C. Sasmal

Direct observation of DNA dynamics in semidilute solutions in extensional flow

The dynamic behavior of semidilute polymer solutions is governed by an interplay between solvent quality, concentration, molecular weight, and flow type. Semidilute solutions are characterized by large fluctuations in polymer concentration, wherein polymer coils interpenetrate but may not be topologically entangled at equilibrium. In nonequilibrium flows, it is generally thought that polymer chains can “self-entangle” in semidilute solutions, thereby leading to entanglements in solutions that are nominally unentangled at equilibrium. Despite recent progress in the field, we still lack a complete molecular-level understanding of the dynamics of polymer chains in semidilute solutions. In this work, we use single molecule techniques to investigate the dynamics of dilute and semidilute solutions of λ-phage deoxyribonucleic acid in planar extensional flow, including polymer relaxation from high stretch, transient stretching dynamics in step-strain experiments, and steady-state stretching in flow. Our results a...

Parameter-free prediction of DNA dynamics in planar extensional flow of semidilute solutions

The dynamics of individual Deoxyribonucleic acid (DNA) molecules in semidilute solutions undergoing planar extensional flow is simulated using a multiparticle Brownian dynamics algorithm, which incorporates hydrodynamic and excluded volume interactions in the context of a coarse-grained bead-spring chain model for DNA. The successive fine-graining protocol [P. Sunthar and J. R. Prakash, Macromolecules 38, 617–640 (2005); R. Prabhakar et al., J. Rheol. 48, 1251–1278 (2004)], in which simulation data acquired for bead-spring chains with increasing values of the number of beads Nb, is extrapolated to the number of Kuhn steps NK in DNA (while keeping key physical parameters invariant), is used to obtain parameter-free predictions for a range of Weissenberg numbers and Hencky strain units. A systematic comparison of simulation predictions is carried out with the experimental observations of Hsiao et al. [J. Rheol. (in press)], who have recently used single molecule techniques to investigate the dynamics of dil...

Laminar Free Convection in Power-Law Fluids from a Heated Hemisphere

Extensive results are presented on the laminar free convection heat transfer in power-law fluids from a heated hemisphere in two orientations, namely, its flat base oriented upward (inverted) or downward (upright). The coupled field equations have been numerically solved over wide ranges of conditions as follows: Grashof number (10≤Gr≤105), Prandtl number (0.72≤Pr≤100) and power-law index (0.3≤n≤1.5). Detailed flow and temperature fields are visualized in terms of the streamline and isotherm contours, respectively. At the next level, the results are analyzed in terms of the total drag coefficient and local Nusselt number variation along the surface of the hemisphere, together with its surface averaged value. The average Nusselt number increases with both the Grashof and Prandtl numbers. Furthermore, for fixed values of the Grashof and Prandtl numbers, and for a given orientation, shear-thinning behavior (n 1) impedes it with reference to th...

Brownian dynamics simulations of planar mixed flows of polymer solutions at finite concentrations

Abstract Periodic boundary conditions for planar mixed flows are implemented in the context of a multi-chain Brownian dynamics simulation algorithm. The effect of shear rate γ , and extension rate ϵ , on the size of polymer chains, 〈 R e 2 〉 , and on the polymer contribution to viscosity, η , is examined for solutions of FENE dumbbells at finite concentrations, with excluded volume interactions between the beads taken into account. The influence of the mixedness parameter, χ , and flow strength, Γ , on 〈 R e 2 〉 and η , is also examined, where χ → 0 corresponds to pure shear flow, and χ → 1 corresponds to pure extensional flow. It is shown that there exists a critical value, χ c , such that the flow is shear dominated for χ χ c , and extension dominated for χ > χ c .

Effect of stretching-induced changes in hydrodynamic screening on coil-stretch hysteresis of unentangled polymer solutions

Extensional rheometry and Brownian Dynamics simulations of flexible polymer solutions confirm predictions based on blob concepts that coil--stretch hysteresis in extensional flows increases with concentration, reaching a maximum at the critical overlap concentration

Effect of aspect ratio on natural convection in power-law liquids from a heated horizontal elliptic cylinder

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Effect of orientation on laminar natural convection from a heated square cylinder in power-law liquids

before progressively vanishing in the semidilute regime. These observations demonstrate that chain stretching strengthens intermolecular hydrodynamic screening in dilute solutions, but weakens it in semidilute solutions. Flow can thus strongly modify the concentration dependence of viscoelastic properties of polymer solutions.