17 research outputs found
Sudden collapse of a colloidal gel
Metastable gels formed by weakly attractive colloidal particles display a
distinctive two-stage time-dependent settling behavior under their own weight.
Initially a space-spanning network is formed that for a characteristic time,
which we define as the lag time \taud, resists compaction. This solid-like
behavior persists only for a limited time. Gels whose age \tw is greater than
\taud yield and suddenly collapse. We use a combination of confocal
microscopy, rheology and time-lapse video imaging to investigate both the
process of sudden collapse and its microscopic origin in an refractive-index
matched emulsion-polymer system. We show that the height of the gel in the
early stages of collapse is well described by the surprisingly simple
expression, h(\ts) = \h0 - A \ts^{3/2}, with \h0 the initial height and
\ts = \tw-\taud the time counted from the instant where the gel first yields.
We propose that this unexpected result arises because the colloidal network
progressively builds up internal stress as a consequence of localized
rearrangement events which leads ultimately to collapse as thermal equilibrium
is re-established.Comment: 14 pages, 11 figures, final versio
Gels under stress: the origins of delayed collapse
Attractive colloidal particles can form a disordered elastic solid or gel
when quenched into a two-phase region, if the volume fraction is sufficiently
large. When the interactions are comparable to thermal energies the
stress-bearing network within the gel restructures over time as individual
particle bonds break and reform. Typically, under gravity such weak gels show a
prolonged period of either no or very slow settling, followed by a sudden and
rapid collapse - a phenomenon known as delayed collapse. The link between local
bond breaking events and the macroscopic process of delayed collapse is not
well understood. Here we summarize the main features of delayed collapse and
discuss the microscopic processes which cause it. We present a plausible model
which connects the kinetics of bond breaking to gel collapse and test the model
by exploring the effect of an applied external force on the stability of a gel.Comment: Accepted version: 10 pages, 7 figure
Non-aqueous microgel particles:Synthesis, properties and applications
Advances in microgel particles swollen in non-aqueous solvents and the challenges in their characterisation, synthesis and potential applications are discussed.</p
Protein-Polymer Mixtures in the Colloid Limit: Aggregation, Sedimentation and Crystallization
While proteins have been treated as particles with a spherically symmetric
interaction, of course in reality the situation is rather more complex. A
simple step towards higher complexity is to treat the proteins as
non--spherical particles and that is the approach we pursue here. We
investigate the phase behavior of enhanced green fluorescent protein (eGFP)
under the addition of a non--adsorbing polymer, polyethylene glycol (PEG). From
small angle x-ray scattering we infer that the eGFP undergoes dimerization and
we treat the dimers as spherocylinders with aspect ratio .
Despite the complex nature of the proteins, we find that the phase behaviour is
similar to that of hard spherocylinders with ideal polymer depletant,
exhibiting aggregation and, in a small region of the phase diagram,
crystallization. By comparing our measurements of the onset of aggregation with
predictions for hard colloids and ideal polymers [S.V. Savenko and M. Dijkstra,
J. Chem. Phys 124, 234902 (2006) and F. lo Verso et al., Phys. Rev. E 73,
061407 (2006)] we find good agreement, which suggests that the eGFP proteins
are consistent with hard spherocylinders and ideal polymer.Comment: 12 page