Attraction of minute particles to invariant regions of volume preserving flows by transients

We find that tracer material can be concentrated into invariant regions of hows due exclusively to transient effects, as are produced when tracers tem porarily become more buoyant than the surrounding fluid. This can occur eit her as a single event, e.g., if the tracer is initially weakly buoyant, or under periodic forcing, e.g., when external effects: (such as solar heating ) change the tracer density periodically. We study both cases in experiment s, in a model, and in direct numerical simulations of laminar flow in a sti rred tank. Focusing occurs for very small tracer size and inertia in flows that are instantaneously strictly volume conserving.