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  • 1
    Electronic Resource
    Electronic Resource
    Anomalous thermal conductivity enhancement in nanotube suspensions (2001)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 79 (2001), S. 2252-2254 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have produced nanotube-in-oil suspensions and measured their effective thermal conductivity. The measured thermal conductivity is anomalously greater than theoretical predictions and is nonlinear with nanotube loadings. The anomalous phenomena show the fundamental limits of conventional heat conduction models for solid/liquid suspensions. We have suggested physical concepts for understanding the anomalous thermal behavior of nanotube suspensions. In comparison with other nanostructured materials dispersed in fluids, the nanotubes provide the highest thermal conductivity enhancement, opening the door to a wide range of nanotube applications. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1408272
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  • 2
    Electronic Resource
    Electronic Resource
    Effect of initial conformation, flow strength, and hydrodynamic interaction on polymer molecules in extensional flows (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 3397-3403 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Brownian dynamics simulations are used to study the unraveling process of polymer molecules in dilute solutions under strong elongational flows. We follow chain extension, segmental alignment, and viscosity contribution behavior of individual, randomly coiled, freely jointed bead–rod chain model molecules. In the absence of hydrodynamic shielding, segmental orientation at an intermediate strain rate begins only when aided by overall chain extension. However, at a very high strain rate, rapid initial segmental orientation and lateral chain compression precedes overall chain extension, resulting in the formation of sharp folds in most chains. Fold formation during the extension process is characterized by a sudden decrease in the rate of overall chain extension, an intermediate plateau in birefringence, and a disproportionately low chain end-to-end distance. Hydrodynamic screening generally slows down the uncoiling process, sometimes enough to avoid the formation of folded conformations. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1287175
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  • 3
    Electronic Resource
    Electronic Resource
    Stability of bicontinuous cubic phases in ternary amphiphilic systems with spontaneous curvature (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 3792-3802 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We study the phase behavior of ternary amphiphilic systems in the framework of a curvature model with nonvanishing spontaneous curvature. The amphiphilic monolayers can arrange in different ways to form micellar, hexagonal, lamellar, and various bicontinuous cubic phases. For the latter case we consider both single structures (one monolayer) and double structures (two monolayers). Their interfaces are modeled by the triply periodic surfaces of constant mean curvature of the families G, D, P, C(P), I-WP, and F-RD. The stability of the different bicontinuous cubic phases can be explained by the way in which their universal geometrical properties conspire with the concentration constraints. For vanishing saddle-splay modulus κ¯, almost every phase considered has some region of stability in the Gibbs triangle. Although bicontinuous cubic phases are suppressed by sufficiently negative values of the saddle-splay modulus κ¯, we find that they can exist for considerably lower values than obtained previously. The most stable bicontinuous cubic phases with decreasing κ¯〈0 are the single and double gyroid structures since they combine favorable topological properties with extreme volume fractions. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480529
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  • 4
    Electronic Resource
    Electronic Resource
    Hydrodynamic shielding induced stability of zipping macromolecules in elongational flows (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 6055-6061 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Enhanced shear stability of associating polymers during drag reduction observed recently has been attributed to the breakage of reversible associations (e.g., hydrogen bonds) in preference to covalent bonds in the polymer backbone. A simple mechanistic analysis of a perfectly "zipped'' assembly of fully extended bead rod chain model of two macromolecules in steady elongational flow is presented. It explains the enhanced stability as a result of (i) distribution (near the vulnerable chain center) of the drag tension into the two parallel "zipped'' chains, and (ii) reduction of the drag force due to the enhanced hydrodynamic shielding. Some guidelines for optimum design of shear stable and effective drag reducing macromolecules have been deduced.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467116
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  • 5
    Electronic Resource
    Electronic Resource
    Brownian dynamics simulation of a polymer molecule in solution under elongational flow (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 1610-1617 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We use Brownian dynamics simulation to study coil–stretch transition of macromolecules in solution. Into a simple elongational flow field, we introduce freely jointed bead-rod chain model molecules in their coiled and stretched states, and follow the conformational changes. We find good agreement of our simulation results with the available theoretical predictions for low and high strain rates (ε(overdot)). At the intermediate elongation rates (near the onset of coil–stretch transition) of the flow field, we find that the residence time required for stretching of an initially coiled chain can be extremely large as compared to predicted (1+ln((square root of)N))ε(overdot)−1, especially for the non-free-draining case. Hence, the chain conformation is dependent on the initial state of the chain molecule for residence time as long as 100ε(overdot)−1. Thus, hysteresis is predicted when chain residence time in such an elongational flow field is limited, as in practical situations. Further, at such intermediate ε(overdot), the chain molecule is seen to undergo Brownian fluctuation induced jumps between a randomly coiled state and another partially stretched state. This suggests the existence of more than one equilibrium conformation that is unstable to Brownian fluctuations. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475531
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  • 6
    Electronic Resource
    Electronic Resource
    NMR evidence of metal-support interaction in syngas conversion catalyst Co-TiO2 (1990)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 67 (1990), S. 5847-5849 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: To examine the relation between catalytic and magnetic properties, the zero-field NMR spectra and hysteresis loops of cobalt supported on silica, alumina, magnesia, titania, and ZSM-5 with and without the promoter thoria were investigated. Cobalt was incorporated on the support by simple physical admixture of precipitated cobalt and support, and by aqueous impregnation technique. Our studies indicate that the particle sizes are consistently lower in the presence of thoria. Of all the catalysts examined, the Co/Th/TiO2 catalyst exhibits a high saturation magnetization value—about 20% higher than pure cobalt. In addition, the NMR spectrum of the aqueous impregnation Co/TiO2 catalyst is distinctly different from the rest. All the NMR lines are shifted to a higher frequency by about 4 MHz. These two features—enhancement of the magnetic moment of cobalt atoms and increases in the hyperfine field at the Co nucleus—clearly indicate that there occurs strong metal-support interaction between cobalt and titania support. The higher hydrocarbon yields observed by the earlier investigators with Co/TiO2 catalysts might be related to this phenomenon.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.345985
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  • 7
    Electronic Resource
    Electronic Resource
    Simulation of diffusion-limited step-growth polymerization in 2D: Effect of shear flow and chain rigidity (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 3067-3074 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Multimolecule Brownian dynamics simulation results for diffusion controlled polymerization of bead–rod chain molecules in 2D solution are presented. Reaction between any two molecules undergoing Brownian diffusion takes place if the reactive chain ends approach each other to within a certain reaction radius, and if the chain end carrying segments are collinear within certain specified limits. The second order reaction rate constant is found to decrease with time as the molecular lengths increase and the diffusivities decrease. Application of a shear flow is seen to result in alignment of the molecules along the flow direction, thereby enhancing the concentration of molecular pairs with parallel orientation of reactive-end carrying chain segments, and hence the overall reaction rate. This effect is found to be more pronounced in the case of long rigid molecules as compared to flexible molecules because of the slow rotation and high level of orientation under flow of the former. Even the molecular weight distribution (MWD) obtained during polymerization may be affected. For example, longer molecules have lower diffusivities and hence lower reactivities, resulting in a narrower MWD in the absence of flow, as compared to the results with the usual assumption of molecular reactivity being independent of chain length. Furthermore, in the presence of an external flow, the longer molecules orient to a higher degree and hence display a higher enhancement in reactivity. This results in a wider MWD of the polymer. The simulation results are in qualitative agreement with previous experimental data for solution polymerization of rod-like molecules.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466195
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  • 8
    Electronic Resource
    Electronic Resource
    Competition effects in surface diffusion controlled reactions: Theory and Brownian dynamics simulations (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 9237-9247 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Surface diffusion controlled reactions on a heterogeneous catalyst surface comprising randomly placed circular reactive sites are considered. The diffusing species adsorbs onto the surface following Langmuir–Hinshelwood kinetics and reacts instantaneously on contact with a reactive site. Approximate theories are formulated to describe the process for high concentrations of the reactive sites, when competition between the sites is significant, following three different approaches: (i) modification of the single sink theory; (ii) using a cell model; and (iii) using an effective medium theory. The predictions of the theories are compared with the results of multiparticle Brownian dynamics simulations for the overall reaction rate, the bulk concentration of the reactive species, and the ensemble averaged concentration profile around a reactive site. The effective medium theory is found to give the best results among the theories considered, and the predictions are in good agreement with the computational results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465540
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  • 9
    Electronic Resource
    Electronic Resource
    Diffusion-limited polymerization of rigid rodlike molecules: Dilute solutions (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 7125-7134 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A theoretical investigation of rates of diffusion-limited polymerization between rigid rodlike molecules in dilute solutions is presented, incorporating both translational and rotational diffusion of the molecules. Simplified criteria for reaction are proposed according to which molecules must approach within a specified reaction radius, and have a relative orientation angle in a specified range to react. The translational diffusion of the molecules is assumed to be isotropic. The results of the exact analytical solution, finite element computations, as well as approximate solutions for low rotational diffusivity, are presented. The match between the solutions is satisfactory in the appropriate limits, although the exact solution is found to be computationally intensive for low rotational diffusivities and rapid intrinsic kinetics. The reaction rate is found to decrease to a limiting value with decreasing rotational diffusivity, and the magnitude of the decrease in rate increases with intrinsic rate of reaction. The limiting value of rate is entirely due to the translational diffusive flux of the correctly oriented molecules from the far field, and an asymptotic expression for the rate in this limit is given. Comparison with reported experimental data is qualitative, since the data is only available for the semidilute regime for such systems, where the anisotropic translational diffusion is expected to play a prominent role.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462546
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  • 10
    Electronic Resource
    Electronic Resource
    Diffusion-limited polymerization of rigid rodlike molecules: Semidilute solutions (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 1382-1392 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The rotational diffusivity and the translational diffusivity perpendicular to the rod axis of rigid rodlike (RRL) molecules decreases rapidly with increasing molecular length, during polymerization in semidilute solutions. This can result in slowing of step-growth polymerizations of RRL molecules with reactive groups at the rod ends and a near-collinearity requirement for reaction. Here a theoretical analysis of the rate of RRL polymerization in semidilute solutions, based on Smoluchowski's approach, and incorporating the rotational and anisotropic translational diffusion of the molecules is presented. The work is an extension of our analysis of polymerization in dilute solutions in which the translational diffusion was assumed to be isotropic [J. Chem. Phys. 96, 7125 (1992)]. The effective second order rate constant for the system is obtained for different parameter values using a numerical finite element method. With reduction in rotational diffusivity, for a fixed translational diffusivity, the effective reaction rate constant is found to decrease to a limiting value determined by only the translational flux of the correctly oriented molecules. Similarly, for a given rotational diffusivity, with reduction in translational diffusivity perpendicular to the rod axis, the reaction rate constant is found to decrease to a limiting value determined by the flux only due to translational diffusion parallel to the rod axis, aided by rotational diffusion. An asymptotic analysis for this case is presented. For low rotational diffusivities, reduction in the translational diffusion perpendicular to the rod axis results in a significant decrease in the effective rate constant, even for reactions with relatively slow intrinsic kinetics. A qualitative comparison of the theoretical predictions with experimental results is presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465382
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