An inherent structure approach to yield behaviors in colloidal gels.

Abstract

While gels and glasses display similar time and temperature dependency in their stress-strain response, there is a qualitative difference in the underlying structure of high- and low-volume fraction amorphous solids. We quantify the structure of yielding suspensions by calculating the distribution of particle free volumes. For a binary Lennard-Jones system, aggregates below the limit of mechanical stability (&phis; &ap; 0.43) show a tail that corresponds to a few particles with very high free volumes. Above the limit of mechanical stability, two-peaked distributions appear at the onset temperature of slowed dynamics <italic>kT</italic>/epsilon &ap; 1.0. We compare the purely repulsive inverse power system with the Lennard-Jones system and observe that at low volume fractions the attractive interaction determines the characteristic distribution, while at high volume fractions repulsion dominates. We confirm the role of Brownian motion in the time-dependency of the yield stress by studying creep in constant stress simulations. Creep is observed only in simulations with Brownian motion, indicating that Brownian motion is the only existing mechanism for time-dependent relaxation in our simulations. In order to obtain a time-independent measurement of the yield stress, we study the stress-strain response of amorphous solids at zero-temperature. We find that a steep interparticle repulsion leads to a brittle yielding response, and a lower overall yield stress. Increasing the volume fraction increases the sustainable yield stress. We conclude that the measurable yield stress value is a result of both structural and Brownian effects. The dimensionless temperature determines the relative importance of Brownian to interparticle contributions. The volume fraction affects the relative importance of the attractive and repulsive interparticle interactions to yield. At low volume fractions a mechanically unstable solid may exist in the presence of an attractive interaction. At high volume fractions a mechanically stable solid state may be formed with purely repulsive interactions.