Dynamical Heterogeneities in the crossover region from colloidal gels to colloidal glasses

Using molecular dynamics simulations, we study a DLVO-type model for attractive colloidal systems. In particular we focus on the crossover from the gel-like regime, at low volume fraction, to the glassy-like regime, at higher volume fraction. In such crossover region, some features of chemical gels and some features of glassy systems are present. The former are due to long living clusters the latter to crowding effects, which both slow down the relaxation process. The contribution of the long living clusters is obtained from the fluctuations of the self-overlap, chi(4)(t), by summing only over pairs of particles, linked by persistent bonds (present at both times 0 and time t), the crowding contribution is instead obtained from the remaining pairs of particles. For small volume fractions and low enough temperature, where the lifetime of the bonds is dominant, essentially the cluster contribution is present. At higher volume fraction, where bond lifetime and relaxation time start to be of the same order of magnitude, both contributions are manifest. The dependence of these two contributions on the length scale a, which appears in the definition of chi(4)(t), is also interesting. The clustering contribution is highly suppressed as a decreases, like in chemical gels, while the contribution due to crowding has a non-monotonic behaviour, and it is maximum at intermediate values of a.