LARGE-DEFORMATION RHEOLOGICAL BEHAVIOR OF A MODEL PARTICLE GEL

We report on some aspects of the large deformation rheology of model t hree-dimensional networked particle gels. Model gels with a particle v olume fraction of 5% are formed by aggregation in a Brownian dynamics simulation from soft spherical particles incorporating flexible surfac e-to-surface bonds that restrict the subsequent angular reorganization and infer structural stability on the resulting percolating, fractal structure. The interaction potential allows some control over the fina l fractal dimension of the gel and bonds may be either breakable or es sentially permanent depending on the choice of parameters. The use of continuous potentials allows the rheology to be studied at constant st rain-rate and at constant stress by the incorporation of a homogeneous strain algorithm. For systems with 'permanent' bonds, strain hardenin g is observed when the strain-rate is very low compared with the struc tural relaxation time. At relatively high strain-rates the stress resp onse is more nearly proportional to the strain. These systems also sho w strain recovery when the stress is removed. For systems with short b reakable bonds, a yield stress is observed at slow constant strain. He re, we have studied the yielding behaviour of these systems by applyin g a steadily increasing stress and we find that, under these condition s, the structure degrades in three distinct stages. The initial breaka ge of bonds does not immediately disrupt the gel but allows some visco elastic flow. This is followed by breakdown into a small number of rel atively large aggregates. The ensuing viscoplastic flow causes the fur ther rupture of aggregates that culminates in a catastrophic break-up to smaller entities at a critical point that presages true viscous flo w. These transitions between viscoelastic and viscoplastic flow and be tween viscoplastic and viscous flow correspond to the static and dynam ic yield stresses that have been observed experimentally in colloidal systems at high volume fraction. The oscillatory response for systems with permanent bonds shows non-linear behaviour expressed as overtone modes for strain amplitudes in excess of 0.05. The effective modulus f or these systems also increases with strain amplitude while for system s with breakable bonds the modulus decreases or passes through a maxim um as a consequence of structural decay. This behaviour compares favou rably with experimental studies on chemical and physical gels.