NER: Multi-timescale Molecular Simulation of Fluidity of Ultrathin Nanoconfined Aqueous Systems

卡明斯，Peter T.Vanderbilt大学，“NER：超薄纳米水系统流动性的多时间尺度分子模拟”纳米限水系统的性质在粘土膨胀、生物润滑、生命系统中的电荷迁移以及微/纳米机电系统（M/NEMS）的摩擦控制等领域具有相当大的兴趣和重要性，包括自然和人造的。虽然在过去的几十年里，我们对固体表面附近液体的物理性质，特别是纳米尺度约束下的液体的物理性质的理解取得了重大进展，但很少有理论研究对纳米约束缔合水膜的吸附结构和摩擦特性进行研究。为了了解两个云母表面水合结合层表面力平衡（SFB）实验中观察到的现象背后的摩擦动力学，我们提出了一个纳米级探索性研究（NER）项目，该项目涉及开发和应用“多时间尺度”分子模拟来涵盖SFB实验时间尺度。该方法结合了复杂云母-液体系统的真实分子模型和微观分子弛豫与宏观力学系统动力学相结合的混合模拟技术。传统的分子动力学和混合模拟技术将在提案中进行探索。此外，将非平衡分子动力学（NEMD）的瞬态时间相关函数（TTCF）方法扩展到研究非常低的剪切速率，从而可以将分子动力学、混合模拟结果和实验数据直接进行比较。新的物理见解结合水化层的结构，界面动力学（分子的扩散率，水化鞘的流动性），以及驱动云母板的宏观动力学将出现。这些见解是迫切需要的，因为最近发现的实验设备表面可能存在铂纳米颗粒污染，以及污染的潜在影响，对过去十年中许多公认的实验发现产生了怀疑。在这个项目中建议开发的那种建模和模拟工具可以在即将到来的关于这种污染影响的辩论中发挥重要作用。这项工作的智力价值来自于将Cummings、Leng和Delhommelle（他将于2004年6月1日加入Cummings的团队）在计算纳米科学、固体力学和纳米摩擦学以及NEMD方面的广泛背景整合在一起的努力。这些努力将大大提高我们对纳米约束下含水液体性质的理解。这将在理解自然界的微/纳米机电系统（M/NEMS）（例如细菌中的鞭毛马达）方面发挥关键作用，并将为合成M/NEMS的设计提供见解。所提出的研究的广泛影响包括发展多时间尺度分子模拟方法和使用真实分子模型来研究自然环境中的纳米限制水系统。所提出的混合分子模拟方法克服了传统分子动力学（MD）模拟的时间尺度问题，可应用于其他纳米尺度过程，如纳米压痕、原子力显微镜（AFM）和化学力显微镜（CFM）、生命系统中生物分子的结合/解结合以及地下储层石油的开采（能源开采）等。

Cummings, Peter T.Vanderbilt University"NER: Multi-timescale Molecular Simulation of Fluidity of Ultrathin Nanoconfined Aqueous Systems."The properties of nanoconfined aqueous system are of considerable interest and importance in areas as diverse as clay swelling, bio-lubrication, charge migration in living systems, and the friction control in micro/nano electro-mechanical systems (M/NEMS), both natural and man-made. Although significant progress toward our understanding of the physics of liquids in the vicinity of solid surfaces, and particularly under nanoscale confinement, has been made in the past few decades, very few theoretical studies have been conducted to investigate the adsorbed structure and frictional properties of nanoconfined associative aqueous films. To understand the frictional dynamics underlying the observed phenomena in surface force balance (SFB) experiments of hydration bound layers between two mica surfaces, we propose a nanoscale exploratory research (NER) project that involves the development and application of "multi-timescale" molecular simulations to encompass the SFB experimental timescales. The approach incorporates a realistic molecular model for complex mica-liquid system and a hybrid simulation technique that combines microscopic molecular relaxation with macroscopic dynamics of mechanical systems. Both conventional molecular dynamics and hybrid simulation techniques will be explored in the proposal. Moreover, the transient time correlation function (TTCF) method of non-equilibrium molecular dynamics (NEMD) will be extended to study very low shear rates, allowing a direct comparison between molecular dynamics, the hybrid simulation results and experimental data. New physical insights into the structure of bound hydration layers, the interfacial dynamics (diffusivity of molecules, fluidity of hydration sheath), and the macroscopic dynamics of the driven mica plate will emerge. These insights are urgently needed since recent findings of possible Pt nanoparticle contamination on the surfaces of the experimental apparatuses, and the potential impact of the contamination, have thrown doubt on much of the accepted experimental findings of the past decade. Modeling and simulation tools of the kind proposed for development in this project can play an essential role in the coming debate over the impact of such contamination. The intellectual merit of the proposed work results from integrating the efforts of Cummings, Leng, and Delhommelle (who will join Cummings group beginning from June 1, 2004) with extensive backgrounds in computational nanoscience, solid mechanics and nanotribology, and NEMD. These efforts will greatly enhance our understanding of the properties of aqueous liquids in nanoconfinement. This will play a key role in the understanding nature's micro/nano electro-mechanical systems (M/NEMS) (e.g., flagellar motors in bacteria) and will provide insight into the design of synthetic M/NEMS. The broader impacts of the proposed research include the development of multi-timescale molecular simulation methodology and using the real molecular model to investigate nanoconfined aqueous system in naturally occurring environment. The hybrid molecular simulation method proposed will overcome the timescale issue encountered in conventional molecular dynamics (MD) simulations, and can be applied to other nanoscale processes, such as nanoindentation, atomic force microscopy (AFM) and chemical force microscopy (CFM), and binding/unbinding bio-molecules in living systems, as well as recovery of petroleum from underground reservoirs (energy extraction), etc.