A Scaling Law for Diffusion in Charged Fluid Mixtures

Pure Yukawa plasmas are well studied for their applications in dusty plasmas, astrophysics, and charged fluids. These systems of point particles with screened Coulomb interactions have only two free parameters, the screening length and coupling parameter. However, realistic mixtures are much more complicated and universal scaling laws for describing the microphysics remains elusive, despite their importance. This talk will report on molecular dynamics simulations of realistic astrophysical mixtures of strongly coupled Yukawa fluids and present results for diffusion coefficients across coupling regimes. We demonstrate that diffusion coefficients for a species of charge Z_i in the mixture can be approximated by rescaling the diffusion coefficients for an equivalent one-component plasma with the same charge as the mixture average by (Z_i /<Z>)^0.6. This empirical law is best understood by recognizing that the global viscosity of the mixture applies equally to all particles within the mixture and thus the species-wise diffusion coefficients should scale with the effective charge radius. While specific mixtures discussed in this work have astrophysical relevance, this law has broad applications in charged fluid physics.