Patrick David L, Flanagan John F, Kohl Patrick, Lynden-Bell Ruth M
Department of Chemistry, Western Washington University, 516 High Street, Bellingham, WA 98225, USA.
J Am Chem Soc. 2003 Jun 4;125(22):6762-73. doi: 10.1021/ja0345367.
Chemical force microscopy and related force measurement techniques have emerged as powerful tools for studying fundamental interactions central to understanding adhesion and tribology at the molecular scale. However, detailed interpretation of these interactions requires knowledge of chemical and physical processes occurring in the region of the tip-sample junction that experiments cannot provide, such as atomic-scale motions and distribution of forces. In an effort to address some of these open issues, atomistic molecular dynamics simulations were performed modeling a chemical force microscope stylus covered with a planar C12 alkylthiolate self-assembled monolayer (SAM) interacting with a solid wall. A complete loading-unloading sequence was simulated under conditions of near-constant equilibrium, approximating the case of infinitely slow tip motion. In the absence of the solid wall, the stylus film existed in a fluid state with structural and dynamic properties similar to those of the analogous planar SAM at an elevated temperature. When the wall was brought into contact with the stylus and pressed against it, a series of reversible changes occurred culminating with solidification of the SAM film at the largest compressive force. During loading, the chemical composition of the contact changed, as much of the film's interior was exposed to the wall. At all tip heights, the distribution of forces within the contact zone was uneven and subject to large local fluctuations. Analysis using the Johnson-Kendall-Roberts, Derjaguin-Muller-Toporov, and Hertz contacts mechanics models revealed significant deviations from the simulation results, with the JKR model providing best overall agreement. Some of the discrepancies found would be overlooked in an actual experiment, where, unlike the simulations, contact area is not separately known, possibly producing a misleading or incorrect interpretation of experimental results. These shortcomings may be improved upon by using a model that correctly accounts for the finite thickness of the compliant components and nonlinear elastic effects.
化学力显微镜及相关力测量技术已成为研究分子尺度下理解粘附和摩擦学核心基本相互作用的有力工具。然而，对这些相互作用的详细解释需要了解实验无法提供的针尖 - 样品接触区域中发生的化学和物理过程，例如原子尺度的运动和力的分布。为了解决其中一些未解决的问题，进行了原子分子动力学模拟，对覆盖有平面十二烷基硫醇自组装单分子层（SAM）并与固体壁相互作用的化学力显微镜探针进行建模。在接近恒定平衡的条件下模拟了完整的加载 - 卸载序列，近似于针尖无限缓慢运动的情况。在没有固体壁的情况下，探针膜以流体状态存在，其结构和动力学性质类似于高温下类似平面SAM的性质。当壁与探针接触并压向它时，发生了一系列可逆变化，最终在最大压缩力下SAM膜固化。在加载过程中，接触的化学成分发生了变化，因为膜的大部分内部暴露于壁。在所有针尖高度下，接触区内力的分布不均匀且存在较大的局部波动。使用约翰逊 - 肯德尔 - 罗伯茨、德亚金 - 穆勒 - 托波罗夫和赫兹接触力学模型进行的分析表明，与模拟结果存在显著偏差，其中JKR模型总体一致性最佳。在实际实验中，一些发现的差异可能会被忽略，与模拟不同的是，实际实验中接触面积并非单独已知，这可能会对实验结果产生误导性或错误的解释。通过使用正确考虑柔顺部件有限厚度和非线性弹性效应的模型，这些缺点可能会得到改善。
