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Benjamin P. Lee

Publications and source records attributed to Benjamin P. Lee.

11 recordsLinked to original sources
arXiv1999

Filler-Induced Composition Waves in Phase-Separating Polymer Blends

The influence of immobile filler particles (spheres, fibers, platelets) on polymer blend phase separation is investigated computationally using a generalization of the Cahn-Hilliard-Cook (CHC) model. Simulation shows that the selective affinity of one of the polymers for the filler surface leads to the development of concentration waves about the filler particles at an early stage of phase separation in near critical composition blends. These "target" patterns are overtaken in late stage phase separation by a growing "background" spinodal pattern characteristic of blends without filler particles. The linearized CHC model is used to estimate the number of composition oscillations emanating from isolated filler particles. In far-off-critical composition blends, an "encapsulation layer" grows at the surface of the filler rather than a target pattern. The results of these simulations compare favorably with experiments on filled phase separating blend films.

cond-mat.stat-mech↗
arXiv1999

Fluctuation Effects in Steric Reaction-Diffusion Systems

We propose a simple model for reaction-diffusion systems with orientational constraints on the reactivity of particles, and map it onto a field theory with upper critical dimension d_c=2. To two-loop level the long-time particle density N(t) is given by the same universal expression as for a nonsteric system, with N(t)~t^{-d/2} for d<=2. For slow rotations of the particles we find an intermediate regime where N(t)~t^{-d/4}, with the crossover to the nonsteric asymptotics determined by the rates of rotations and reactions. Consequences for experiments are discussed.

cond-mat.stat-mech↗
arXiv1998

Persistence, Poisoning, and Autocorrelations in Dilute Coarsening

We calculate the exact autocorrelation exponent lambda and persistence exponent theta, and also amplitudes, in the dilute limit of phase ordering for dimensions d >= 2. In the Lifshitz-Slyozov-Wagner limit of conserved order parameter dynamics we find theta = gamma_d*epsilon, a universal constant times the volume fraction. For autocorrelations, lambda = d at intermediate times, with a late time crossover to lambda >= d/2 + 2. We also derive lambda and theta for globally conserved dynamics and relate these to the q->infinity -state Potts model and soap froths, proposing new poisoning exponents.

cond-mat.stat-mech↗
arXiv1997

Critique of Primitive Model Electrolyte Theories

Approximate theories for the restricted primitive model electrolyte are compared in the light of Totsuji's lower bound for the energy (an improvement over Onsager's), Gillan's upper bound for the free energy, and thermal stability requirements. Theories based on the Debye-Hueckel (DH) approach and the mean spherical approximation (MSA), including extensions due to Bjerrum, Ebeling, Fisher and Levin, and Stell, Zhou, and Yeh (PMSA1,2,3) are tested. In the range T* = (k_B T)Da/q^2 \lesssim 10 T*_c \simeq 0.5, all DH-based theories satisfy Totsuji's bound, while the MSA possesses a significant region of violation. Both DH and MSA theories violate Gillan's bound in the critical region and below unless ion pairing and the consequent free-ion depletion are incorporated. However, the PMSA theories, which recognize pairing but not depletion, fail to meet the bound. The inclusion of excluded-volume terms has only small effects in this respect. Finally, all the pairing theories exhibit negative constant-volume specific heats when T* \gtrsim 2T*_c \simeq 0.1; this is attributable to the treatment of the association constant.

cond-mat.stat-mech↗
arXiv1997

Charge Oscillations in Debye-Hueckel Theory

The recent generalized Debye-Hueckel (GDH) theory is applied to the calculation of the charge-charge correlation function G_{ZZ}(r). The resulting expression satisfies both (i) the charge neutrality condition and (ii) the Stillinger-Lovett second-moment condition for all T and rho_N, the overall ion density, and (iii) exhibits charge oscillations for densities above a "Kirkwood line" in the (rho_N,T) plane. This corrects the normally assumed DH correlations, and, when combined with the GDH analysis of the density correlations, leaves the GDH theory as the only complete description of ionic correlation functions, as judged by (i)-(iii), (iv) exact low-density (rho_N,T) variation, and (v) reasonable behavior near criticality.

cond-mat.stat-mech↗
arXiv1996

Ginzburg Criterion for Coulombic Criticality

To understand the range of close-to-classical critical behavior seen in various electrolytes, generalized Debye-Hueckel theories (that yield density correlation functions) are applied to the restricted primitive model of equisized hard spheres. The results yield a Landau-Ginzburg free-energy functional for which the Ginzburg criterion can be explicitly evaluated. The predicted scale of crossover from classical to Ising character is found to be similar in magnitude to that derived for simple fluids in comparable fashion. The consequences in relation to experiments are discussed briefly.

cond-mat↗
arXiv1996

Density Fluctuations in an Electrolyte from Generalized Debye-Hueckel Theory

Near-critical thermodynamics in the hard-sphere (1,1) electrolyte is well described, at a classical level, by Debye-Hueckel (DH) theory with (+,-) ion pairing and dipolar-pair-ionic-fluid coupling. But DH-based theories do not address density fluctuations. Here density correlations are obtained by functional differentiation of DH theory generalized to {\it non}-uniform densities of various species. The correlation length $ξ$ diverges universally at low density $ρ$ as $(Tρ)^{-1/4}$ (correcting GMSA theory). When $ρ=ρ_c$ one has $ξ\approxξ_0^+/t^{1/2}$ as $t\equiv(T-T_c)/T_c\to 0+$ where the amplitudes $ξ_0^+$ compare informatively with experimental data.

cond-mat↗
arXiv1995

Renormalization Group Study of the A+B->0 Diffusion-Limited Reaction

The $A + B\to 0$ diffusion-limited reaction, with equal initial densities $a(0) = b(0) = n_0$, is studied by means of a field-theoretic renormalization group formulation of the problem. For dimension $d > 2$ an effective theory is derived, from which the density and correlation functions can be calculated. We find the density decays in time as $a,b \sim C\sqrt{\D}(Dt)^{-d/4}$ for $d < 4$, with $\D = n_0-C^\prime n_0^{d/2} + \dots$, where $C$ is a universal constant, and $C^\prime$ is non-universal. The calculation is extended to the case of unequal diffusion constants $D_A \neq D_B$, resulting in a new amplitude but the same exponent. For $d \le 2$ a controlled calculation is not possible, but a heuristic argument is presented that the results above give at least the leading term in an $ε= 2-d$ expansion. Finally, we address reaction zones formed in the steady-state by opposing currents of $A$ and $B$ particles, and derive scaling properties.

cond-mat↗
arXiv1994

Scaling of Reaction Zones in the A+B->0 Diffusion-Limited Reaction

We study reaction zones in three different versions of the A+B->0 system. For a steady state formed by opposing currents of A and B particles we derive scaling behavior via renormalization group analysis. By use of a previously developed analogy, these results are extended to the time-dependent case of an initially segregated system. We also consider an initially mixed system, which forms reaction zones for dimension d<4. In this case an extension of the steady-state analogy gives scaling results characterized by new exponents.

cond-mat↗
arXiv1993

Renormalization Group Calculation for the Reaction $kA\rightarrow\emptyset$

The diffusion-controlled reaction $kA\rightarrow\emptyset$ is known to be strongly dependent on fluctuations in dimensions $d\le d_c=2/(k-1)$. We develop a field theoretic renormalization group approach to this system which allows explicit calculation of the observables as expansions in $ε^{1/(k-1)}$, where $ε=d_c-d$. For the density it is found that, asymptotically, $n\sim A_k t^{-d/2}$. The decay exponent is exact to all orders in $ε$, and the amplitude $A_k$ is universal, and is calculated to second order in $ε^{1/(k-1)}$ for $k=2,3$. The correlation function is calculated to first order, along with a long wavelength expansion for the second order term. For $d=d_c$ we find $n \sim A_k (\ln t/t)^{1/(k-1)}$ with an exact expression for $A_k$. The formalism can be immediately generalized to the reaction $kA\rightarrow\ell A$, $\ell < k$, with the consequence that the density exponent is the same, but the amplitude is modified.

cond-mat↗