Microscale flow control using non-uniform electroosmotic flow
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 fieldeffect 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.
Electroosmotic flow is a well-established and efficient method for driving microchannel flows that relies on the interaction of an externally applied electric field with charge arising at the interface between the liquid and the channel walls. However, its relatively low velocities together with its dependence on the pH of the liquid severely limit its utility. Here, we experimentally demonstrate fast electroosmotic flow over microstructured superhydrophobic surfaces. By suspending the electrolyte in a Cassie-Baxter state over hierarchical surfaces, we create stable gas-liquid interfaces on which we induce charge through a gate electrode. We provide a detailed investigation and characterization of the electroosmotic velocity as a function of the surface geometry by utilizing particle tracking velocimetry in a microfluidic device, and show that the resulting electroosmotic velocity scales with the ratio of slip length to double-layer thickness. Compared to no-slip surfaces, we demonstrate an order of magnitude enhancement in velocity, and complete pH independency, enabling wider utility of EOF in manipulation of microscale flows.
We present a new concept for on-chip separation that leverages bidirectional flow, to tune the dispersion regime of molecules and particles. The system can be configured so that low diffusivity species experience a ballistic transport regime and are advected through the chamber, whereas high diffusivity species experience a diffusion dominated regime with zero average velocity and are retained in the chamber. We detail the means of achieving bidirectional electroosmotic flow using an array of alternate-current (AC) field-effect electrodes, experimentally demonstrate the separation of particles and antibodies from dyes, and present a theoretical analysis of the system, providing engineering guidelines for its design and operation.
Select Publications
Boyko E., Rubin S., Gat A.D., and Bercovici M., (2015), “Flow Patterning in Hele-Shaw Configurations using Non-Uniform Electroosmotic Slip”, Physics of Fluids 27, 102001.
Paratore F., Boyko E., Kaigala G. V. and Bercovici M. (2019), "Electroosmotic flow dipole: Experimental observation and flow field patterning”, Physical Review Letters, 122, 224502.
Paratore F., Bacheva V., Kaigala G. V. and Bercovici M. (2019), "Dynamic microscale flow patterning using electrical modulation of zeta potential", Proceedings of the National Academy of Sciences, 116, 10258-10263.
Dehe S., Rofman B., Bercovici M., and Hardt S., (2020),”Electroosmotic flow enhancement over superhydrophobic surfaces“, Physical Review Fluids, 5, 053701.
Bacheva V., Paratore F., Kaigala G.V., and Bercovici M. (2020), “Tunable bidirectional electroosmotic flow for diffusion-based separation", Angewandte Chemie International Edition, 132, 2-8.
Boyko E., Bacheva V. Eigenbrod M., Paratore F., Gat A.D., Hardt S., and Bercovici M., (2021) “Microscale Hydrodynamic Cloaking and Shielding via Electro-Osmosis”, Physical Review Letters, 126, 184502.