Fluidic Dielectrophoresis (fDEP): The Electrical Manipulation of Liquid-Liquid Interfaces

Zachary R Gagnon

Johns Hopkins University
Email: zgagnon1@jhmi.edu

Development of robust tools that can manipulate small volumes of fluid and their contents quickly, easily and without the need for an experienced operator is an important area of microfluidic research. Indeed, the last decade in particular has witnessed rapid advances in the ability to manipulate fluids at small length scales. A popular method involves the use of electric field-induced forces, or electrokinetic phenomena, to perform fluidic manipulations. Because microfluidic length scales are on the order of tens of microns, a small (~5 volts) voltage can be applied across these micro lengths to create a large (~ 5 kV/m) electric field to deliver precise electrokinetic forces to liquid and particles. Such electrokinetic techniques involve no moving parts and are capable of inducing precise and tunable forces on suspending particles, cells and liquid in small microfluidic volumes, and are thus an attractive method for microfluidic manipulations. Here, we discuss how to use a new type of dielectrophoresis, known as fluidic dielectrophoresis, to precisely manipulate and electrically characterize liquids in small microfluidic space.

Dielectrophoresis is an electrokinetic technique commonly used to manipulate, sort, and characterize particles in microfluidic systems. Let us consider for a moment what physical mechanism produces this field-induced particle force. For simplicity, we will examine the case of a spherical homogenous particle suspended in an electrolyte solution with a disparaging electrical conductivity and dielectric constant. Both the particle and the electrolyte are considered to be dielectrics, that is, materials that contain charges that can polarize under the application of an external electric field. When an electric field is applied across the interface between the particle and electrolyte, ionic charge in solution will accumulate and the particle-electrolyte interface will polarize. The action of the electric field on this charge gives rise to an electric particle force known as dielectrophoresis, or DEP1.

Figure 1. L/L interface displaces in an electric field (a) Top view of a L/L interface between an array of microelectrodes created using a microfluidic “T-channel”. [3D VIEW] Confocal microscopy reveals a sharp (< 2 µm) boundary between two co-flowing red and green fluorescently labeled streams. (b) The L/L interface polarizes and electrokinetically displaces when exposed to an AC electric field.
When an alternating current (AC) electric field is applied across the interface, the magnitude and sign of the induced charge, and the resulting DEP force, becomes dependent on the electric field frequency. At low frequency, below the charge relaxation time, the material with greater electrical conductivity will conduct charge to the interface at a rate faster than can be removed by the adjacent lower conductivity phase. As such, low frequency interfacial polarization will be driven by material differences in electrical conductivity. At high frequency, above the charge relaxation timescale, the electric field oscillates faster than charges can electro-migrate to the interface. Since conductive charging does not have enough time to occur over every field half-cycle, the interfacial polarization will be driven by differences in dielectric constant. If neither material has both a greater electrical conductivity and dielectric constant, the net sign of the induced charge that results from interfacial polarization will reverse at a frequency high enough to prevent conductive charging. This mechanism is known as Maxwell-Wagner (MW) polarization.

While DEP has been exploited to manipulate bubbles, particle, biomolecules and cells, research and application in these areas have been primarily limited to particle systems. In this article, we describe how to utilize fDEP manipulate and electrically characterize liquid systems.

In fDEP, an electric field is applied across a liquid electrical interface created between two co-flowing fluid streams with disparaging electrical properties. The interface is created using a microfluidic “T-channel” type device, and the field is delivered across this interface using integrated microelectrodes (Fig 1a – TOP VIEW). These two fluids are readily labeled with a fluorescent dye for imaging, and the interfacial structure can be analyzed using confocal microscopy (Fig 1b – 3D VIEW).

Shown in Figure 1a, two fluids flow side-by-side - the right-most stream (green) consists of deionized water (σ = 20 × 10-3 S/m and ε = 78) treated with NaCl to increase electrical conductivity, and then dyed with fluorescent dye (Alexa 488) to observe the fluid interface. The left stream is dyed with Alexa 594, and consists of a 0.8 M solution of 6-aminohexanoic acid in deionized water (Sigma) to increase the dielectric constant (σ = 15 × 10-6 S/m and ε = 110]. Hence, when flowing side-by-side, one liquid stream has a higher dielectric constant and lower electrical conductivity than its neighbor. When an electric field is applied across the liquid interface, ions in solution migrate and accumulate at the electrical mismatch created between the two fluids, and create a region of diffuse charge. Similar to particle-based DEP, the electric field exerts a force on this induced interfacial charge, and the interface is observed to displace from its initial “flat” position (Figure 1b) to a “tilted” configuration (Figure 1c).

Figure 2. Liquid interface response over a range applied AC frequencies (a) Interface tilts to the left below the (b) crossover frequency (7.6MHz) and (c) tilts to the right above the crossover.


Figure 3. The liquid crossover frequency is linearly proportional to the conductivity difference at the interface, and is well-predicted with our polarization model2.
Unlike conventional DEP applications involving colloidal particles with fixed electrical properties, the electrical properties (conductivity and dielectric constant) of both liquid interfacial phases can be readily tuned with the addition of soluble salts and polarizable zwitterions. Hence, the electrical characteristics of a liquid interface can be readily engineered to produce a desired electrokinetic response. For example, the magnitude and direction of interfacial displacement is dependent on the AC electric field frequency, and on the electrical characteristics of the interface. Most importantly, aqueous liquid interfaces exhibit a crossover frequency similar to that of classical particle DEP; the interface “tilts” to the left at a frequency of 4 MHz (Fig 2a), remains fixed at 7.6 MHz (Fig 2b), or “tilts” to the right at 25 MHz (Fig 2c). The observed crossover occurs where no “tilt” is observed (see Fig 2b) at a frequency of 7.6 MHz. Moreover, this fDEP crossover frequency is highly sensitive to the electrical properties at the interface, and exhibits a linear response with the difference in electrical conductivity (σ) difference between each co-flowing fluid stream, [σ12] (Fig 3), and is well described by our Maxwell-Wagner polarization model. Please see our article in Physical Review Letters2 for a more complete description of this polarization model, and a through description of the relevant physics responsible for fDEP.

In this article we have presented fluidic dielectrophoresis (fDEP), a newly discovered electrokinetic phenomena, which can be utilized to manipulate electrical liquid interfaces created between co-flowing electrolyte fluids. The liquid-liquid DEP behavior is similar to that of conventional particle DEP in that the interface displaces under exposure to an AC electric field and displays a crossover frequency dependent on the fluid electrical properties. For more information on the governing physics surrounding both DEP and fDEP please see the listed resources below.


References
  1. Z. R. Gagnon, “Cellular dielectrophoresis: applications to the characterization, manipulation, separation and patterning of cells.,” Electrophoresis, vol. 32, no. 18, pp. 2466–2487, Sep. 2011
  2. M. Desmond, N. Mavrogiannis, and Z. Gagnon, “Maxwell-Wagner Polarization and Frequency-Dependent Injection at Aqueous Electrical Interfaces,” Phys. Rev. Lett., 2012