Length scale dependent relaxation in colloidal gels

Emanuela Del Gado* and Walter Kob
Dipartimento di Scienze Fisiche, Universitri di Napoli "Federico II", Italia

Laboratoire de Colloi'des, Verres et Nanomateriaux, Universiti Montpellier II, France

Although gels are ubiquitous in fundamental science, technological applications and also in our daily life, their structural and dynamical properties are not well understood. In contrast to other systems that show a slow relaxation, such as glass-forming liquids, the structure of gels is given by an open network that is thought to be responsible for the unusual dynamical properties of these systems. Apart from the dramatic slowing down of the relaxation dynamics with increasing interaction strength between the particles, one finds a complex dependence of the dynamics on the length scale considered: The relaxation functions are often stretched and/or, most remarkably, compressed, i.e. the time correlators decay faster than an exponential. Due to the large variety of gel-forming systems (colloidal gels, chemical gels, ... ) it has so far not been possible to obtain a clear picture on which ones of these phenomena are universal and which ones are specific to just a subclass of these systems. The same is true for their theoretical description since various mechanisms have been proposed to rationalize certain observations from experiments or computer simulations, but so far no unifying picture has emerged yet. We present the re suIts of a recent study based on a simple model that does indeed have the characteristics of (colloidal) gelforming systems at a finite temperature. By means of molecular dynamics computer simulations, we investigate the gel formation from the equilibrium sol phase. At low volume fraction and low T particles are linked by long-living bonds and form an open percolating network. Our results on the structural and dynamical properties [1, 2] shed some light on the mechanism that is responsible for the slow dynamics in these systems. In particular we show that the strong length scale dependence of the dynamics in gel forming systems is tightly related to the formation of the gel structure and is therefore a general feature. This study allows for the first time to investigate on a microscopic level the relaxation processes in the incipient gel and to understand why they must strongly depend on the length scale investigated. In our model the mesh-size of the incipient gel network corresponds to a crossover length between dramatically different relaxation processes, from stretched to compressed exponentials. Moreover our results link the super-exponential relaxation at low temperature to the motion of pieces of the incipient gel structure.

* Corresponding author: delgado@na. infn. it

[1] E. Del Gado and W. Kob, Europhys. Lett. (2005, in press; cond-mat/0507085).

[2] E. Del Gado and W. Kob, cond-mat/0510690.