1386 Nicholas Mavrogiannis Mitchell Desmond Zachary R. Gagnon Department of Chemical and Biomolecular Engineering, Johns Hopkins University Baltimore, Baltimore, MD, USA
Received September 24, 2014 Revised November 29, 2014 Accepted December 3, 2014
Electrophoresis 2015, 36, 1386–1395
Research Article
Fluidic dielectrophoresis: The polarization and displacement of electrical liquid interfaces Traditional particle-based dielectrophoresis has been exploited to manipulate bubbles, particles, biomolecules, and cells. In this work, we investigate analytically and experimentally how to utilize Maxwell–Wagner polarization to initiate fluidic dielectrophoresis (fDEP) at electrically polarizable aqueous liquid–liquid interfaces. In fDEP, an AC electric field is applied across a liquid electrical interface created between two coflowing fluid streams with different electrical properties. When potentials as low as 2 volts are applied, we observe a frequency-dependent interfacial displacement that is dependent on the relative differences in the electrical conductivity (⌬ ) and dielectric constant (⌬ ε) between the two liquids. At low frequency this deflection is independent of dielectric constant, while at high frequency it is independent of electrical conductivity. At intermediate frequencies, we observe an fDEP cross-over frequency that is independent of applied voltage, sensitive to both fluid electrical properties, and where no displacement is observed. An analytical fDEP polarization model is presented that accurately predicts the liquid interfacial cross-over frequency, the dependence of interfacial displacement on liquid electrical conductivity and dielectric constant, and accurately scales the measured fDEP displacement data. The results show that miscible aqueous liquid interfaces are capable of polarizing under AC electric fields, and being precisely deflected in a direction and magnitude that is dependent on the applied electric field frequency. Keywords: Fluidic dielectrophoresis / Interfacial polarization / Liquid interface / Micro channel DOI 10.1002/elps.201400454
1 Introduction
Correspondence: Dr. Zachary Gagnon, Department of Chemical and Biomolecular Engineering, Johns Hopkins University, 3400 North Charles, St. Maryland Hall 220A, Baltimore, MD 21218, USA E-mail: [email protected] Fax: +1-410-516-5510
that drive fluid motion [12], suspending particles [13], cells [14], and droplets [15]. Owing to the ease at which metal electrodes can be fabricated and integrated into microfluidic channels [16], AC and DC electric fields have been widely utilized to manipulate particle and fluid contents of microfluidic systems. In terms of the electrokinetic phenomena, the major electrical forces acting on particles in an electrolyte solution are electrophoresis (EP) and dielectrophoresis (DEP). Electrophoresis arises from the coupling of the electric field with the fixed surface charge on the particle surface, while DEP occurs when the electric field induces surface charge at the particle/liquid interface and only results in a net particle motion when the electrical field is nonuniform. For liquids, EHD motion is known to occur where spatial gradients in the electric properties are present. Electrical gradients are a common occurrence in lab-on-a-chip applications. Examples include isoelectric focusing [17], amplified field sample stacking [18], thermally induced gradients [19], and mixing of coflowing laminar streams [20]. Different types of electrokinetic flow resulting from these processes include electrothermal [21], DC electroosmosis [22], and AC EOF [23, 24]. Due to the prevalence of electrical gradients in microfluidic systems, work has been done to
Abbreviations: AHA, 6-aminohexanoic acid; CM, Clausius– Mossatti; DEP, dielectrophoresis; EHD, electrohydrodynamics; fDEP, fluidic dielectrophoresis; MW, Maxwell–Wagner
Colour Online: See the article online to view Figs. 1, 3, 4 and 6 in colour.
Development of techniques that can transport and manipulate liquid at nanoliter length scales is an important area of microfluidic research [1]. Small-scale liquid routing [2], mixing [3], and pumping [4,5], in particular, are essential components in many lab-on-a-chip microfluidic applications including immunoassays [6], capillary electrophoresis [7], dropletbased flow assays [8], cellular analysis [9], and automated sample processing [10]. While many different liquid transport strategies have been developed, a popular method for microfluidic liquid actuation involves the use of electrokinetic phenomena [11]. Electrokinetics is a field of study within electrohydrodynamics (EHD) that describes the coupling between interfacial charge and electric fields to produce electrical body forces
C 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.electrophoresis-journal.com
Microfluidics and Miniaturization
Electrophoresis 2015, 36, 1386–1395
1387
Figure 1. (A) A microfluidic T-channel with integrated electrodes. Two streams with different electrical properties flow side-by-side to create an electrical interface. The left-most stream (green) has a greater conductivity. The right stream (red) has a greater permittivity. (B) 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 (
Received September 24, 2014 Revised November 29, 2014 Accepted December 3, 2014
Electrophoresis 2015, 36, 1386–1395
Research Article
Fluidic dielectrophoresis: The polarization and displacement of electrical liquid interfaces Traditional particle-based dielectrophoresis has been exploited to manipulate bubbles, particles, biomolecules, and cells. In this work, we investigate analytically and experimentally how to utilize Maxwell–Wagner polarization to initiate fluidic dielectrophoresis (fDEP) at electrically polarizable aqueous liquid–liquid interfaces. In fDEP, an AC electric field is applied across a liquid electrical interface created between two coflowing fluid streams with different electrical properties. When potentials as low as 2 volts are applied, we observe a frequency-dependent interfacial displacement that is dependent on the relative differences in the electrical conductivity (⌬ ) and dielectric constant (⌬ ε) between the two liquids. At low frequency this deflection is independent of dielectric constant, while at high frequency it is independent of electrical conductivity. At intermediate frequencies, we observe an fDEP cross-over frequency that is independent of applied voltage, sensitive to both fluid electrical properties, and where no displacement is observed. An analytical fDEP polarization model is presented that accurately predicts the liquid interfacial cross-over frequency, the dependence of interfacial displacement on liquid electrical conductivity and dielectric constant, and accurately scales the measured fDEP displacement data. The results show that miscible aqueous liquid interfaces are capable of polarizing under AC electric fields, and being precisely deflected in a direction and magnitude that is dependent on the applied electric field frequency. Keywords: Fluidic dielectrophoresis / Interfacial polarization / Liquid interface / Micro channel DOI 10.1002/elps.201400454
1 Introduction
Correspondence: Dr. Zachary Gagnon, Department of Chemical and Biomolecular Engineering, Johns Hopkins University, 3400 North Charles, St. Maryland Hall 220A, Baltimore, MD 21218, USA E-mail: [email protected] Fax: +1-410-516-5510
that drive fluid motion [12], suspending particles [13], cells [14], and droplets [15]. Owing to the ease at which metal electrodes can be fabricated and integrated into microfluidic channels [16], AC and DC electric fields have been widely utilized to manipulate particle and fluid contents of microfluidic systems. In terms of the electrokinetic phenomena, the major electrical forces acting on particles in an electrolyte solution are electrophoresis (EP) and dielectrophoresis (DEP). Electrophoresis arises from the coupling of the electric field with the fixed surface charge on the particle surface, while DEP occurs when the electric field induces surface charge at the particle/liquid interface and only results in a net particle motion when the electrical field is nonuniform. For liquids, EHD motion is known to occur where spatial gradients in the electric properties are present. Electrical gradients are a common occurrence in lab-on-a-chip applications. Examples include isoelectric focusing [17], amplified field sample stacking [18], thermally induced gradients [19], and mixing of coflowing laminar streams [20]. Different types of electrokinetic flow resulting from these processes include electrothermal [21], DC electroosmosis [22], and AC EOF [23, 24]. Due to the prevalence of electrical gradients in microfluidic systems, work has been done to
Abbreviations: AHA, 6-aminohexanoic acid; CM, Clausius– Mossatti; DEP, dielectrophoresis; EHD, electrohydrodynamics; fDEP, fluidic dielectrophoresis; MW, Maxwell–Wagner
Colour Online: See the article online to view Figs. 1, 3, 4 and 6 in colour.
Development of techniques that can transport and manipulate liquid at nanoliter length scales is an important area of microfluidic research [1]. Small-scale liquid routing [2], mixing [3], and pumping [4,5], in particular, are essential components in many lab-on-a-chip microfluidic applications including immunoassays [6], capillary electrophoresis [7], dropletbased flow assays [8], cellular analysis [9], and automated sample processing [10]. While many different liquid transport strategies have been developed, a popular method for microfluidic liquid actuation involves the use of electrokinetic phenomena [11]. Electrokinetics is a field of study within electrohydrodynamics (EHD) that describes the coupling between interfacial charge and electric fields to produce electrical body forces
C 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.electrophoresis-journal.com
Microfluidics and Miniaturization
Electrophoresis 2015, 36, 1386–1395
1387
Figure 1. (A) A microfluidic T-channel with integrated electrodes. Two streams with different electrical properties flow side-by-side to create an electrical interface. The left-most stream (green) has a greater conductivity. The right stream (red) has a greater permittivity. (B) 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 (
