arXiv subjects

Jack F. Douglas

Publications and source records attributed to Jack F. Douglas.

41 records · Page 3Linked to original sources

arXiv2003

Structural Signatures of Vitrification in Hard Core Fluids

Computer simulations of hard spheres and disks are used to estimate the most probable cavity size, $ξ_{\rm cavity}$, and a ``rattle'' size, $ξ_{\rm rattle}$, over which a particle can translate holding all other particles fixed. Both of these measures of free volume appear to extrapolate to zero at the random close packed density, $ρ_{\rm rcp}$, close to the density where extrapolations of the viscosity diverge. We also identify the onset of caging as the density at which cavities cluster. These results suggest that vitrification in hard core fluids can be viewed as a geometrical phenomenon, and that {\em local} free volume measures can identify the location of the onset of liquid-like dynamics, the complex dynamics of caged liquids, and vitrification.

cond-mat.soft↗

arXiv2002

What do we learn from the local geometry of glass-forming liquids?

We examine the local geometry of a simulated glass-forming polymer melt. Using the Voronoi construction, we find that the distributions of Voronoi volume $P(v_V)$ and asphericity $P(a)$ appear to be universal properties of dense liquids, supporting the use of packing approaches to understand liquid properties. We also calculate the average free volume $ $ along a path of constant density and find that $ $ extrapolates to zero at the same temperature $T_0$ that the extrapolated relaxation time diverges. We relate $ $ to the Debye-Waller factor.

cond-mat↗

arXiv1999

Dynamic Entropy as a Measure of Caging and Persistent Particle Motion in Supercooled Liquids

The length-scale dependence of the dynamic entropy is studied in a molecular dynamics simulation of a binary Lennard-Jones liquid above the mode-coupling critical temperature $T_c$. A number of methods exist for estimating the entropy of dynamical systems and we utilize an approximation based on calculating the mean first-passage time (MFPT) for particle displacement because of its tractability and its accessibility in real and simulation measurements. The MFPT dynamic entropy $S(ε)$ is defined to equal the inverse of the average first-passage time for a particle to exit a sphere of radius $ε$. This measure of the degree of chaotic motion allows us to identify characteristic time and space scales and to quantify the increasingly correlated particle motion and intermittency occurring in supercooled liquids. In particular, we identify a ``cage'' size defining the scale at which the particles are transiently localized, and we observe persistent particle motion at intermediate length scales beyond the scale where caging occurs. Furthermore, we find that the dynamic entropy at the scale of one interparticle spacing extrapolates to zero as the mode-coupling temperature $T_c$ is approached.

cond-mat.soft↗

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↗

arXiv1997

String-like Clusters and Cooperative Motion in a Model Glass-Forming Liquid

A large-scale molecular dynamics simulation is performed on a glass-forming Lennard-Jones mixture to determine the nature of dynamical heterogeneities which arise in this model fragile liquid. We observe that the most mobile particles exhibit a cooperative motion in the form of string-like paths (``strings'') whose mean length and radius of gyration increase as the liquid is cooled. The length distribution of the strings is found to be similar to that expected for the equilibrium polymerization of linear polymer chains.

cond-mat.dis-nn↗