download report - Sapienza

Scientific Report 2007-2009
Condensed matter physics and biophysics
C9. Soft Matter: Arrested states in colloidal systems
In recent years, dynamical arrest in colloidal systems,
and more generally in soft matter, has gained increasing
scientific attention. Colloidal suspensions have unambiguous
advantages with respect to their atomic counterparts.
Characteristic space and time scales are much
larger, allowing for experimental studies in the light scattering
regime and for a better time resolution. The size
of the particles allows for direct observation with confocal
microscopy techniques, down to the level of singleparticle
resolution. In addition, particle-particle interactions
can be tuned by changing the solution conditions
or by additives, as well as by synthesis of functionalized
colloids. Colloidal suspensions, despite being very complex
in nature and number of components, can be well
described theoretically via simple effective potentials.
The variety of interactions reflects also in a variety
of dynamically arrested states, which can be of gel or
glass type. Gels are low density structures, stabilized
by strong inter-particle bonds which create a percolating
network, while glasses are generically found at larger
density and stabilized by caging. The most famous colloidal
glasses are certainly the so-called attractive and
repulsive glasses observed in colloids with short-range
depletion attractions, induced by the addition of nonadsorbing
polymers in solution. The glass-glass interplay
has been recently studied by simulations, showing
that there is a long-time relaxation from the attractive
to the repulsive glass [1].
At low densities the situation is more complex, and a
variety of scenarios emerge when different inter-particle
interactions are at hand. It has long been debated
whether —for colloids with short-range attractions —
the attractive glass line could extend continuously to
lower densities, since a liquid-gas phase separation is
encountered. To clarify the interplay between arrest
and phase separation, we carried out a joint experimental/numerical
work[2] in collaboration with Harvard University.
Thanks to the single-particle resolution achieved
by confocal microscopy, we compared the distribution of
aggregates (clusters) in the fluid prior to gelation and
built a mapping between the experimental control parameters
and the thermodynamic parameters. In this
way, we have provided unambiguous evidence that gelation
occurs exactly at the spinodal threshold, as illustrated
in Figure 1.
When depletion interactions are competing with electrostatic
repulsion, the situation changes and phase separation
can be suppressed. In this case, an equilibrium
fluid of clusters exists at low densities. These clusters become
the building blocks of dynamical arrest. As shown
in Figure 2, with increasing packing fraction ϕ, clusters
branch in a network gel structure, while at lower
densities repulsive interactions dominate, originating a
Wigner glass of clusters[3]. Wigner glasses are lowdensity
disordered solids in which particles arrest despite
being very far apart due to the soft repulsive cages[4].
Figure 1: 3d reconstructions (a,b) of the fluid and gel
states observed by confocal miscroscopy (c,d). The mapping
of experimental onto numerical data (e) allows to identify
that gelation takes place in coincidence with thermodynamic
phase separation. From [2].
Figure 2: Simulation snapshots of Wigner glasses of clusters
at low ϕ, turning into a percolating gel when ϕ increases, and
related phase diagram. From [3].
References
1. E. Zaccarelli et al., PNAS 106, 15203 (2009).
2. P. J. Lu et al., Nature 453, 499-503 (2008).
3. J.C.F. Toledano et al, Soft Matter 5, 2390-2398 (2009).
4. E. Zaccarelli et al., Phys. Rev. Lett. 100, 195701 (2008).
Authors
C. De Michele 1 , F. Romano 1 , J. Russo 1 , F. Sciortino 1,2 , P.
Tartaglia 1,3 , E. Zaccarelli 1,2
http://soft.phys.uniroma1.it/
Sapienza Università di Roma 62 Dipartimento di Fisica