Flow-reversal hydrodynamic metamaterial

In passive viscous systems, the local flow field is governed by the prescribed hydraulic properties and the applied pressure boundary conditions. Hydrodynamic metamaterials based on spatially engineered permeability or viscosity distributions can redistribute, concentrate, conceal, and even redirect flow fields. Transformation-based rotators can, in principle, produce large rotations of the local velocity direction, but such transport states are encoded in the prescribed material architecture and generally require increasingly strong anisotropy as the rotation angle increases. This motivates an alternative strategy for achieving dynamically tunable localized reverse transport without redesigning the passive structure. Here we demonstrate a flow-reversal hydrodynamic metamaterial that creates a target region with velocity opposite to the imposed background flow while retaining an approximately uniform external flow. The device combines an inner annular array of active source-sink dipoles with a surrounding passive Hele-Shaw compensation shell. The active dipoles supply the hydraulic work required to overcome the background transport within the target region, whereas the passive shell suppresses the exterior disturbance by hydraulic-resistance matching. This active-passive coupled design enables complete internal flow reversal while maintaining an almost undisturbed external flow field. This work uses active transport control on hydrodynamic metamaterials, enabling programmable localized transport in pressure-driven viscous flows.