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 spacespanning
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:
(1) the accumulation of dense ‘debris’ (compact clusters) at the top, which then
fall through the bulk, and (2) 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 gelair
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.