  We performed two-dimensional numerical simulations of passive stirring of two
liquids which generated two spatially distinguishable paradigmatic velocity
flows, viz. few large extended vortices and rapid oscillations. Using the
Okubo- Weiss criterion, we mapped regions of stretching and rotation in the
flow field. We find that an elementary oscillatory sequence of
stretching-rotation-stretching induces spatial redistribution and steepening of
concentration gradients, and the aggregation of such sequences determines fluid
mixing at a downstream position. Furthermore, we quantify the paradigms of
'stretching' and 'folding' by developing measures based on flow topology which
would facilitate the design of stirring protocols.
