Multi-Timescale Molecular Simulation Study of Hydration Force, Hydrophobic Interaction and Shear Dynamics in Nanometer Confined Aqueous Systems

在水环境中，纳米尺度距离的两个分子光滑固体表面之间产生的力在许多不同的系统中是普遍存在的，其潜在机制是许多拟议技术的核心。在本项目中，pi将开展基础和创新的分子模拟研究，探索这些力的潜在机制，可分为两大类：致密电解质溶液中两个亲水表面之间的短程排斥水合力和水中两个疏水表面之间的远程疏水吸引力。我们将开发一种多时间尺度的混合分子模拟方法来研究纳米尺度的方法和两个表面之间的剪切。了解这些物理现象对许多技术应用至关重要，从微/纳米机电系统（MEMS/NEMS）的摩擦和润滑，生物界面的生物润滑，到离子通道（或通过拥挤的细胞内环境）和蛋白质折叠中的纳米流体。研究目标包括：(1)发展一种分子模拟方法，该方法结合了纳米水系统中表面-水-离子复合物的真实分子模型和机械分子系，以代表表面力装置（SFA）实验中的真实情况；(2)亲水性表面之间的水合力、疏水表面之间的脱湿、疏水吸引和坍缩的潜在机制的基础研究；(3)结合适用于低剪切速率的瞬态时间相关函数（TTCF）形式，研究纳米尺度表面接近和剪切的多时间尺度混合分子模拟。这项工作的智力价值在于其目标是在分子水平上阐明纳米水系统的水合作用、疏水相互作用和剪切动力学的潜在机制，整合Leng和Cummings的互补专业知识，并协调Jacob Klein小组的理论研究和实验研究。所提出的研究的更广泛的影响在于它增强了我们对SFA实验中观察到的水合力和疏水效应相关的长期基本问题的理解。该研究项目的新发现将通过出版物、会议报告和网站向SFA科学界发布，并将用于指导生物润滑和bioMEMS/NEMS设备的工程设计。这些发现也将对生物学中蛋白质折叠、自组装和纳米力学的理解产生潜在的影响。本项目将培养1名博士生。本科生将参加这个跨学科的研究项目，由范德比尔特大学暑期本科生研究项目资助。此外，K-12教师将有机会通过范德比尔特教师暑期研究体验项目参与该项目。PI和联合PI将根据这项研究成果在化学工程研究生分子模拟课程、初级机械工程动力学课程（ME190，由PI教授）和ES101机械工程前沿课程（旨在激发机械工程前沿研究领域的兴趣的入门课程）中授课。

The forces induced between two molecularly smooth solid surfaces at nanometer scale distances in an aqueous environment are ubiquitous in many diverse systems, and the underlying mechanism is at the heart of numerous proposed technologies. In this proposed project, the PIs will carry out a fundamental and innovative molecular simulation study to explore the underlying mechanisms of these forces, which can be classified into two main categories: The short-range repulsive hydration force between two hydrophilic surfaces in dense electrolyte solutions and the long-range hydrophobic attraction force between two hydrophobic surfaces in water. A multi-timescale hybrid molecular simulation method will be developed to investigate the nanoscale approach and shearing between two surfaces. Understanding these physical phenomena is critical to many technological applications, ranging from friction and lubrication in micro/nano electro-mechanical systems (MEMS/NEMS) and biolubrication at biological interfaces, to nanofluidics in ion channels (or through crowded intracellular environments) and protein folding. The research objectives include: (1) Development of a molecular simulation methodology that incorporates realistic molecular models for the surface-water-ion complex in nanoconfined aqueous system and mechanomolecular ensembles to represent the real situation in surface force apparatus (SFA) experiments; (2) Fundamental studies of the underlying mechanisms of hydration force between hydrophilic surfaces, the dewetting, hydrophobic attraction and collapse between hydrophobic surfaces; and (3) Multi-timescale hybrid molecular simulation study of nanoscale surface approach and shearing, combined with the transient time correlation function (TTCF) formalism applicable at low shear rates. The intellectual merit of the proposed work lies in its goals to elucidate, at molecular level, the underlying mechanisms of hydration forces, hydrophobic interaction and shear dynamics of nanometer confined aqueous systems, to integrate the complementary expertise of Leng and Cummings, and to coordinate the theoretical studies with experimental studies in Jacob Klein's group. The broader impacts of the proposed research lie in its enhancement of our knowledge in understanding the long-standing fundamental questions related to the hydration force and hydrophobic effects observed in SFA experiments. The new findings from the research program will be communicated to SFA science community via publication, conference presentation and website, and will be used to guide the engineering design for biolubrication and bioMEMS/NEMS devices. These findings will also have potential impact on the understanding of protein folding, self-assembly, and nanomechanics in biology. One PhD student will be trained through this project. Undergraduate students will participate in this interdisciplinary research program, funded by the Vanderbilt Summer Undergraduate Research Program. Further, K-12 teachers will have the opportunity to participate in the project through Vanderbilt Summer Research Experience for Teachers programs. The PI and co-PI will offer lectures based on the insights derived from this research work in a chemical engineering graduate molecular simulation course, in the junior-level mechanical engineering dynamics course (ME190, being taught by the PI), and in ES101, Frontiers in Mechanical Engineering, an introductory course for freshmen mechanical engineering students designed to excite their interest in frontier research areas in mechanical engineering.