Computer simulations of colloidal gels: how hindered particle rotation affects structure and rheology

The effects of particle roughness and short-ranged non-central forces on colloidal gels are studied using computer simulations in which particles experience a sinusoidal variation in energy as they rotate. The number of minima $n$ and energy scale $K$ are the key parameters; for large $K$ and $n$, particle rotation is strongly hindered, but for small $K$ and $n$ particle rotation is nearly free. A series of systems are simulated and characterized using fractal dimensions, structure factors, coordination number distributions, bond-angle distributions and linear rheology. When particles rotate easily, clusters restructure to favor dense packings. This leads to longer gelation times and gels with strand-like morphology. The elastic moduli of such gels scale as $G' \propto \omega^{0.5}$ at high shear frequencies $\omega$. In contrast, hindered particle rotation inhibits restructuring and leads to rapid gelation and diffuse morphology. Such gels are stiffer, with $G'\propto\omega^{0.35}$. The viscous moduli $G''$ in the low-barrier and high-barrier regimes scale according to exponents $0.53$ and $0.5$, respectively. The crossover frequency between elastic and viscous behaviors generally increases with the barrier to rotation. These findings agree qualitatively with some recent experiments on heterogeneously-surface particles and with studies of DLCA-type gels and gels of smooth spheres.