The study of vortices have captivated fluid dynamicists for centuries. This image shows the calculated backward Finite Time Lyapunov Exponent (FTLE), based on experimental data, for a vortex ejected out of a cylinder. Ridges in the backward FTLE can be thought of physically as attracting structures within the flow. The stronger the color intensity, the stronger the attraction. Different colors correspond to different times after ejection and are super-imposed: 1.5 sec in orange, 3.75 sec in blue, and 6 sec in green.