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John R Melrose

Publications and source records attributed to John R Melrose.

2 recordsLinked to original sources
PubMed2003

Brownian dynamics simulations of aging colloidal gels.

The aging of colloidal gels is investigated using very long duration Brownian dynamics simulations. The Asakura-Oosawa description of the depletion interaction is used to model a simple colloid polymer mixture. Several regimes are identified during gel formation. The intermediate scattering function displays a double decay characteristic of systems where some kinetic processes are frozen. The beta relaxation at short times is explained in terms of the Krall-Weitz model for the decorrelation due to the elastic modes present. The alpha relaxation at long times is well described by a stretched exponential, showing a wide spectrum of relaxation times for which the q dependence is tau(alpha)=q(-2.2), lower than for diffusion. For the shortest waiting times, a combination of two stretched exponentials is used, suggesting a bimodal distribution. The extracted relaxation times vary with waiting time as tau(alpha)=tau(0.66)(w), more slowly than in the simple aging case. The real space displacements are found to be strongly non-Gaussian, correlated in space and time. We were unable to find clear evidence that the gel aging was driven by internal stresses. Rather, we hypothesize that in this case of weakly interacting gels, the aging behavior arises due to the thermal diffusion of strands, constrained by the percolating network, which ruptures discontinuously. Although the mechanisms differ, the similarity of some of the results to aging of glasses is striking.

Journal Article↗
PubMed2003

Colloid flow during thickening--a particle level understanding for core-shell particles.

The flow curve of sheared concentrated colloids can show a region of shear thickening. The underlying mechanisms involved in this effect has long been an issue. Recently the author and co-workers have used Stokesian dynamics approximated for high concentrations to simulate models of polymer coated particles in the thickening regime. This work continues the search for a particle level of understanding these systems in the thickening regime. Previous work found that shear thickening is associated with the formation of a network of contacts between polymer coats. Previous work by the author has examined the fabric (geometry) and texture (density distribution) of the contact network. However despite this many-body effect, it was also found that a very simple mean-field type argument at the level of a pair of particles in contact relates the particle interaction laws to the thickening part of the flow curve up to an unknown factor. In the argument this factor is determined by aspects of the destruction of particle contacts. In this paper several new results are reported. The first concerns the definition of the network and its coordination. The paper contrasts systems with different thickness of polymer coats and gives tentative evidence that the network formed sits close to what is known as the iso-static coordination. The second set of issues concerns the analysis of the lifetimes of particle contacts--in effect an examination of the unknown factor and assumptions of the pair argument. A surprising result is that some particle pairs have relatively long lifetimes. Finally, data is reported for the stress fluctuations in the thickening regime.

Journal Article↗