  The mixing of passive scalars of decreasing diffusivity, advected in each
case by the same three-dimensional Navier-Stokes turbulence, is studied. The
mixing becomes more isotropic with decreasing diffusivity. The local flow in
the vicinity of steepest negative and positive scalar gradients are in general
different, and its behavior is studied for various values of the scalar
diffusivity. Mixing approaches monofractal properties with diminishing
diffusivity. We consider these results in the context of possible singularities
of scalar dissipation.
