Hybrid Molecular Simulation Studies of Nano-Scale Adhesion and Friction: Chemical Termination and Solvent Effects

粘附和摩擦对微纳机电系统（MEMS/NEMS）的性能至关重要。在干燥或液体环境（例如，BioMEMS/NEMS）中，考虑表面化学终止是非常重要的。然而，在这种尺度下，人们对控制粘附和摩擦特性的一般规律知之甚少。根据最近的几个实验结果，粘附和摩擦对界面和溶剂性质的依赖似乎没有普遍的规律可循。因此，这项工作的第一个目标是研究界面和溶剂性质如何影响纳米尺度的粘附和摩擦，这对bioMEMS/NEMS至关重要。化学力显微镜（CFM）提供了一种利用化学灵敏度探测分子相互作用的方法。通过用终止于不同官能团的自组装单层（sam）共价修饰原子力显微镜（AFM）尖端和衬底，人们能够应用该技术来测量具有特定表面化学性质的各种探针尖端和衬底之间的粘附力和摩擦力。由于系统的复杂性，很难解释CFM结果，也很难检测CFM实验中隐藏的界面。此外，传统分子动力学（MD）模拟与AFM（或CFM）实验在时间尺度上的差异是6个数量级或更大。因此，本工作的第二个目标是在实验时间尺度上模拟具有各种界面和溶剂的CFM。在这项工作中，PI将应用PI小组最近开发的时间混合分子模拟技术来模拟两个表面之间或烷硫醇sam改性AFM尖端与浸入溶剂的表面之间的粘附和摩擦，从快速（在MEMS/NEMS设备中）到慢速（在CFM实验中）。纳米级触点的界面性质将是疏水/疏水、亲水/亲水和疏水/亲水，同时将考虑从极性到非极性的各种溶剂。对于两个表面之间的受限流体，将在一个新的集合中进行模拟，其中受限流体与体接触。该集成更接近于bioMEMS/NEMS和表面力装置（SFA）实验。然后，模拟将扩展到检查SAM缺陷和表面凹凸对粘附和摩擦的影响，以及技术上重要的硅烷基单层的纳米摩擦学。这项工作的成功将为了解表面化学终止和溶剂极性如何影响纳米尺度的粘附和摩擦提供基本的理解。仿真结果将用于指导bioMEMS/NEMS的设计，并解释CFM和SFA实验。它将对BioMEMS/NEMS技术、CFM技术、生物分子识别和生物力学产生重大影响。研究生和本科生将参与这个跨学科的研究项目。这项工作产生的新的模拟能力将有利于学生学习特殊课程（例如，大一的eng100）和PI提供的计算纳米技术课程。有了这种支持，PI的小组将继续为许多对分子模拟和建模感兴趣的研究小组提供服务。

Adhesion and friction are critical to the performance of micro- or nanoelectromechanical systems (MEMS/NEMS). It is of great importance to consider surface chemical terminations for applications in dry or liquid environment (e.g., BioMEMS/NEMS). However, there is little understanding of the general laws that govern adhesion and frictional properties at this scale. The dependence of adhesion and friction on interfacial and solvent properties does not seem to have general rules to follow based on several recent experimental results. Thus, the first objective of this work is to study how interfacial and solvent properties affect nano-scale adhesion and friction important for bioMEMS/NEMS.Chemical force microscopy (CFM) provides a method of probing molecular interactions with chemical sensitivity. By covalently modifying atomic force microscopic (AFM) tips and substrates with self-assembled monolayers (SAMs) that terminate in distinct functional groups, one is able to apply this technique to measure adhesion and frictional forces between various probe tips and substrates with specific surface chemistry. Due to the complexity of the system, it is hard to interpret CFM results and to examine interfaces buried in CFM experiments. Furthermore, the difference in time scale between conventional molecular dynamics (MD) simulations and AFM (or CFM) experiments is six orders of magnitude or larger. Thus, the second objective of this work is to simulate CFM with various interfaces and solvents at the experimental time scale. In this work, the PI will apply the temporally hybrid molecular simulation technique that the PI's group developed recently to simulate adhesion and friction at a wide range of time scales from fast (in MEMS/NEMS devices) to slow (in CFM experiments) either between two surfaces or between alkanethiol SAM-modified AFM tips and surfaces immerged in solvents. Interfacial properties of nano-scale contacts will be hydrophobic/hydrophobic, hydrophilic/hydrophilic, and hydrophobic/hydrophilic while various solvents from polar to non-polar will be considered. For confined fluids between two surfaces, simulations will be performed in a new ensemble, in which confined fluids are in contact with the bulk. This ensemble is closer to bioMEMS/NEMS and surface force apparatus (SFA) experiments. Simulations will then be extended to examine the effect of SAM defects and surface asperities on adhesion and friction, and the nanotribology of the technologically important alkyl monolayers on silicon.The success of this work will provide a fundamental understanding of how surface chemical terminations and solvent polarities affect nano-scale adhesion and friction. Simulation results will be used to guide the design of bioMEMS/NEMS and to interpret CFM and SFA experiments. It will have significant impacts on BioMEMS/NEMS technology, CFM technique, biomolecular recognition, and biomechanics. Graduate students and undergraduate students will participate in this interdisciplinary research project. New simulation capacities generated from this work will be beneficial to students in special courses (e.g., ENG 100 for freshmen) and the course on computational nanotechnology offered by the PI. With this support, the PI's group will continue to serve many research groups on and off campus interested in molecular simulation and modeling.