A Lagrangian marker particle (LMP) method is applied to measure the toroidal large-scale eddies (LSEs) and their enveloping stagnation surfaces in a 2801 bottom-sweeping model crystallizer. The trajectories of a 0.4 cm diameter LMP show that these stagnation surfaces inhibit transport. Analysis shows that the velocity component normal to stagnation surfaces vanish. Therefore, stagnation surfaces act as a semi-permeable barriers to particle transport. Microconductivity measurements show that the stagnation surfaces are leaky at the molecular scale. Thus particle transport through stagnation surfaces is size-dependent. The LMP measurements reveal the structure of the LSEs. This consists of (1) an upward-swirling flow adjacent to the tank perimeter extending from the bottom to the top of the tank, (2) a central, quiescent zone, and (3) a downward return flow between (1) and (2) through a system of nested, smaller diameter, secondary toroidal flows concentric with the impeller axis. A cylindrical stagnation surface surrounds the central quiescent zone. These results are corroborated by measurements of inhomogeneous concentration profiles in an industrial scale 20001 batch crystallizer. This leads to an understanding of the effects of LSEs on silver halide microcrystal particle size distribution in the industrial scale crystallizer. (C) 2009 Elsevier Ltd. All rights reserved.