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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 997-1008 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structural and dynamical changes that take place in simple liquids and glasses during extreme shear thinning are investigated by nonequilibrium molecular dynamics (NEMD). Simulations performed on the Lennard-Jones liquid at states close to the triple point reveal long range order accompanying extreme shear thinning which is characterized by linear lines of molecules spontaneously forming along the shear flow lines. These "strings'' of molecules pack into a triangular lattice. The density of strings increases with shear rate, as does the difference in density in various directions with respect to the flow field. Local structural asymmetry with lifetimes comparable to the simulation are observed at the highest shear rates considered, leading eventually to structural disorder via dislocations in the triangular lattice.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 117 (2002), S. 2377-2388 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Brownian dynamics simulations have been carried out to model attractive polymers in solution. Bead–spring polymer chains with attractions between the end beads were modeled over a wide concentration range on either side of the overlap concentration, ρ*, for the corresponding random coil polymers. The polymers were treated as beads linked by finitely extensible nonlinear elastic springs and the excluded volume repulsion between unlinked beads was represented by a pair potential with a Gaussian analytic form. For the sticky end-beads the potential has an attractive tail of Gaussian form. In addition to chains with purely repulsive bead–bead nonbonded interactions, three different systems with attactive end beads were modeled. There were those with (a) head–head (H–H) attractions, (b) with both H–H and tail–tail (T–T) attractions, and (c) with head–tail (H–T) attractions. The dimensions of the chains, the bead–bead radial distribution functions, as well as the dynamic properties such as stress tensor time-correlation functions, infinity frequency elastic modulus, and specific viscosity of the solution were calculated as functions of solution density and end-bead attraction class. We show that with the three classes of attractive end-bead functionality, the model polymers all depart from random coil statistics and show evidence of enhanced association, even in the dilute regime, especially for the H–T systems, which can form necklacelike structures at low dilution and micellelike states with increasing concentration. (Not all of the polymer statistics measures show major differences though.) However, only the rheology if the H–T system is markedly different from the random coil case. The rheology of the H–T system is quite different in qualitative and quantitative behavior to the other classes studied. There is a progressive retardation and increasingly near-exponential decay in the shear stress relaxation function. The viscosity of the H–T class of polymers is typically at least an order of magnitude higher than that of the others, even at concentrations far below the overlap concentration ρ* for such polymers. The infinite frequency elastic modulus is also typically about five times larger for the H–T class across the density range when compared with the other three types modeled. © 2002 American Institute of Physics.
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 10694-10705 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Brownian dynamics simulations have been carried out for model polymer chains in a good solvent over a wide concentration range. The polymers were treated as beads linked by finitely extensible nonlinear elastic (FENE) springs and the repulsion between any two unlinked beads was modeled by a pair potential with a Gaussian analytic form, βu(r)=A exp(−r2/σ2), where β=1/kT, A and σ are characteristic energy and distance scales, respectively. The effects of the bead–bead repulsion on the structure and time-correlation functions of the chains in the polymer solution were studied as a function of polymer concentration. Three concentration regimes are distinguished, a dilute region where intrachain bead–bead repulsions dominate, a concentrated region where interchain bead–bead repulsions dominate, and a highly concentrated region where the net repulsion on any bead tends to zero owing to substantial cancellation of the effects from nearest neighbors. The pair radial distribution function, the relaxation time for the rotation of the coil, the mean square displacement of the middle-bead and that of the center of mass of the chain, the infinity frequency elastic modulus, and the viscosity of the system are examined in all the density regions. Our results show that the excluded volume repulsion strongly affects the behavior of the system in the concentrated region and that the structural features return to the Rouse-limit behavior at high density more rapidly than the dynamical properties. © 1999 American Institute of Physics.
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 7567-7577 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Brownian dynamics, (BD), simulation has been used to follow the phase separation of Lennard-Jones-type particles quenched from a supercritical state point into the vapor–liquid or vapor–solid co-existence parts of their phase diagrams. Calculations were performed with spherical particles interacting via 12:6, 24:12, and 36:18 interaction laws at subcritical temperatures and low-volume fractions (φ≤0.2). Structural properties were followed as the systems evolved using pictures of the configurations, radial distribution function, and the low-angle scattering peak of the structure factor. The time dependence of the interaction energy was also followed. The scaling behavior of these quantities as a function of time was found to be similar to that observed in light scattering experiments during the phase separation of real colloidal systems. The aggregate structure that developed with time was sensitive to the range of the attractive part of the potential and its underlying phase diagram (the 36:18 system does not have a liquid phase). The 12:6 systems soon formed compact structures, whereas the systems generated using the shorter-ranged potentials persisted in a more diffuse, tenuous network for the duration of the simulations. Apart from at very short times for all potential laws, the only convincing evidence for a long-lived fractal structure was for the 36:18 systems at the lowest quench temperatures (kT/ε=0.3, where ε is the depth of the potential). The local structure in the dense regions of the network was sensitive to the range of the potential, exhibiting in the vapor–solid co-existence part of the phase diagram glassylike features for the 12:6 systems and crystalline local order for the 24:12 and 38:18 states. The 12:6 systems close to the metastable region of the vapor–liquid two-phase part of the phase diagram exhibited latency in the appearance and growth of the small angle scattering peak. The 24:12 and 36:18 systems also displayed latency at the higher temperatures both in the growth of the peak height and its movement to lower scattering vectors. © 1998 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1749-6632
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Natural Sciences in General
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 9039-9049 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular dynamics simulations have been performed on a system of flexible extended simple point charge (SPC/E) model water molecules which include intramolecular stretching and bending terms. A series of densities, 0.70, 0.80, 0.90, and 1.00 g cm−3 was simulated at 298 K, thereby investigating the early stages of bulk water "stretching" leading to cavitation. The local structural changes were followed using atom-resolved pair radial distribution functions which revealed that the local water structure deforms inhomogeneously as the density decreases below 1.00 g cm−3. Snapshots of the configurations and the radial distribution functions revealed that the decrease in density was accommodated in the sample by the appearance of "cavities" coexisting with clusters of more bulklike water but which were increasingly strained as the density decreased. Power spectra in the region of 0–5000 cm−1 were obtained from bond length, bond angle, and atom velocity autocorrelation functions. Combined with the power spectrum derived from the hydrogen atom velocity autocorrelation function, the vibrational frequencies of the gas and liquid states and the spectral features have been more clearly assigned. The power spectra are largely dominated by these near normal bulklike regions, which explains their observed insensitivity to density in the wide range studied. Another sequence of simulations was carried out at temperatures of 259, 273, and 350 K at a density of 1.00 g cm−3. © 1998 American Institute of Physics.
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 1963-1969 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Analytic expressions for the thermodynamic properties and elastic moduli of molecular fluids interacting with steeply repulsive potentials are derived using Rowlinson's hard-sphere perturbation treatment which employs a softness parameter, λ specifying the deviation from the hard-sphere potential. Generic potentials of this form might be used to represent the interactions between near-hard-sphere stabilized colloids. Analytic expressions for the equivalent hard-sphere diameter of inverse power [ε(σ/r)n where ε sets the energy scale and σ the distance scale] exponential and logarithmic potential forms are derived using the Barker–Henderson formula. The internal energies in the hard-sphere limit are predicted essentially exactly by the perturbation approach when compared against molecular dynamics simulation data using the same potentials. The elastic moduli are similarly accurately predicted in the hard-sphere limit, as they are trivially related to the internal energy. The compressibility factors from the perturbation expansion do not compare as favorably with simulation data, and in this case the Carnahan–Starling equation of state prediction using the analytic effective hard-sphere diameter would appear to be a preferable route for this thermodynamic property. A more refined state point dependent definition for the effective hard-sphere diameter is probably required for this property. © 1997 American Institute of Physics.
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 6096-6100 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Strongly sheared colloids often form strings oriented along the shear direction, which in turn often order into a quasi-2D hexagonal lattice. We examine this ordering process in detail to elucidate its strong system-size dependence, which is a major obstacle to establishing a connection between simulation and experiment. We catalog the possible lattices and use an energy criterion to predict which one a particular system is likely to crystallize in. The process is similar to nucleation and growth in a true 2D system except that strings crystallize without point defects (vacancies and interstitials), unlike 2D disks, because particle (string) number is not conserved.
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  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 2149-2153 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We examine the infinite frequency elastic moduli of steeply repulsive inverse power, r−n, potential fluids. Using molecular dynamics simulation we show that these are proportional to n and therefore diverge in the hard-sphere n→∞ limit, which we also prove independently for the case of hard spheres.
    Type of Medium: Electronic Resource
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