Biomedical subjects

Th Voigtmann

Publications and source records attributed to Th Voigtmann.

7 recordsLinked to original sources

PubMed2006

Glass transition in fullerenes: mode-coupling theory predictions.

We report idealized mode-coupling theory results for the glass transition of ensembles of model fullerenes interacting via phenomenological two-body potentials. Transition lines are found for C60, C70, and C96 in the temperature-density plane. We argue that the observed glass transition behavior is indicative of kinetic arrest that is strongly driven by the interparticle attraction in addition to excluded-volume repulsion. In this respect, these systems differ from most standard glass-forming liquids. They feature arrest that occurs at lower densities and that is stronger than would be expected for repulsion-dominated hard-sphere-like or Lennard-Jones-type systems. The influence of attraction increases with increasing the number of carbon atoms per molecule. However, unrealistically large fullerenes would be needed to yield behavior reminiscent of recently investigated model colloids with strong short-ranged attraction (glass-glass transitions and logarithmic decay of time-correlation functions).

Journal Article↗

PubMed2006

Sensitivity of arrest in mode-coupling glasses to low-q structure.

We quantify, within mode coupling theory, how changes in the liquid structure affect that of the glass. Apart from the known sensitivity to the structure factor S(q) at wave vectors around the first sharp diffraction peak q0, we find a strong (and inverted) response to structure at wave vectors below this peak: an increase in S(q0/2) lowers the degree of arrest over a wide q-range. This strong sensitivity to "caged cage" packing effects, on length scales of order 2d, is much weaker in attractive glasses where short-range bonding dominates the steric caging effect.

Journal Article↗

PubMed2004

Tagged-particle dynamics in a hard-sphere system: mode-coupling theory analysis.

The predictions of the mode-coupling theory of the glass transition (MCT) for the tagged-particle density-correlation functions and the mean-squared displacement curves are compared quantitatively and in detail to results from Newtonian- and Brownian-dynamics simulations of a polydisperse quasi-hard-sphere system close to the glass transition. After correcting for a 17% error in the dynamical length scale and for a smaller error in the transition density, good agreement is found over a wide range of wave numbers and up to five orders of magnitude in time. Deviations are found at the highest densities studied, and for small wave vectors and the mean-squared displacement. Possible error sources not related to MCT are discussed in detail, thereby identifying more clearly the issues arising from the MCT approximation itself. The range of applicability of MCT for the different types of short-time dynamics is established through asymptotic analyses of the relaxation curves, examining the wave-number and density-dependent characteristic parameters. Approximations made in the description of the equilibrium static structure are shown to have a remarkable effect on the predicted numerical value for the glass-transition density. Effects of small polydispersity are also investigated, and shown to be negligible.

Journal Article↗

PubMed2004

alpha-Relaxation processes in binary hard-sphere mixtures.

Molecular-dynamics simulations are presented for two correlation functions formed with the partial density fluctuations of binary hard-sphere mixtures in order to explore the effects of mixing on the evolution of glassy dynamics upon compressing the liquid into high-density states. Partial-density-fluctuation correlation functions for the two species are reported. Results for the alpha-relaxation process are quantified by parameters for the strength, stretching, and time scale, where the latter varies over almost four orders of magnitude upon compression. The parameters exhibit an appreciable dependence on the wave vector, and this dependence is different for the correlation function referring to the smaller and that for the larger species. These features are shown to be in semiquantitative agreement with those calculated within the mode-coupling theory for ideal liquid-glass transitions.

Journal Article↗

PubMed2003

Dynamics of colloidal glass-forming mixtures.

Recent experimental results from dynamic light scattering on two-component colloidal mixtures close to the glass transition are compared to theory. In the framework of the mode-coupling theory of the glass transition, close agreement is found in general. Discrepancies are identified for the minority-particle correlation function, and possible reasons for them are discussed.

Journal Article↗

PubMed2003

Mixing effects for the structural relaxation in binary hard-sphere liquids.

We report extensive molecular-dynamics-simulation results for binary mixtures of hard spheres for different size disparities and different mixing percentages, for packing fractions up to 0.605, and over a characteristic time interval spanning up to 5 orders in magnitude. We explore the changes in the evolution of glassy dynamics due to mixing and discover two opposite scenarios: For large size disparity, increasing the mixing percentage of small particles leads to a speed up of long-time dynamics, while small disparity leads to a slowing down. These results agree with predictions based on the mode-coupling theory for ideal-glass transitions.

Journal Article↗

PubMed2003

Effect of composition changes on the structural relaxation of a binary mixture.

Within the mode-coupling theory for idealized glass transitions, we study the evolution of structural relaxation in binary mixtures of hard spheres with size ratios delta of the two components varying between 0.5 and 1.0. We find two scenarios for the glassy dynamics. For small size disparity, the mixing yields a slight extension of the glass regime. For larger size disparity, a plasticization effect is obtained, leading to stabilization of the liquid due to mixing. For all delta, a decrease of the elastic moduli at the transition due to mixing is predicted. A stiffening of the glass structure is found as is reflected by the increase of the Debye-Waller factors at the transition points. The critical amplitudes for density fluctuations at small and intermediate wave vectors decrease upon mixing, and thus the universal formulas for the relaxation near the plateau values describe a slowing down of the dynamics upon mixing for the first step of the two-step relaxation scenario. The results explain the qualitative features of mixing effects reported by Williams and van Megen [Phys. Rev. E 64, 041502 (2001)] for dynamical light-scattering measurements on binary mixtures of hard-sphere-like colloids with size ratio delta=0.6.

Journal Article↗