On the Long-Term Stability of Colloidal Gels
Executive summary
Colloidal gels are used in industrial formulations to solve the ‘gravity problem’.
Particles are typically heavier than their suspending media, and will settle out over time. A strong enough short-range attraction will cause the formation of space-spanning networks that are strong enough to support their own weight. Such gel states are, however, metastable, and will, in time, evolve towards thermodynamic equilibrium. This is manifested in products as the collapse of the gel structure and the appearance of dense sediments. Our project is concerned with understanding such gravitational instabilities.
To do so, we set up a very well-defined experimental model system in which a short-range attraction between nearly-hard-sphere colloids was induced by added non-adsorbing polymers via the ‘depletion’ mechanism. Careful comparison between experimental observations and simulations allowed us to establish that gelation in our system was due to ‘arrested spinodal decomposition’, which gave rise to gels with bicontinuous texture.
Studying such gels using magnetic resonance and optical imaging and again comparing our findings with simulations, we have made a number of important, perhaps paradigm-shifting, discoveries. Two mechanisms operate in gel collapse:
- the accumulation of dense ‘debris’ (compact clusters) at the top, which then fall through the bulk, and
- the rise of solvent ‘bubbles’ from the bulk of the gel to the top.
In both cases, solvent back flow plays an essential role in the break up of spatial structures. Perhaps surprisingly, processes occurring right at the top of gels are vital in determining their fate. In particular, curved menisci at gel-air interfaces seem to generate copious ‘debris’, leading to continuous collapse without any latency (or delay) times, while filling a sample cuvette gives rise to gels that have finite gravitational stability before collapse.