The Relationship Between Contact Mechanics and Adhesion in Nanoscale Contacts Between Non-Polar Molecular Monolayers
The Relationship Between Contact Mechanics and Adhesion in Nanoscale Contacts Between Non-Polar Molecular Monolayers
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DOI：
10.1007/s11249-013-0105-2
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发表时间：
2013-01
影响因子：
3.2
通讯作者：
Nikos Nikogeorgos;G. Leggett
中科院分区：
文献类型：
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作者：
Nikos Nikogeorgos;G. Leggett
采用原子力显微镜和摩擦力显微镜研究纯介质中以及在双组分庚烷/丙酮混合物中十二烷基乙基醚自组装单分子膜之间的粘附和摩擦。在不含氢键供体的介质中，发现拉脱力与基于Lifshitz理论的理论预测非常一致。随着介质的氢键供体能力的增加，粘附能被发现越来越被低估的模型，说明在确定的纳米级接触的粘附性能的重要性的介质-介质的相互作用的接触区域外。例外的是，在正辛醇中，拉脱力大大低于预测值，并观察到双斜率线性摩擦-载荷关系。这些观察结果是合理的物理吸附层的表面上形成的辛醇。在其他介质中的摩擦-载荷关系被认为是依赖于粘附的大小。对于弱粘附系统，摩擦-载荷关系是线性的，但随着粘附力的增加，观察到一个亚线性关系。通过将摩擦视为以表面剪切强度τ为特征的粘附相关剪切项和以摩擦系数μ为特征的分子犁耕项之和，使数据合理化。因此，Amontons定律似乎描述了非常弱的粘附力的极限情况，其中粘弹性犁削主要负责能量耗散，而在其他情况下，由于剪切粘合剂接触中的能量耗散，出现了次线性摩擦-载荷关系。
Atomic and friction force microscopy were employed to examine adhesion and friction between dodecanethiol self-assembled monolayers in pure media as well as in two-component heptane/acetone mixtures. In media that did not contain hydrogen bond donors, the pull-off forces were found to be in very good agreement with theoretic predictions based on the Lifshitz theory. As the hydrogen bond donor ability of the medium increased, the adhesion energy was found to be increasingly underestimated by the model, illustrating the importance of the medium–medium interactions outside the contact area in determining the adhesive properties of the contact at the nanoscale. Exceptionally, inn-octanol, the pull-off forces were considerably lower than predicted and a dual slope linear friction–load relation was observed. These observations were rationalized by the formation of physisorbed layers of octanol on the surfaces. The friction–load relationship in the other media was found to be dependent on the magnitude of adhesion. For weakly adhering systems, the friction–load relationship was linear, but as adhesion increased, a sublinear relationship was observed. The data were rationalized by treating the friction as the sum of an adhesion-dependent shear term characterized by a surface shear strengthτand a molecular plowing term characterized by a coefficient of frictionμ. Thus, Amontons’ law appears to describe the limiting case of very weak adhesion where viscoelastic plowing is primarily responsible for energy dissipation, while a sublinear friction–load relationship emerges in other situations due to the dissipation of energy in shearing adhesive contacts.