  This work provides an experimental method for simultaneously measuring finite
time Lyapunov exponent fields for multiple particle groups, including non-flow
tracers, in three-dimensional multiphase flows. From sequences of particle
images, e.g., from experimental fluid imaging techniques, we can directly
compute the flow map and coherent structures, with out performing the
computationally costly numerical integration. This is particularly useful to
find three-dimensional Lagrangian coherent structures for inertial particles,
that do not follow the bulk fluid velocity, as we demonstrate for a grid
turbulence experiment. The technique described may provide a new means for
exploring the physics of experimental multi-phase flows.
