Abstract
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.
Highlights
The ability to move fluids at the microscale is at the core of many scientific and technological advancements
Electroosmotic flow (EOF) is the motion of an electrolyte resulting from the interaction of an external electric field with the net charge of an electric double layer (EDL)
Several names were proposed for this phenomenon including “flow field-effect transistor” [17] and “fieldeffect flow control” [18]; we will refer to it as “field-effect electroosmosis” or FEEO as it best captures the physical phenomenon, as was originally proposed in 1989 by Ghowsi and Gale [12]
Summary
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. We demonstrate the interaction of multiple gate electrodes with an externally generated flow field, allowing spatial modulation of streamlines in real time. An alternative approach to control the EOF relies on the application of a perpendicular electric field to the surface, enabling modification of the zeta potential in a dynamic fashion. We note that Eq 3 is not valid outside the electrode region, where Δφ is not defined In such regions the zeta potential can be assumed to be constant in time and related only to the native surface charge, and the time-averaged EOF vanishes.
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