  The Stokes-Einstein relation, relating the diffusion and viscosity
coefficients D and eta, is tested in two dimensions. An equilibrium
molecular-dynamics simulation was used with a Yukawa pair potential. Regimes
are identified where motion is diffusive and D is meaningful. The
Stokes-Einstein relation, D ~ kT, was found to be violated near the disordering
transition; under these conditions collective particle motion exhibits
dynamical heterogeneity. At slightly higher temperatures, however, the
Stokes-Einstein relation is valid. These results may be testable in
strongly-coupled dusty plasma experiments.
