Microscale hydrodynamic cloaking and shielding via electro-osmosis*

We demonstrate theoretically and experimentally that injection of momentum in a region surrounding an object in microscale flow can yield both 'cloaking' conditions, where the flow field outside the cloaking region is unaffected by the object, and 'shielding' conditions, where the hydrodynamic forces on the object are eliminated. The experimental setup is based on a cylindrical obstacle in a Hele-Shaw cell exposed to pressure-driven flow. Momentum injection is performed using field-effect electro-osmosis in a region around the obstacle. We present a theoretical framework employing a shape-perturbation approach and analytical solutions for a range of geometrical shapes of the obstacle. Good agreement between experiments and theory is found. We also demonstrate the ability to dynamically switch between the cloaking and shielding states, which corresponds to a control principle with real-time adaptivity.