J.H.J. van Opheusden

Effects of structural rearrangements on the rheology of rennet-induced casein particle gels.

During ageing of casein or skim milk gels, structural changes take place that affect gel parameters, such as pore size and storage modulus. These changes can be explained in terms of rearrangements of the gel network at various length scales. In this paper, rheological experiments on rennet-induced casein gels and a general model on rearrangements are presented. The results of experiments (e.g. microscopy, permeametry) and computer simulations, the model, and recent literature on casein gels and other types of particle gels are compared to each other. Experiments presented include measurements of storage and loss moduli and maximum linear strain of the casein gels. Parameters varied were pH (5.3 and 6.65) and temperature (25 and 30 degrees C). In addition, the casein volume fraction (5-9 vol.%) was varied, which enables application of fractal scaling models. For rennet-induced casein gels, it is demonstrated that at the lower pH, all types of rearrangements proceed significantly faster. The rearrangements include: an increase in the size of compact building blocks; partial disappearance of fractal structure; and the formation of straightened strands, some of which eventually break. All of these rearrangements seem to be a consequence of particle fusion. There are indications of universality of the relation between particle fusion and gel syneresis for gels composed of viscoelastic particles.

Categorization of rheological scaling models for particle gels applied to casein gels

Rennet-induced casein gels made from skim milk were studied rheologically. A scaling model or framework for describing the rheological behavior of gels is discussed and used for classification of the structure of casein gels. There are two main parameters in the model that describe the number of deformable links in a strand and the bendability of the links. In the model at least five types of gel structure can be distinguished. Application of the model to experimental data on rennet-induced casein gels at pH of 6.0–6.6 and 25 °C shows that they contain straight strands with a large number of deformable links. Analysis of the experimental data of the storage modulus, maximum linear strain and yield stress as a function of the volume fraction results in the same information about the gel structure.

Relating colloidal particle interactions to gel structure using Brownian dynamics simulations and the Fuchs stability ratio

Brownian dynamics simulations of aggregation of hard-sphere dispersions at intermediate volume fractions (∼3–10 vol%) have been performed. A long-range activation energy for aggregation was incorporated. The bonds formed were irreversible and flexible. Cluster growth rate and fractal properties of the gel matrix could be related to particle interactions by using a Fuchs stability ratio WF. Although this approach is expected to apply only to the very early stages of gelation, WF was shown to be a useful parameter, especially for predicting gel matrix parameters like the fractal dimensionality Df (which is a measure of the compactness of the clusters in the intermediate or fractal length scale regime) and the correlation length ξ (which is a measure of the average gel pore size). The number of aggregates, Nagg, was found to be a convenient measure of the stage of aggregation for the range of volume fractions and interactions studied. For high values of WF, the value of Df was more generic (i.e., less depend...

Large shear deformation of particle gels studied by Brownian dynamics simulations

Brownian dynamics (BD) simulations have been performed to study structure and rheology of particle gels under large shear deformation. The model incorporates soft spherical particles, and reversible flexible bond formation. Two different methods of shear deformation are discussed, namely affine and nonaffine deformation, the second being novel in simulation studies of gels. Also two dynamic descriptions of the model are presented, with and without inertia effects. Nonaffine deformation resulted in a slower increase of the stress at small deformation than affine deformation. At large deformation both models gave similar stress responses, although the inertia model resulted in lower stresses. The particle gels, regardless of the model used, were observed to fracture into lumps that compactified due to local reorganization. A reversible yielding transition, as observed in polymer gels, was not found. Fractal properties of the gels were irreversibly lost at large deformation.