The ability to move fluids at the microscale is at the core of many scientific and technological advancements. Despite its importance, microscale flow control remains highly limited by the use of discrete channels and mechanical valves, and relies on fixed geometries. Here we present an alternative mechanism that leverages localized field-effect electroosmosis to create dynamic flow patterns, allowing fluid manipulation without the use of physical walls. We control a set of gate electrodes embedded in the floor of a fluidic chamber using an ac voltage in sync with an external electric field, creating nonuniform electroosmotic flow distributions. These give rise to a pressure field that drives the flow throughout the chamber. We demonstrate a range of unique flow patterns that can be achieved, including regions of recirculating flow surrounded by quiescent fluid and volumes of complete stagnation within a moving fluid. We also demonstrate the interaction of multiple gate electrodes with an externally generated flow field, allowing spatial modulation of streamlines in real time. Furthermore, we provide a characterization of the system in terms of time response and dielectric breakdown, as well as engineering guidelines for its robust design and operation. We believe that the ability to create tailored microscale flow using solid-state actuation will open the door to entirely new on-chip functionalities.

The field of microfluidics has enabled a wide range of discoveries and technologies in the biological and chemical sciences. However, despite three decades of research, the vision of lab-on-a-chip, a microscale device capable of replacing large-scale chemical and biological laboratories, remains elusive. Here we argue that a major gap toward achieving this goal is the lack of reconfigurability and programmability of existing microfluidic platforms. We portray a vision of a fully reconfigurable microfluidic device, which can change its shape and function dynamically, thus allowing researchers to 'put their hands' into a microscale experiment and enabling real-time decision making. We review existing technologies that can dynamically control microscale flows, suggest additional physical mechanisms that could be leveraged towards the goal of reconfigurable microfluidics, and call on the broad scientific community to join in this effort.

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. Using field-effect electro-osmosis as a mechanism for injection of momentum, we present a theoretical framework and analytical solutions for a range of geometrical shapes, validate these both numerically and experimentally, and demonstrate the ability to dynamically switch between the different states.