Large scale simulations of two-dimensional bidisperse granular fluids allow us to determine spatial correlations of slow particles via the four-point structure factor $S_4(q,t)$. Both cases, elastic ($varepsilon=1$) as well as inelastic ($varepsilon
< 1$) collisions, are studied. As the fluid approaches structural arrest, i.e. for packing fractions in the range $0.6 le phi le 0.805$, scaling is shown to hold: $S_4(q,t)/chi_4(t)=s(qxi(t))$. Both the dynamic susceptibility, $chi_4(tau_{alpha})$, as well as the dynamic correlation length, $xi(tau_{alpha})$, evaluated at the $alpha$ relaxation time, $tau_{alpha}$, can be fitted to a power law divergence at a critical packing fraction. The measured $xi(tau_{alpha})$ widely exceeds the largest one previously observed for hard sphere 3d fluids. The number of particles in a slow cluster and the correlation length are related by a robust power law, $chi_4(tau_{alpha}) approxxi^{d-p}(tau_{alpha})$, with an exponent $d-papprox 1.6$. This scaling is remarkably independent of $varepsilon$, even though the strength of the dynamical heterogeneity increases dramatically as $varepsilon$ grows.
Large scale simulations and analytical theory have been combined to obtain the non-equilibrium velocity distribution, $f(v)$, of randomly accelerated particles in suspension. The simulations are based on an event-driven algorithm, generalised to incl
ude friction. They reveal strongly anomalous but largely universal distributions which are independent of volume fraction and collision processes, which suggests a one-particle model should capture all the essential features. We have formulated this one-particle model and solved it analytically in the limit of strong damping, where we find that $f(v)$ decays as $1/v$ for multiple decades, eventually crossing over to a Gaussian decay for the largest velocities. Many particle simulations and numerical solution of the one-particle model agree for all values of the damping.
We investigate the dynamics of an intruder pulled by a constant force in a dense two-dimensional granular fluid by means of event-driven molecular dynamics simulations. In a first step, we show how a propagating momentum front develops and compactifi
es the system when reflected by the boundaries. To be closer to recent experiments cite{candelier2010journey,candelier2009creep}, we then add a frictional force acting on each particle, proportional to the particles velocity. We show how to implement frictional motion in an event-driven simulation. This allows us to carry out extensive numerical simulations aiming at the dependence of the intruders velocity on packing fraction and pulling force. We identify a linear relation for small and a nonlinear regime for high pulling forces and investigate the dependence of these regimes on granular temperature.
We study the velocity autocorrelation function (VACF) of a driven granular fluid in the stationary state in 3 dimensions. As the critical volume fraction of the glass transition in the corresponding elastic system is approached, we observe pronounced
cage effects in the VACF as well as a strong decrease of the diffusion constant. At moderate densities the VACF is shown to decay algebraically in time (t^{-3/2}) like in a molecular fluid, as long as the driving conserves momentum locally.
We use event driven simulations to analyze glassy dynamics as a function of density and energy dissipation in a two-dimensional bidisperse granular fluid under stationary conditions. Clear signatures of a glass transition are identified, such as an i
ncrease of relaxation times over several orders of magnitude. As the inelasticity is increased, the glass transition is shifted to higher densities and the precursors of the transition become less and less pronounced -- in agreement with a recent mode-coupling theory. We analyze the long-time tails of the velocity autocorrelation and discuss its consequences for the nonexistence of the diffusion constant in two dimensions.
