On the Long-Term Stability of Colloidal Gels

Publication Reference: 
FRR-65-08
Author Last Name: 
Poon
Authors: 
W. C. K. Poon
Report Type: 
FRR
Research Area: 
Wet Systems
Publication Year: 
2016
Publication Month: 
12
Country: 
United Kingdom

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 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.