Nicolas G Green

Ac electrokinetics: a review of forces in microelectrode structures

Ac electrokinetics is concerned with the study of the movement and behaviour of particles in suspension when they are subjected to ac electrical fields. The development of new microfabricated electrode structures has meant that particles down to the size of macromolecules have been manipulated, but on this scale forces other than electrokinetic affect particles behaviour. The high electrical fields, which are required to produce sufficient force to move a particle, result in heat dissipation in the medium. This in turn produces thermal gradients, which may give rise to fluid motion through buoyancy, and electrothermal forces. In this paper, the frequency dependency and magnitude of electrothermally induced fluid flow are discussed. A new type of fluid flow is identified for low frequencies (up to 500 kHz). Our preliminary observations indicate that it has its origin in the action of a tangential electrical field on the diffuse double layer of the microfabricated electrodes. The effects of Brownian motion, diffusion and the buoyancy force are discussed in the context of the controlled manipulation of sub-micrometre particles. The orders of magnitude of the various forces experienced by a sub-micrometre latex particle in a model electrode structure are calculated. The results are compared with experiment and the relative influence of each type of force on the overall behaviour of particles is described.

Electrohydrodynamics and dielectrophoresis in microsystems: scaling laws

The movement and behaviour of particles suspended in aqueous solutions subjected to non-uniform ac electric fields is examined. The ac electric fields induce movement of polarizable particles, a phenomenon known as dielectrophoresis. The high strength electric fields that are often used in separation systems can give rise to fluid motion, which in turn results in a viscous drag on the particle. The electric field generates heat, leading to volume forces in the liquid. Gradients in conductivity and permittivity give rise to electrothermal forces and gradients in mass density to buoyancy. In addition, non-uniform ac electric fields produce forces on the induced charges in the diffuse double layer on the electrodes. This causes a steady fluid motion termed ac electro-osmosis. The effects of Brownian motion are also discussed in this context. The orders of magnitude of the various forces experienced by a particle in a model microelectrode system are estimated. The results are discussed in relation to experiments and the relative influence of each type of force is described.

Separation of Submicron Bioparticles by Dielectrophoresis

Submicron particles such as latex spheres and viruses can be manipulated and characterized using dielectrophoresis. By the use of appropriate microelectrode arrays, particles can be trapped or moved between regions of high or low electric fields. The magnitude and direction of the dielectrophoretic force on the particle depends on its dielectric properties, so that a heterogeneous mixture of particles can be separated to produce a more homogeneous population. In this paper the controlled separation of submicron bioparticles is demonstrated. With electrode arrays fabricated using direct write electron beam lithography, it is shown that different types of submicron latex spheres can be spatially separated. The separation occurs as a result of differences in magnitude and/or direction of the dielectrophoretic force on different populations of particles. These differences arise mainly because the surface properties of submicron particles dominate their dielectrophoretic behavior. It is also demonstrated that tobacco mosaic virus and herpes simplex virus can be manipulated and spatially separated in a microelectrode array.

Single cell dielectric spectroscopy

Over the last century a number of techniques have been developed which allow the measurement of the dielectric properties of biological particles in fluid suspension. The majority of these techniques are limited by the fact that they only provide an average value for the dielectric properties of a collection of particles. More recently, with the advent of microfabrication techniques and the Lab-on-a-chip, it has been possible to perform dielectric spectroscopic experiments on single biological particles suspended in physiological media. In this paper we review current methods for single cell dielectric spectroscopy. We also discuss alternative single cell dielectric measurement techniques, specifically the ac electrokinetic methods of dielectrophoresis and electrorotation. Single cell electrical impedance spectroscopy is also discussed with relevance to a microfabricated flow cytometer. We compare impedance spectroscopy data obtained from measurements made using a microfabricated flow cytometer with simulation data obtained using an equivalent circuit model for the device.

Electrothermally induced fluid flow on microelectrodes

Planar microelectrodes, used for the electrokinetic manipulation of particles, generate high strength AC electric fields, resulting not only in forces on the particles but also on the suspending fluid. Observations of electrolytes on microelectrode structures at applied signal frequencies of the order of 1 MHz have shown the importance of the illumination in generating fluid flow. In this paper, these experiments are analysed in terms of the theory of electrothermally induced fluid flow. Numerical calculations are made of the electric field, temperature field and fluid flow, arising both from Joule heating and from light heating. The results verify that Joule heating is not important under the experimental conditions. The temperature gradient generated by the light that is required in order to match the experimental fluid velocities is determined.

Ac electrokinetics: a survey of sub-micrometre particle dynamics

Particles suspended in fluid exhibit motion when subjected to ac electric fields. The applied field results in forces on both the particles and the fluid, the study of which is referred to as ac electrokinetics. The ac electrokinetic techniques can be used for the controlled manipulation and characterization of particles, and the separation of mixtures. For sub-micrometre particles, Brownian motion is important and strong electric fields are required to overcome these effects. Planar micro-electrode arrays, fabricated using semiconductor manufacturing processes, can generate electric fields of the required strength from low potentials over a wide range of frequencies. This paper reviews and discusses sub-micrometre particle dynamics under the influence of dielectrophoretic and electrohydrodynamic forces. New experimental observations of the movement of sub-micrometre particles are also presented.

The dielectrophoretic and travelling wave forces generated by interdigitated electrode arrays: analytical solution using Fourier series

In alternating current electrokinetics, electric fields are used to generate forces on particles. Techniques have been applied for the manipulation of particles and the measurement of their dielectric properties. The fields are typically generated by microelectrode structures fabricated on planar surfaces. One particular design, using interdigitated bar electrodes, is used both in dielectrophoretic field flow fractionation and travelling wave dielectrophoresis. This paper presents a Fourier series analysis of the dielectrophoretic force on a particle generated by this type of electrode array, for both dielectrophoresis and travelling wave dielectrophoresis. Simple expressions are derived for the force at a distance of the order of the electrode spacing from the electrodes. A full analytical expression is given for the dielectrophoretic force in two dimensions. Comparisons are made with previously published experimental observations.

Dielectrophoretic manipulation of rod-shaped viral particles

The dielectrophoretic manipulation of a rod-shaped virus, tobacco mosaic virus (TMV) by means of non-uniform AC electric fields is described. An expression is derived for the dielectrophoretic force on a homogeneous dielectric cylinder suspended in a liquid medium which shows that the dielectrophoretic force varies over a wide range of frequencies and medium conductivities. Measurements of the dielectrophoretic behaviour of TMV particles in microelectrode arrays have been made as a function of frequency and applied field strength. The results are shown to be in quantitative agreement with the derived expression for the dielectrophoretic force. By measuring the threshold field strength for the onset of dielectrophoresis, the force on a single TMV particle has been estimated. The results point to the potential use of dielectrophoresis for the collection and manipulation of sub-micron particles using microelectrode arrays.

Numerical solution of the dielectrophoretic and travelling wave forces for interdigitated electrode arrays using the finite element method

AC electrokinetics is the study of the movement of polarisable particles under the influence of AC electric fields. The fields are applied to a suspension of particles by planar microelectrode structures and one particular design, the interdigitated bar electrode has been used in both dielectrophoretic (DEP) field flow fractionation and travelling wave dielectrophoresis. This paper presents, numerical solutions of the DEP and travelling wave forces for an interdigitated electrode array energised with either a 2- or 4-phase signal. The electrorotational torque experienced by the particle in the 4-phase travelling wave array is also calculated. The solutions are compared with previous results.

Manipulation and trapping of sub-micron bioparticles using dielectrophoresis

A non-uniform alternating electric field induces motion in polarisable particles called dielectrophoresis. The effect is governed by the relative magnitudes of the dielectric properties of the medium and the particles. The technology has been used to manipulate particles for biotechnological applications, including purification, fractionation and concentration of cells and microorganisms. However, the lower size limit for the dielectrophoretic manipulation of particles was believed to be about 1 micron, but recent work has proved otherwise. The dielectrophoretic movement and properties of latex beads and a simple rod-shaped virus, tobacco mosaic virus (TMV), have been measured using microfabricated electrode structures. Measurements have been made over a range of suspending medium conductivities, applied frequencies and electric field strengths. It is shown that under appropriate conditions both latex beads and tobacco mosaic virus particles can be selectively attracted to regions of high electric field strength located at the tips of microfabricated electrode structures. The ability to selectively trap and separate bio-particles has many potential applications in the area of biotechnology.