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Relaxation dynamics of unentangled polymer melts (with a chain length below N = 100 for polyethylene) can be studied by performing either Rouse mode or relaxation mode analysis, such as proper orthogonal decomposition. The former mode analysis can be derived analytically from the linear equation of motion of polymers with the inclusion of inertia term, whereas the latter is designed for the case when the numerical solution to a highly nonlinear equation of motion is known, such as MD simulation data. This article presents a summary of both approaches and their applications in the evaluation of zero-shear viscosity of polymer melts. [ABSTRACT FROM AUTHOR]

Understanding stress variations in pulsating flows due to periodic pressure gradients is fundamentally significant for the design and optimization of various medical apparatuses and for understanding biological phenomena. Oscillating flows have been dealt analytically and experimentally in great detail in the past. In this paper, a three-dimensional lattice boltzmann model is used to simulate the pulsating flows in a rectangular duct due to low, moderate, and high periodic pressure gradients. Asymptotic forms for velocity (valid for high frequencies) valid in different regions of the channel have been matched with the analytical results. The evolution of annular effects in the flow along with the Stokesian layer characteristics have been followed. Stress analysis at different points of the wall of the duct has been carried out to show regions of high stress and stress rates. [ABSTRACT FROM AUTHOR]

Steady state pressure driven flow of liquid argon through a finite length cylindrical nanopore was investigated numerically by classical Navier-Stokes (NS) hydrodynamic models and nonequilibrium molecular dynamics (MD) simulations. In both approaches, the nanopore was nominally 2.2 nm in diameter and 6 nm long. For the MD simulations, the intermolecular properties of the walls were specified independently from the liquid. Comparisons between the approaches were made in terms of the gross feature of total flow rate through the nanopore, as well as the more refined considerations of the spatial distributions of pressure, density, and velocity. The results showed that for the NS equations to predict the same trends in total flow rate with increasing pressure difference as the MD simulation, submodels for variations in density and viscosity with pressure are needed to be included. The classical NS boundary conditions quantitatively agreed with the flow rate predictions from MD simulations only under the condition of having a neutral-like solid-liquid interaction. Under these conditions, the NS and MD models also agreed well in streamwise distributions of pressure, density, and velocity, but not in the radial direction. [ABSTRACT FROM AUTHOR]

Fluid transport through a nanopore in a membrane was investigated by using a novel molecular dynamics approach proposed in this study. The advantages of this method, relative to dual-control-volume grand-canonical molecular dynamics method, are that it eliminates disruptions to the system dynamics that are normally created by inserting or deleting particles from control volumes, and that it functions well for dense systems due to the number of particles being fixed in the system. Using the proposed method, we examined liquid argon transport through a nanopore by performing nonequilibrium molecular dynamics (NEMD) simulations under different back pressures. Validation of the code was performed by comparing simulation results to published experimental data obtained under equilibrium conditions. NEMD results show that constant pressure difference across the membrane was readily achieved. [ABSTRACT FROM AUTHOR]

For a model system of polyethylene of chain lengths 40 and 100 carbon atoms, we calculated the pressure at different densities and compared them with the experimental values. The simulation was conducted on the second nearest neighbor diamond lattice, and the pressure was calculated using the virtual-volume-variation method after the system was reverse mapped to its fully atomistic form in continuous space and energy minimized. In addition, the pressure was also calculated from the virial route by conducting a short molecular dynamics simulation starting from the energy minimized structure. We show that the pressure obtained from our simulations is quite reasonable in the length of simulation time (in Monte Carlo steps) normally employed in our group. These results provide additional evidence for the equilibration of our model systems, and methodology to calculate the pressure in our lattice models. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An amorphous 50/50 atactic polypropylene (aPP)/isotactic polypropylene (iPP) mixture at 125 °C was simulated using a second nearest neighbor diamond lattice and a three states rotational isometric state model. The result suggests that at the liquidlike density that corresponds to the atmospheric pressure, aPP prefers to interact with other aPP chains rather than with iPP chains. The result is consistent with the inference of Keith and Padden [J. Appl. Phys. 35, 1286 (1964)] that aPP and iPP will tend to separate from one another in their melt at 125 °C, before the onset of crystallization of iPP. The tendency for immiscibility of the amorphous aPP/iPP blend is likely attributed to the presence of short syndiotactic sequences in the aPP chains adopting all-trans conformations. The attractive intermolecular interaction of pairs of such subchains at 125 °C promotes the separation of aPP from iPP. This interaction is weakened at higher temperature, where aPP and iPP become miscible. The result also shows that miscibility of the blend increases with increasing pressure. However, the origin of the pressure effect is not clear. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations using a modified Drieding 2.21 force field were carried out to study the coalescence behavior of nanometer-sized water droplets in vacuum and in n-heptane. The coalescence mechanisms of the water droplets in the above-noted environments are fairly similar in a sense that the water droplets form a bridge linking the droplets before they merge. However, in the latter situation, due to the presence of n-heptane molecules in between the water droplets, the coalescence was observed to be slowed down considerably, especially in the first 10 ps of the process. However, once the bridge is formed, the water droplets, in both situations, spend about the same amount of time to form a single droplet. The maximum distance between the droplets above which coalescence does not occur was found to be 10 Å. In terms of the dynamics, the diffusion coefficient of n-heptane in the emulsion system was very close to its value in the pure liquid form. This may be because n-heptane is the continuous phase. Nonetheless, the dynamic behavior of water in n-heptane is different from that of pure water during and after the coalescence. In particular, the self-diffusion coefficient of water molecules in n-heptane is about 20% higher than the experimental value of pure water. Due to the lack of strong attraction forces between water and n-heptane molecules, the n-heptane molecules were observed to orient themselves perpendicularly to the water/n-heptane interfaces so that the contacting area is minimized. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A three-dimensional lattice Boltzmann model is used for simulating the pulsating flows in a rectangular duct due to low, moderate, and high periodic pressure gradients. The studies have shown that stress is maximum in the middle of the duct walls and decreases to zero towards the corners, which has helped in designing and optimizing medical apparatuses and understanding the medical flow phenomena.

Surface tensions (gamma) of normal alkanes and methyl methacrylate (MMA) oligomers at various molecular weights in the low molecular weight range are calculated using a new molecular dynamics simulation strategy. The MD simulations, even with the use of a generic force field, reproduced the experimentally observed molecular weight dependence of gamma for both series of oligomers.

Molecular dynamics (MD) simulation is used to study the stability and molecular origin of nanometer-sized water clusters in n-heptane, with the full coverage of adsorbed 6-methyl-2-naphthatenepropionic acid at the water/n-heptane interfaces, at two temperatures. The results show that the coalescence of two water clusters fully covered by naphthenic acid molecules is a low-probability event even at evaluated temperatures and the mobility of naphthenic acids is enhanced with more n-heptane molecules present outside a water droplet.