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.
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.
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 stretchedexponential
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.
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 in-
sight 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).
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.
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.