On the Long-Term Stability of Colloidal Gels

Author Last Name: 
Wilson Poon
Report Type: 
Research Area: 
Wet Systems
Publication Year: 
Publication Month: 
United Kingdom

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.