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

粘着和摩擦对微纳机电系统(MEMS/NEMS)的性能至关重要。在干燥或液体环境中(例如，生物微机械系统/新能源系统)，考虑表面化学终止是非常重要的。然而，人们对控制这种规模的粘附性和摩擦性的一般规律知之甚少。根据最近的几个实验结果，附着力和摩擦力对界面和溶剂性质的依赖似乎没有遵循的一般规则。因此，这项工作的第一个目标是研究界面和溶剂的性质如何影响纳米尺度的粘着和摩擦，这对生物MEMS/NEMS非常重要。化学力显微镜(CFM)提供了一种探测具有化学敏感性的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分子膜(SAM)共价修饰原子力显微镜(AFM)尖端和衬底，人们能够应用这项技术来测量具有特定表面化学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，也很难检验埋藏在CFM实验中的界面。此外，传统的分子动力学(MD)模拟和原子力显微镜(或CFM)实验之间的时间尺度相差六个数量级或更大。因此，这项工作的第二个目标是在实验时间尺度上模拟具有不同界面和溶剂的CFM。在这项工作中，PI将应用PI团队最近开发的时间杂化分子模拟技术来模拟从快(在MEMS/NEMS器件中)到慢(在CFM实验中)两个表面之间或烷硫醇SAM修饰的AFM尖端和浸入溶剂中的表面之间的粘着和摩擦。纳米接触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，将在一种新的系综中进行模拟，其中受限流体与主体接触。这个系综更接近于生物MEMS/NEMS和表面力装置(SFA)的实验。模拟结果将被扩展到研究SAM缺陷和表面粗糙度对粘着和摩擦的影响，以及具有重要技术意义的硅烷基单分子膜的纳米摩擦学。这项工作的成功将提供对表面化学末端和溶剂极性如何影响纳米级粘着和摩擦的基本理解。模拟结果将用于指导生物MEMS/NEMS的设计，并用于解释CFM和SFA实验。它将对生物MEMS/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一跨学科研究项目。这项工作产生的新的模拟能力将有益于特殊课程(例如，新生的ENG 100)和PI提供的计算纳米技术课程的学生。在这一支持下，PI的团队将继续为校内外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.