  Steady laminar flows through porous media spontaneously generate Lagrangian
chaos at pore scale, with qualitative implications for a range of transport,
reactive and biological processes. The characterization and understanding of
mixing dynamics in these opaque environments is an outstanding challenge. We
address this issue by developing a novel technique based upon high-resolution
imaging of the scalar signature produced by push-pull flows through porous
media samples. Owing to the rapid decorrelation of particle trajectories in
chaotic flows, the scalar image measured outside the porous material is
representative of in-situ mixing dynamics. We present a theoretical framework
for estimation of the Lyapunov exponent based on extension of Lagrangian
stretching theories to correlated aggregation. This method provides a full
characterization of chaotic mixing dynamics in a large class of porous
materials.
