Biomedical subjects

Sanat K Kumar

Publications and source records attributed to Sanat K Kumar.

13 recordsLinked to original sources

PubMed2006

Multiscale modeling of polymer rheology.

We propose a simulation method which can be used to readily parallelize simulations on systems with a large spatial extent. We simulate small parts of the system with independent molecular dynamics simulations, and only occasionally pass information between them through a constitutive model free continuum approach. We illustrate the power of this method in the case of a polymeric fluid undergoing rapid one-dimensional shear flow. Since we show that this flow problem cannot be modeled by using a steady-state constitutive model, this method offers the unique capability for accessing the nonlinear viscoelasticity of complex fluids.

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PubMed2006

Nature of the breakdown in the Stokes-Einstein relationship in a hard sphere fluid.

Molecular dynamics simulations of high density hard sphere fluids clearly show a breakdown of the Stokes-Einstein equation (SE). This result has been conjectured to be due to the presence of mobile particles, i.e., ones which have the propensity to "hop" distances that are integer multiples of the interparticle distance. We conclusively show that the sedentary particles, i.e., ones complementary to the "hoppers," obey the SE relationship to a good approximation, even though the fluid as a whole violates the SE equation at high densities. These results support the notion that the unusual diffusive behavior of supercooled liquids is dominated by the hopping particles.

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PubMed2006

Analysis of uncertainties in polymer viscoelastic properties obtained from equilibrium computer simulations.

We critically evaluate the uncertainties in the stress autocorrelation function obtained from equilibrium molecular dynamics simulation of model polymer melts. This quantity is central to evaluating transport properties, e.g., the complex modulus and the viscosity. In contrast to the intuitive expectation that simulations have to be run five to six orders of magnitude longer than the chain relaxation time to reduce uncertainties to acceptable levels, our analysis shows that the majority of the uncertainty is associated with rapidly oscillating bonded interactions. These fluctuations occur on time scales which are approximately 10(4) times shorter than the relaxation time of a chain of length 80. Consequently, the effects of these oscillations on the stress autocorrelation function can be dramatically reduced by (i) conducting long simulations (typically 10(6) times longer than the bond relaxation times or only 10(2) chain relaxation times) and (ii) by performing running averages with time windows whose time scales are much longer than these oscillations. Conducting such long simulations also allows for the accurate determination of the melt viscosity and the low-frequency complex modulus, but performing running averages do not impact these quantities since they are time integrals of the stress autocorrelation function.

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PubMed2006

Molecular-dynamics simulations of the transport properties of a single polymer chain in two dimensions.

Molecular-dynamics simulations are conducted to elucidate the critical factors affecting the transport properties of isolated polymer chains in strictly two dimensions. The relevance of surface inhomogeneity is critically examined. We unequivocally find that surface inhomogeneity is critical in obtaining transport behavior consistent with the recent measurements of surface diffusion for polymers adsorbed at the solid-liquid interface. For a systematic investigation of this point, heterogeneity was introduced by decorating the surface with impenetrable elements and we find that chain diffusivity crossed over from Rouse-type behavior to reptationlike with increasing surface coverage of obstacles. This transition in behavior occurred when the mean distance between obstacles is approximately equal to the end-to-end distance, Re, of the two-dimensional chain. Our results underscore the importance of surface disorder (not only literal obstacles but by reasonable extension also to other types of disorder) in determining the transport behavior of chains adsorbed to solids.

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PubMed2005

Quantitative equivalence between polymer nanocomposites and thin polymer films.

The thermomechanical responses of polymers, which provide limitations to their practical use, are favourably altered by the addition of trace amounts of a nanofiller. However, the resulting changes in polymer properties are poorly understood, primarily due to the non-uniform spatial distribution of nanoparticles. Here we show that the thermomechanical properties of 'polymer nanocomposites' are quantitatively equivalent to the well-documented case of planar polymer films. We quantify this equivalence by drawing a direct analogy between film thickness and an appropriate experimental interparticle spacing. We show that the changes in glass-transition temperature with decreasing interparticle spacing for two filler surface treatments are quantitatively equivalent to the corresponding thin-film data with a non-wetting and a wetting polymer-particle interface. Our results offer new insights into the role of confinement on the glass transition, and we conclude that the mere presence of regions of modified mobility in the vicinity of the particle surfaces, that is, a simple two-layer model, is insufficient to explain our results. Rather, we conjecture that the glass-transition process requires that the interphase regions surrounding different particles interact.

Elasticity↗

PubMed2005

Molecular dynamics simulations of polymer transport in nanocomposites.

Molecular dynamics simulations on the Kremer-Grest bead-spring model of polymer melts are used to study the effect of spherical nanoparticles on chain diffusion. We find that chain diffusivity is enhanced relative to its bulk value when polymer-particle interactions are repulsive and is reduced when polymer-particle interactions are strongly attractive. In both cases chain diffusivity assumes its bulk value when the chain center of mass is about one radius of gyration R(g) away from the particle surface. This behavior echoes the behavior of polymer melts confined between two flat surfaces, except in the limit of severe confinement where the surface influence on polymer mobility is more pronounced for flat surfaces. A particularly interesting fact is that, even though chain motion is strongly speeded up in the presence of repulsive boundaries, this effect can be reversed by pinning one isolated monomer onto the surface. This result strongly stresses the importance of properly specifying boundary conditions when the near surface dynamics of chains are studied.

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PubMed2005

Mesoscale model of polymer melt structure: self-consistent mapping of molecular correlations to coarse-grained potentials.

Development and application of coarse-graining methods to condensed phases of macromolecules is an active area of research. Multiscale modeling of polymeric systems using coarse-graining methods presents unique challenges. Here we apply a coarse-graining method that self-consistently maps structural correlations from detailed molecular dynamics (MD) simulations of alkane oligomers onto coarse-grained potentials using a combination of MD and inverse Monte Carlo methods. Once derived, the coarse-grained potentials allow computationally efficient sampling of ensemble of conformations of significantly longer polyethylene chains. Conformational properties derived from coarse-grained simulations are in excellent agreement with experiments. The level of coarse graining provides a control over the balance of computational efficiency and retention of chemical identity of the underlying polymeric system. Challenges to extension and application of this and similar structure-based coarse-graining methods to model dynamics and phase behavior in polymeric systems are briefly discussed.

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PubMed2004

Multiscale modeling of the surfactant mediated synthesis and supramolecular assembly of cobalt nanodots.

Multiscale simulations are used to bridge the surfactant templated assembly of individual approximately 1-10 nm cobalt dots, to their ordering into supramolecular arrays. Potential energy surfaces derived from ab initio calculations are input to lattice Monte Carlo simulations at atomic scales. By this process we quantitatively reproduce the experimental cobalt nanoparticle sizes. Crucially, we find that there is an effective short range attraction between pairs of nanodots. Mesoscale simulations show that these attractive interdot potentials are so short ranged that the dots can assemble only into orientally ordered hexatic phases as in the experiments.

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PubMed2004

Effect of the hydrophilic size on the structural phases of aqueous nonionic gemini surfactant solutions.

Aggregate structures of aqueous nonionic Gemini surfactant solutions, alpha,alpha'-[2,4,7,9-tetramethyl-5-decyne-4,7-diyl]bis[omega-hydroxyl-polyoxyethylene] with three different length polyoxyethylenes (i.e., 10, 20, and 30 ethylene oxide monomers, denoted from now on as S-10, S-20, and S-30, respectively), are investigated using small angle neutron scattering, dynamic light scattering, and fluorescence spectroscopy. For S-10 at low surfactant concentrations (Cs < 0.9 wt %), large "clusters", with an average hydrodynamic radius (RH) > 40 nm, are found to coexist with monomers. At intermediate Cs (0.9 < Cs < 2 wt %), some clusters break down forming micelles, with an (RH) approximately 2-3 nm, while the remaining clusters coexist with micelles. Increasing Cs further (>2 wt %) results in a pure micellar phase with little or no clusters present. S-20 and S-30 mixtures, on the other hand, differ from S-10 in that irrespective of surfactant concentration, large clusters and small monomers/dimers are found to coexist, while there is no direct evidence for the presence of micelles.

Light↗

PubMed2004

Modeling diffusion in miscible polymer blend films.

Recent experiments designed to probe polymer transport in the bulk and in the vicinity of surfaces have examined the interdiffusion of multilayer sandwiches of isotopically labeled polymers. The measured time dependent concentration profiles normal to the surface are typically fit to Fick's law, with a single fitting parameter, the mutual binary diffusion coefficient (MBDC). The resulting MBDCs are found to vary over a broad range of film thicknesses and time, with the time dependence being viewed as a unique signature of the reptation mechanism of long chain motion, and the thickness dependence being attributed to the slowing down of chain dynamics near surfaces. Since the experiments are conducted at finite concentration, the MBDC, which is a product of the bare mobility and the concentration derivative of the chemical potential, could be dominated by the time and thickness dependence of this second term (which is ignored in Fick's law). To quantify this conjecture we consider the more rigorous Cahn formulation of the diffusion problem in terms of chemical potential gradients. We use square gradient theory to evaluate chemical potentials, and fit the resulting time dependent concentration profiles to the analytical solution of Fick's law. By thus mimicking the experimental analysis we find that the apparent MBDCs vary with time as t(-1/2) at short times, in good agreement with existing experiments. We show that this time dependence reflects the system's desire to minimize concentration gradients, a fact ignored in Fick's law. Since these arguments make no reference to the mechanism of chain motion, we argue that the time dependence of MBDC derived from interdiffusion experiments does not provide unequivocal support for the reptation mechanism of long chain transport. The MBDC values, which also vary with the degree of confinement, are predicted to increase with decreasing thickness for model parameters corresponding to experimental systems. In contrast, since the experimental fits yield an opposite trend, we suggest that the bare mobility of the chains decreases strongly with decreasing thickness. These findings strongly support the idea that the chains are "pinned" irreversibly to the surfaces, in good agreement with other, independent experiments.

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PubMed2003

Monte carlo study of coulombic criticality in polyelectrolytes.

The role of charges in determining the water solubility of polyelectrolytes, a question of considerable relevance to biology, is currently unresolved. We use computer simulations to study the purely Coulombic phase separation of flexible polyelectrolytes with monovalent counterions in an athermal solvent. In agreement with recent theories we find that the critical temperature for this transition increases with chain length, but that the critical density remains unchanged. We therefore stress that the phase behavior of polyelectrolytes is qualitatively different from uncharged polymers, where the critical density decreases towards zero for long chains.

Computer Simulation↗

PubMed2003

Evidence for dynamic heterogeneities in computer simulations of miscible polymer blends.

The controversial origins of the unusual dynamics of miscible polymer blends are incisively probed through computer simulations. The distribution of mobilities experienced by a probe monomer in a miscible blend of chains with disparate glass transition temperatures is found to be much broader than in the pure polymers, providing clear evidence for local concentration variations in the mixture. These concentration fluctuations yield distinctly different temperature dependences for the dynamics of the two different components, in a manner that closely mimics experiments.

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PubMed2002

Amorphous solidification in polymer-platelet nanocomposites.

Computer simulations are used to understand the molecular basis of the rheology changes in polymer melts when loaded with platelet filler particles, specifically when the polymer and nanofiller interact attractively. With decreasing temperature, there is increasing aggregation between chains and filler and an increase in the polymer matrix structural relaxation time. These lifetimes are predicted to diverge at an extrapolated temperature, which we identify with the emergence of an amorphous solid state. Our findings suggest that filled polymers are phenomenologically similar to solutions of associating polymers and to supercooled liquids near their glass transition.

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