Fractures

The particles or droplets of colloidal dispersions to which a non-adsorbing polymer has been added aggregate into networks. For certain compositions these systems also exhibit the phenomenon of transient gelation, in which a long-lived viscoelastic gel state forms and which is apparently quiescent in the bulk, but which undergoes a relatively sudden collapse after a time the depends on system composition.

Networks of aggregated particles have been seen. Vertically oriented features observed in hard particle suspensions using dark field microscopy (Poon, Starrs and others) have been interpreted as “channels” forming in the transient gel prior to collapse.

Starrs et al. J Phys. Cond. Matter 14 2002 p2485 - dark field images showing features in a transient gel

Confocal microscopy studies by Verhaegh et al suggested the existence of “fractures” in such a network structure, with a gravity-related anisotropy.

Verhaegh et al. Physica A 264 1999 p64 - confocal images showing voids in an aggregated colloid

Inspired by these observations, and by discussions in the literature, we propose that a viscoelastic transient gel, comprising an ensemble of interconnected particle clusters, evolves by spontaneously developing void regions that grow to span several clusters and give rise to the “directed voids” that we call “fractures”. The mechanism by which voids grow is uncertain, but might result from local network rearrangements. Comprehensive network collapse would then result from the increase in size and / or number of fractures, with gravity implicated as the driving force.

The work described here sets out to test the fractures hypothesis.