Effects of arrested phase separation. Beyond homogeneous colloidal fluids, the resonant frequency distribution also determines the far-and near-field properties of heterogeneous nanoparticle assemblies. At sufficiently large U 0 , thermodynamics favors macroscopic phase separation into a dense colloidal crystal and dilute colloidal fluid. However, very strong attractions can also lead to kinetic arrest, which prevents complete separation into macroscopic phases on experimental or simulation time scales, and instead the particles become locked into a long-lived heterogeneous gel or glassy state (Figure ). The model investigated here, though simple, is known to reproduce basic structural and optical properties of experimental gel assemblies of plasmonic nanoparticles. The evolution of the optical response of Drude attractive particles with U 0 = 10k B T as a function of packing fraction is qualitatively different than for Drude hard-sphere dispersions (U 0 = 0k B T ). Rather than a smooth incremental redshift in σ(ω) and cdf(ω res,i ) with increasing η like for hard spheres, the resulting attractive gels appear to have a preferred resonant frequency around ≈ 0.35ω * p (Figure ), and the fraction of particles resonating at that preferred frequency increases with η. The latter appears as increasingly sharp jumps in the cdf with η at the same ω res,i (Figure ). The preferred frequency corresponds to particles in the interior of gel strands, which are near the coexisting density of the dense phase.