On the Long-Term Stability of Colloidal Gels

The stability of gels of attractive colloidal particles determines the shelf life of products across many sectors. Our project is concerned with coming to a deeper understanding of why and how such gels may collapse under gravity using experiments supported by simulations.

State Diagram Mapping

In our first year, we mapped out the state diagram of a model colloid-polymer mixture of this kind consisting of sterically-stabilized polymethylmethacrylate (PMMA) spheres in which a short-range interparticle attraction was induced by linear polystyrene dispersed in hydrocarbon solvents. Exclusion of polymer in the space between two nearby particles results in a net osmotic pressure pushing the particles together, giving a ‘depletion attraction’ whose range and depth are controlled by the size (i.e., molecular weight) and concentration of the polymer respectively. We showed that the equilibrium phase behavior of this experimental model system could be mapped onto a ‘universal phase diagram’, which we obtained using simulations, via an ‘extended law of corresponding states’. This then permitted us to show that gelation in our model system was due to arrested spinodal decomposition, in which the denser part of a coarsening bicontinuous texture underwent dynamical arrest into metastable gel states.

Collapse Mechanisms

We found that these gels could collapse under gravity in two qualitatively distinct ways. After an initial period of stability, gels at moderate colloid concentrations sedimented very rapidly with a constant meniscus speed after a ‘delay time’, before switching abruptly to a stretched-exponential compaction mode, finally arresting suddenly when the sediment reached a volume fraction of f 0:55. At higher colloid volume fractions, however, gels collapsed in a stretched-exponential fashion, asymptotically reaching the limit of random close packing, frcp 0:64. BD simulations never reproduced the rapid collapse regime; we concluded that hydrodynamics are essential in this phenomenon.

Current Research

This year, we have concentrated on elucidating the mechanism of the rapid collapse. Work with the magnetic resonance imaging (MRI) group of Prof Lynn Gladden in Cambridge show that when the gel slowly separates from the top meniscus, dense material gathers at the top. We observe the rapid sinking of this material through the body of a gel just before the onset of macroscopic rapid collapse. Calculations provide support for the idea that rapid collapse is initiated when the gel structure is no longer able to support the weight of these dense clusters. Presumably, at high enough f, the gels are always strong enough to support the weight of such ‘debris’ at the top, so that the gel collapses as a compacting poroelastic continuum (known to follow a stretched-exponential law, as indeed observed in our system). Interestingly, further calculations starting from this insight were able to account for the difference between observed and simulated gel boundaries, as well as the effect of particle size (the latter via comparison with literature data).

Solvent Mixture Experiments

Using a mixture of solvents, we were able to overmatch the density of the particles, creating a system in which gravitational instability consisted of ‘creaming’ upwards rather than sedimentation. This has allowed us to collect preliminary single-particle resolution 3D data of the ‘top’ part of a collapsing gel. Dramatic movies of ‘volcanos’ erupting at the gel-supernatant interface have been obtained.

Finite-Element Analysis

Finally, comparison of finite-element analysis of gels as elastic continua with previous, macroscopic dark-field optical imaging suggests that ‘fault lines’ in the gel correspond to stress concentrations of an elastic body hanging from the walls of the sample container. These ‘fault lines’ are presumably where heavy ‘debris’ first sink through the gel, and/or polymer rich solvent flows to the surface through ‘vents’ for the observed ‘volcanic eruptions’. Detailed working along these lines next year should provide further insights.