报告内容摘要

This talk will present our recent fundamental studies related to surface and interface complex phenomena. Our first topic is related to the structure and dynamics of hydrocarbon or carbon dioxide aqueous complex fluids in clay interlayer. Clay swelling is a complicated interfacial phenomenon that involves the uptake of water molecules into charged clay interlayer. Recent experimental studies showed that in addition to water molecules, other small gas molecules such as methane (CH4) and carbon dioxide (CO2) molecules can also be intercalated into clay interlayers under typical temperature/pressure geological reservoir conditions. We use the equation of state (EOS) theory and grand-canonical Monte Carlo (GCMC) simulation, in combination with molecular dynamics (MD) simulation, to show how this clay swelling process proceeds. Our simulation results demonstrate that initial clay swelling is dominated by water adsorption into the clay interlayer, followed by the intercalation of methane/CO2 as the basal spacing increases. We find that this methane/CO2 intercalation process is strongly influenced by the relative humidity and the total gas pressure in the reservoir. This study has direct applications in the enhanced oil/gas recovery (EOR/EGR) in petroleum industry and CO2 sequestration to ease climate change. Our second topic is related to the mechanical properties of liquid films under nanometers confinement. Our research focuses on the squeezing and shear behaviors of a simple nonpolar liquid film (argon or OMCTS). The liquid-vapor MD simulations unambiguously show that there is a liquid-to-solid phase transition at some n = 7-8 monolayer distance. The stick-slip friction behavior of the solidified film, as well as our recent study on the contact stiffness and damping of the solidlike film in dynamic atomic force microscope (AFM), will also be discussed in this talk. These studies have important applications in nanotribology for nanoscale machines and in surface imaging by AFM for a variety of energy and biological surfaces. Our third topic is related to the driven dynamics simulations of molecular junctions in molecular electronics devices. For the gold-benzenedithiolate-gold molecular junction, we show that there are essentially two distinct breaking force traces corresponding to the Au-Au and Au-S bond ruptures, with the two force quanta at 1.5 and 2.0 nN, respectively. Our findings provide new molecular insights into the gold-thiolate interactions. The intermediate metal-molecule-metal binding structures could be used for further molecular transport calculations.

报告人简介

Prof.Leng was the recipient of the NSF CAREER award (2012). At GW he won the SEAS outstanding junior faculty research award for his contributions to the understanding of friction behavior of liquid films under confinement, and 2014 GW WID (writing in the disciplines) distinguished teaching award. Through theory and computer simulations, his research has been evolving from contact mechanics and tribology to more fundamental problems in the surface and interfacial science, including liquid structure and dynamics under nanometers confinements, hydration force and hydrophobic interactions in aqueous system, self-assembly at metal-organic interface, membrane fouling mechanisms in water purification, clay swelling in oil and gas production, and driven dynamics simulation for molecular junctions in molecular electronics devices (including metal-semiconductor contacts in energy harvesting systems). His recent work on molecular electronics simulations was also highlighted in C&EN news.

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