Transient mixing in a family of two-dimensional vortex flows driven by magneto-hydrodynamic forces is studied systematically in this experiment.

The basic methodology is to label half of the fluid layer with a fluorescent dye, and to measure the subsequent dye distribution with a precision 12-bit CCD camera. Diagnostics are devised that allow us separately to monitor the stretching of fluid elements and development of fine striations, diffusive smearing, and the transport of material across the cell. Various statistical measures are used, including the probability distribution, the scalar gradient PDF, and the power spectrum.

We find major differences between the time-periodic and nonperiodic cases. The former generally show evidence of "KAM surfaces" or barriers to transport, so that mixing, while substantial, is incomplete. After a modest number of cycles (typically 10-20), the flow structure usually reaches a slowly evolving limiting form whose amplitude then decays, as has been proposed in several theoretical studies.