"Experimental measurements of stretching fields in fluid mixing," Greg A. Voth, George Haller, and Jerry Gollub, Preprint, (pdf format, 3.0Mb)

Using precision measurements of particle trajectories, we can experimentally determine the time-dependent stretching fields in 2D fluid mixing. These quantities, which had been previously available only numerically, attain local maxima along lines that coincide with the stable and unstable manifolds of the flow. Contours of a passive impurity field are found at each instant to be oriented parallel to the lines that have recently experienced large stretching.

Look here for Animations that reveal the full time dependence of the stretching fields. The image below shows lines of large past (red) and future (blue) stretching.