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

纳米受限水体系的性质在粘土膨胀、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)中的摩擦控制等领域具有相当大的兴趣和重要性，无论是自然的还是人工的。尽管在过去的几十年里，我们在理解固体表面附近的液体物理方面取得了重大进展，特别是在纳米尺度的限制下，但很少有理论研究来研究纳米受限缔合水膜的吸附结构和摩擦性能。为了理解在两个云母表面之间水化结合层的表面力平衡(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，该项目涉及到包括SFB实验时间尺度的多时间尺度分子模拟的发展和应用。该方法结合了云母-液体复杂体系的真实分子模型和微观分子松弛与力学系统宏观动力学相结合的混合模拟技术。该提案将探索传统的分子动力学和混合模拟技术。此外，非平衡分子动力学(NEMD)的瞬时时间关联函数(TTCF)方法将被扩展到研究极低的剪切速率，允许在分子动力学、混合模拟结果和实验数据之间进行直接比较。对结合水化层的结构、界面动力学(分子的扩散性、水化鞘的流动性)以及被驱动的云母板的宏观动力学将出现新的物理见解。这些洞察力是迫切需要的，因为最近关于实验仪器表面可能存在铂纳米颗粒污染的发现，以及污染的潜在影响，让人对过去十年中公认的大部分实验结果产生了怀疑。本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的影响的辩论中发挥重要作用。这项拟议工作的学术价值源于Cummings、Leng和Delhommelle(他们将于2004年6月1日加入Cummings团队)的努力，他们在计算纳米科学、固体力学和纳米摩擦学以及NEMD方面具有广泛的背景。这些努力将极大地提高我们对纳米限制中水溶液性质的理解。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的设计提供洞察。所提出的研究的更广泛的影响包括发展多时间尺度的分子模拟方法和使用真实的分子模型来研究自然环境中的纳米受限水体系。提出的混合分子模拟方法将克服传统分子动力学(MD)模拟中遇到的时间尺度问题，并可应用于其他纳米尺度的过程，如纳米压痕、原子力显微镜(AFM)和化学力显微镜(CFM)、生命系统中结合/解绑的生物分子，以及从地下储油层中回收石油(能量提取)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.