects crawl on a smooth glass surface, the micro-pads leave traces of self-secreted organic liquid, which suggests that, the thin layer of confined liquid may contribute to the bioadhesive force between the micro-pad and the smooth surface. To investigate the properties of the confined liquid thin film, a series of experiments were carried out on a home-made microtribometer with a trace amount of ionic liquid （ [emim] [Tf2N] ） or PAO（pely-α-olefin） oil confined between a nano-scale smooth steel sphere and a glass plate. For a critical confined volume of pitoliters to nanoliters, and a critical clearance of tens to hundreds of nanometers, confinement-induced spontaneous spreading and abrupt shrinking were observed, accompanied by the presence of a stable interfacial adhesive force of remarkable magnitude. This spreading/shrinking-induced adhesive force was proved to be fundamentally different from the common meniscus capillary force and was considered to stem from confinement-induced solidification, according to our further investigations. The confinement-induced adhesive force of organic liquid may shed significant light on the physical principle “employed” by insects to crawl quickly on smooth vertical surfaces. By subtly adjusting the volume of confined liquid or the clearance between the pads and the surface, insects can control （ i. e., switch on or off） the interracial force at will. The interfacial bonding force of confined liquid thin film discovered in this paper may provide a basis for the principle of biomimetic attachment systems.