Showing papers by "Peter T. Cummings published in 2010"

Supercritical fluid behavior at nanoscale interfaces: Implications for CO2 sequestration in geologic formations

Abstract: Injection of CO2 into subsurface geologic formations has been identified as a key strategy for mitigating the impact of anthropogenic emissions of CO2. A key aspect of this process is the prevention of leakage from the host formation by an effective cap or seal rock which has low porosity and permeability characteristics. Shales comprise the majority of cap rocks encountered in subsurface injection sites with pore sizes typically less than 100 nm and whose surface chemistries are dominated by quartz (SiO2) and clays. We report the behavior of pure CO2 interacting with simple substrates, i.e. SiO2 and muscovite, that act as proxies for more complex mineralogical systems. Modeling of small-angle neutron scattering (SANS) data taken from CO2–silica aerogel (95% porosity; ∼7 nm pores) interactions indicates the presence of fluid depletion for conditions above the critical density. A theoretical framework, i.e. integral equation approximation (IEA), is presented that describes the fundamental behavior of near-...

Phase transitions in nanoconfined fluids: The evidence from simulation and theory

Abstract: N anoconfined fluids — that is, fluids confined between surfaces separated by nanometers — play important roles in many natural and man-made processes and products. One example is hard disk drive lubrication where, as data density has increased exponentially, the distance between the read head and rotating platen has been exponentially decreasing for several decades. This distance is now at 10–12 nm, and in the next generation of disk drives will be at 8 nm; currently, monolayers of lubricant are used to protect disk drives in abnormal situations (e.g., power loss), but in the future it is expected that they will be lubricated at all times, including during read/write operations. Additional examples include the lubrication of microelectromechanical systems (MEMS), and nanoelectromechanical systems (NEMS), and a model for the natural lubrication of synovial joints, all of which can involve moving surfaces separated by distances of the order of nm. The latter exhibit very low-sliding friction at normal pressures up to 5 MPa or more; the model system, consisting of polyzwitterionic brushes polymerized directly onto the mica sheets in a surface force balance (SFB), exhibits very similar low-sliding friction (within a factor of 2 of the natural synovial joints) at pressures as high as 7.55 MPa. These three examples highlight the desirability of being able to lubricate effectively between surfaces moving relative to each other while separated by distances on the order of nm. If the lubricant undergoes a fluid-solid phase transition under nanoconfinement, resulting in a many order of magnitude increase in the effective viscosity and the onset of a nonzero yield stress, then it is clearly not useful as a lubricant. In addition to lubrication at the nanoscale, phase transitions under nanoconfinement are also clearly important in industrial adsorption and catalytic processes (microand mesoporous adsorbents, with pore widths of under 2 nm and 2–50 nm, respectively, are widely used in the chemical, petrochemical, gas processing, and pharmaceutical industries for separations, pollution abatement, and as catalysts and catalyst supports). Additional application areas (e.g., in geology, oil recovery and nanofabrication, including nanotemplating through nanoconfinement) are described in the excellent review article by Gelb et al. Hence, understanding the phase behavior of nanoconfined fluids is key to the rational design and control of many processes and devices, both in the processing industries and in the emerging field of nanotechnology. Specifically, the change in melting temperature as a function of nanoconfinement is an important quantity to understand and predict. Gubbins and coworkers have been leaders in understanding these phenomena

Human Mammary Epithelial Cells Exhibit a Bimodal Correlated Random Walk Pattern

Abstract: Background Organisms, at scales ranging from unicellular to mammals, have been known to exhibit foraging behavior described by random walks whose segments confirm to Levy or exponential distributions. For the first time, we present evidence that single cells (mammary epithelial cells) that exist in multi-cellular organisms (humans) follow a bimodal correlated random walk (BCRW). Methodology/Principal Findings Cellular tracks of MCF-10A pBabe, neuN and neuT random migration on 2-D plastic substrates, analyzed using bimodal analysis, were found to reveal the BCRW pattern. We find two types of exponentially distributed correlated flights (corresponding to what we refer to as the directional and re-orientation phases) each having its own correlation between move step-lengths within flights. The exponential distribution of flight lengths was confirmed using different analysis methods (logarithmic binning with normalization, survival frequency plots and maximum likelihood estimation). Conclusions/Significance Because of the presence of non-uniform turn angle distribution of move step-lengths within a flight and two different types of flights, we propose that the epithelial random walk is a BCRW comprising of two alternating modes with varying degree of correlations, rather than a simple persistent random walk. A BCRW model rather than a simple persistent random walk correctly matches the super-diffusivity in the cell migration paths as indicated by simulations based on the BCRW model.

Resummed thermodynamic perturbation theory for central force associating potential: One-patch model.

Abstract: A resummed thermodynamic perturbation theory for associating fluids with multiply bondable central force associating potential is extended for the fluid with multiple number of multiply bondable associating sites. We consider a multi-patch hard-sphere model for associating fluids. The model is represented by the hard-sphere fluid system with several spherical attractive patches on the surface of each hard sphere. Resummation is carried out to account for blocking effects, i.e., when the bonding of a particle restricts (blocks) its ability to bond with other particles. Closed form analytical expressions for thermodynamical properties (Helmholtz free energy, pressure, internal energy, and chemical potential) of the models with arbitrary number of doubly bondable patches at all degrees of the blockage are presented. In the limiting case of total blockage, when the patches become only singly bondable, our theory reduces to Wertheim's thermodynamic perturbation theory (TPT) for polymerizing fluids. To validate the accuracy of the theory we compare to exact values, for the thermodynamical properties of the system, as determined by Monte Carlo computer simulations. In addition we compare the fraction of multiply bonded particles at different values of the density and temperature. In general, predictions of the present theory are in good agreement with values for the model calculated using Monte Carlo simulations, i.e., the accuracy of our theory in the case of the models with multiply bondable sites is similar to that of Wertheim's TPT in the case of the models with singly bondable sites.

Direct evidence for fluid–solid transition of nanoconfined fluids

Abstract: Atomistically detailed simulations provide direct and reliable evidence that sufficiently nanoconfined fluids undergo a rapid and abrupt first-order transition to an ordered solid-like structure.

Molecular Simulation Studies on the Elongation of Gold Nanowires in Benzenedithiol

Abstract: The bonding geometry at the metal−molecule interface plays an important role in determining the conductance behavior of metal−molecule−metal junctions. This bonding geometry has to be determined a priori in quantum mechanical current−voltage (I−V) calculations. To identify the detailed metal−molecule bonding configurations, we perform classical molecular simulations by combining grand canonical Monte Carlo (GCMC) sampling with molecular dynamics (MD) to explore the dynamic elongations of gold nanowires in the presence of benzenedithiol (BDT) molecules. A specific multitime-scale double reversible reference system propagator algorithm (double-RESPA) has been designed for the metal−organic complex in MD simulations to improve the simulation efficiency. We investigate the variations of bonding sites and bonding angles of BDT molecules on a stretched Au nanowire at a constant chemical potential. The density of BDT and the number of bonded and nonbonded BDT molecules in the simulation box is monitored during t...

Comparative studies on the structure and diffusion dynamics of aqueous and nonpolar liquid films under nanometers confinement

Abstract: Aqueous hydration water confined between two mica surfaces and nonpolar liquid argon confined between two solid crystals have been comparably studied through molecular dynamics simulations. A liquid–vapor molecular ensemble developed in previous studies (Leng 2008 J. Phys.: Condens. Matter 20 354017) has been used to investigate the solvation structures and diffusion dynamics of confined films. We find that water always tends to diffuse even under two-layer extreme confinement (D = 0.73 nm), whereas liquid argon undergoes a spontaneous liquid-to-solid phase transition at an appreciable large distance (n = 9 layers) between the two crystal solids. Vacancy diffusion in the solid phase of argon is observed. We attribute this phase transition of argon to the tendency of argon molecules to form a close-packed structure to maximize the cohesion energy contributed from weak van der Waals attractions.

The Rutile (110)-Water Interface: A comment on "Structure and Dynamics of Liquid Water on Rutile TiO2(110)" by L.-M. Liu, C. Zhang, G. Thornton and A. Michaelides

Abstract: The (110) surface of rutile ( -TiO2) in contact with water is one of the most technologically-important and scientifically-investigated interfaces that exists Liu and coworkers1 (hereafter Liu1) expanded on an excellent review of water-titania interfaces2, by conducting extensive static density functional theory (DFT) and DFT molecular dynamics (DFT-MD) investigations of rutile (110) using a range of cell configurations and DFT functionals We agree with their DFT calculations of the influence of crystal slab thickness on water sorption energies, but find some of their major conclusions unwarranted or overstated, namely a) that there is no dissociation of first-layer sorbed water at ~300K; b) that translational diffusion of water molecules in contact with the surface approaches that of bulk liquid water ; and c) that second layer water structuring and hydrogen bonding to surface oxygens are weak We present published evidence not cited by Liu1 that challenge these assertions