Anisotropic cage evolution in quasi-two-dimensional colloidal fluids

Employing video microscopy, we explore the cage dynamics for colloidal particles confined in quasi-two dimensions (q2D). Our experiments reveal that while ensemble-averaged dynamics of cages are isotropic in the laboratory frame, its evolution in the displacement frame of the caged particle is anisotropic and asymmetric. In turn, this leads to particles in specific regions of the cage contributing either to cage persistence or breaking, influencing the structural relaxation of the fluid. Our findings, thus, provide microscopic insights into cage evolution and dynamics for colloidal fluids in q2D, with direct potential implications for the flow of complex fluids, structural relaxation in dense suspensions, and collective motion in active matter in confined geometries.