Title: Slow and Long-ranged Dynamical Heterogeneities in Dissipative Fluids,
Abstract: A two-dimensional bidisperse granular fluid is shown to exhibit pronounced
long-ranged dynamical heterogeneities as dynamical arrest is approached. Here
we focus on the most direct approach to study these heterogeneities: we
identify clusters of slow particles and determine their size, $N_c$, and their
radius of gyration, $R_G$. We show that $N_c\propto R_G^{d_f}$, providing
direct evidence that the most immobile particles arrange in fractal objects
with a fractal dimension, $d_f$, that is observed to increase with packing
fraction $\phi$. The cluster size distribution obeys scaling, approaching an
algebraic decay in the limit of structural arrest, i.e., $\phi\to\phi_c$.
Alternatively, dynamical heterogeneities are analyzed via the four-point
structure factor $S_4(q,t)$ and the dynamical susceptibility $\chi_4(t)$.
$S_4(q,t)$ is shown to obey scaling in the full range of packing fractions,
$0.6\leq\phi\leq 0.805$, and to become increasingly long-ranged as
$\phi\to\phi_c$. Finite size scaling of $\chi_4(t)$ provides a consistency
check for the previously analyzed divergences of $\chi_4(t)\propto
(\phi-\phi_c)^{-\gamma_{\chi}}$ and the correlation length $\xi\propto
(\phi-\phi_c)^{-\gamma_{\xi}}$. We check the robustness of our results with
respect to our definition of mobility. The divergences and the scaling for
$\phi\to\phi_c$ suggest a non-equilibrium glass transition which seems
qualitatively independent of the coefficient of restitution.