Influence of Humidity on Grip and Release Adhesion Mechanisms for Gecko-Inspired Microfibrillar Surfaces
Influence of Humidity on Grip and Release Adhesion Mechanisms for Gecko-Inspired Microfibrillar Surfaces
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DOI：
10.1021/acsami.7b01624
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发表时间：
2017-04-26
影响因子：
9.5
通讯作者：
Israelachvili, Jacob N.
中科院分区：
文献类型：
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作者：
Cadirov, Nicholas;Booth, Jamie A.;Israelachvili, Jacob N.
壁虎已经进化出了脚垫，使它们能够保持独特的攀爬能力，尽管表面和环境存在巨大差异，从干燥的沙漠到潮湿的雨林。同样，受壁虎启发的成功模仿者应该在同样多样化的气候范围内表现出粘合和摩擦性能。在这项工作中，我们重点研究了相对湿度（RH）对壁虎启发的胶粘剂垫的“摩擦粘附”行为的影响。使用表面力仪定量测量了微纤维交联聚二甲基硅氧烷表面在不同相对湿度（从0到100%）（包括完全淹没在水中）下对光滑半球形玻璃盘的粘附力和摩擦力。几何各向异性倾斜的半圆柱形微纤维产生“抓握状态”（沿纤维倾斜方向剪切后的高附着力和摩擦力，F-ad（+）和f-平行于（+））和“释放状态”（沿纤维倾斜方向剪切后的低附着力和摩擦力，F-ad（-）和f-平行于（-））)。通过适当控制加载路径，这允许在强附着和容易分离之间过渡。改变预紧力和剪切方向会导致每根纤维的有效接触面积以及接触的微尺度和纳米尺度结构的差异，而改变相对湿度会导致范德华力和毛细力的相对贡献的差异。结合起来，这两种效应导致了附着力和摩擦力的有趣趋势。在高达75% RH时，握把状态附着力保持不变，握把与释放附着力的比值不低于4.0。此外，相对湿度在40% ~ 75%之间时，与（+）平行的摩擦力f -和与（-）平行的摩擦力f -和与（-）平行的释放态附着力F-ad（-）最大。
Geckos have developed foot pads that allow them to maintain their unique climbing ability despite vast differences of surfaces and environments, from dry desert to humid rainforest. Likewise, successful gecko-inspired mimics should exhibit adhesive and frictional performance across a similarly diverse range of climates. In this work, we focus on the effect of relative humidity (RH) on the "frictional-adhesion" behavior of gecko-inspired adhesive pads. A surface forces apparatus was used to quantitatively measure adhesion and friction forces of a microfibrillar cross-linked polydimethylsiloxane surface against a smooth hemispherical glass disk at varying relative humidity, from 0 to 100% (including fully submerged under water). Geometrically anisotropic tilted half-cylinder microfibers yield a "grip state" (high adhesion and friction forces after shearing along the tilt of the fibers, F-ad(+) and F-parallel to(+)) and a "release state" (low adhesion and friction after shearing against the tilt of the fibers, F-ad(-) and F-parallel to(-)). By appropriate control of the loading path, this allows for transition between strong attachment and easy detachment. Changing the preload and shear direction gives rise to differences in the effective contact area at each fiber and the microscale and nanoscale structure of the contact while changing the relative humidity results in differences in the relative contributions of van der Waals and capillary forces. In combination, both effects lead to interesting trends in the adhesion and friction forces. At up to 75% RH, the grip state adhesion force remains constant and the ratio of grip to release adhesion force does not drop below 4.0. In addition, the friction forces F-parallel to(+) and F-parallel to(-) and the release state adhesion force F-ad(-) exhibit a maximum at intermediate relative humidity between 40% and 75%.