"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.
