Fluidic Dielectrophoresis: Polarization and Manipulation at Liquid/liquid Interfaces
Embargo until
2015-08-01
Date
2014-08-18
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Publisher
Johns Hopkins University
Abstract
This work presents a new type of alternating current (AC) interfacial polarization and frequency dependent fluid displacement phenomenon at a liquid/liquid electrical interface and how it can be used for microfluidic control. We used two fluid streams, one with a greater electrical conductivity and the other a greater dielectric constant, and flow focused them side-by-side in a microfluidic channel. By using gold electrodes aligned with the microchannel, we applied an AC electric field perpendicular to the interface, and fluid is observed to displace across the liquid/liquid interface. The direction and magnitude of this displacement is frequency dependent. At low AC frequency, below the interfacial inverse charge relaxation time, the high conductive fluid displaces into the high dielectric stream. At high frequency the direction of liquid displacement reverses, and the high dielectric stream injects into the high conductivity stream. The interfacial crossover frequency where the liquid displacement direction reverses is dependent on differences in electrical properties between the two fluid streams, and is well explained by Maxwell-Wagner polarization mechanics.
We demonstrate how the interfacial crossover frequency of a liquid/liquid interface can be measured using impedance spectroscopy. We added an interdigitated gold electrode design downstream from electrodes used for fluid displacement. The impedance of the liquid/liquid interface was characterized over a frequency range of 1kHz to 7 MHz with no fluid displacement. When we apply low frequency to the upstream electrodes to displace the high conductive stream, the impedance decreases, and at high frequency when the low conductive stream displaces, the impedance increased. We determined that the optimum frequency to record impedance on the sensor electrodes to be 400 kHz. By measuring the impedance of the sensor electrodes at this fixed frequency, and sweeping the function generator to displace the interface, the crossover frequency can be quickly determined from the resulting LabView graph.
We also demonstrate how fluidic dielectrophoresis can be used to make an active mixing device in a in a microfluidic system. When two high conductive streams are displaced simultaneously into a center low conductive stream we can create micro-vortices and cause the system to mix on a μs time scale. The two advantages of this device are the speed of mixing, which can be turned on or off very quickly, and the voltage dependence of the mixing, that can be used for tunable injection. This technique keeps the mixing volume constant and can be achieved in a very short distance in the microfluidic channel.
Finally, we demonstrate how the mixing device mentioned above can be applied upstream to a chemotaxis device to create instant gradient control. Based on the voltage dependent mixing of a high conductive stream with a lower conductive stream in an AC field we were able to produce tunable spatial chemical gradients with arbitrary geometry and solute concentration steepness in a microfluidic device for studying directed cell migration. When the electric field is turned on, the high conductive cell buffer mixes with the low conductive buffer and is sent down stream. When the field is off the high conductive cell buffer exits the chip before the chemotaxis chamber. By adding cyclic adenosine monophosphate (cAMP) to the high conductive buffer we can control the strength and direction of the cAMP chemical gradient in the chemotaxis chamber. To change the direction of the chemical gradient, the applied voltage is switched from the electrodes on the first inlet to the electrodes on the second inlet.
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Keywords
microfluidics, dielectrophoresis, impedance, Maxwell-Wagner polarization