Zachary R Gagnon
Johns Hopkins University
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.|
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.|
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.