Gel and Glass Transitions in Short-Range Attractive Colloidal Systems - Food Colloids: Self-Assembly and Material Science - page 328

transition of purely entropic origin, from a liquid to a crystal, was experimen-
tally detected 3 in a system of colloids with a steep repulsive interaction.
Another interesting case is represented by systems with a short-range attrac-
tive interaction - one that is 'short' as compared to the particle size. This is a
case that cannot be studied in atomic systems. For example, when polymers
with radius of gyration much smaller than the diameter of the colloidal particles
are added to a hard sphere system, an effective attractive interaction sets in.
In particular, the radius of gyration of the polymer (or, alternatively, a smaller
colloidal particle) modulates the range of the attraction. In the short-range
attractive colloidal system (SRACS), new thermodynamic behaviour emerges
as predicted by theory 4 and simulations, 5 i.e., when the range of the attraction
is short enough, the liquid-liquid phase separation becomes metastable with
respect to the fluid-crystal phase separation. It is interesting to note that the
metastable liquid-liquid critical point still plays an important role owing to the
density fluctuations that can favour crystal nucleation, a fact that is of great
importance for protein crystallization. 6
Colloidal systems play an important role also in studying dynamics. For
purely hard spheres, for example, a kinetic arrest has been found 7 when a system
of sterically stabilized PMMA particles is prepared at very high densities (i.e.,at
packing fractions above 58%). At these densities the system is frozen into a
disordered structure, and it does not relax anymore towards equilibrium, i.e.,itis
a glass. The existence of this glassy phase has been of great importance to test the
predictions of mode coupling theory (MCT), 8 one of the few predictive theories
of the glass transition. Indeed, in a set of beautiful experiments, van Megen and
Underwood 9 gave the first proof of the predictions of MCT. The phenomenology
of the SRACS is even richer. When the range of the system is short, MCT
predicts a re-entrant line in the density temperature plane. 10-12 This means that,
above a certain density, it is possible to melt the glass on lowering the temper-
ature (i.e., increasing the attraction between particles) and to vitrify it again on
further decreasing it. The general phase diagram for the SRACS is depicted
schematically in Figure 1. In the high-density region, a re-entrant glass line is
generated by the competition between attractive and repulsive forces. Two glassy
phases with different dynamic properties, named repulsive and attractive, are
generated by these two distinct arrest mechanisms. The phenomenology of the
dynamic arrest at large packing fractions is now well established and accepted as
aresultofmanysimulations 13-15 and experiments. 16-18 It is also worth mention-
ing that new phenomena have also been predicted, such as higher-order MCT
singularities and an anomalous logarithmic decay in the density correlators. 12
At low density, the SRACS is known to form an aggregated gel, where a phase-
spanning arrested structure leads to a yield stress (see, for example, the recent
review by Trappe and Sandkuhler 19 and references therein). When the range of
the attraction is reduced enough, MCT predicts that the attractive glass line
moves to low density; 11,20 and it has been speculated 11,21 that this line (depicted as
the dashed line in Figure 1) is indeed coincident with the gel line in the SRACS.
A useful tool for clarifying these phenomena is computer simulation that
allows investigation of the relationship of the interaction to the dynamics. In