Transport phenomena in complex and dynamic microscopic environments are fundamentally shaped by hydrodynamic interactions. In particular, microparticle transport in porous media is governed by the delicate interplay between particle-substrate friction and pressure forces. Here, we systematically investigate the motion of externally driven rotating magnetic microparticles near a substrate patterned with a square lattice of cylindrical obstacles, a model porous medium. Remarkably, we observe a reversal in the direction of particle translation as obstacle spacing decreases, highlighting a sensitive competition between shear-induced forward rolling and pressure-driven backward sliding due to flow-field symmetry breaking. These results demonstrate the crucial role of structured environments in determining microscale active particle transport, offering strategies for microfluidic design, targeted cargo delivery, and tunable active materials. [Full Article]
