Antonio Ramos

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.

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.

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.

Pumping of liquids with traveling-wave electroosmosis

Net flow of electrolyte induced by a traveling-wave electric potential applied to an array of microelectrodes is reported. Two fluid flow regimes have been observed: at small-voltage amplitudes the fluid flow follows the direction of the traveling wave, and at higher-voltage amplitudes the fluid flow is reversed. In both cases, the flow seems to be driven at the level of the electrodes. The experiments have been analyzed with a linear electroosmotic model based upon the Debye–Huckel theory of the double layer. The electrical problem for the experimental interdigitated electrode array is solved numerically using a truncated Fourier series. The observations at low voltages are in qualitative accordance with the electroosmotic model.

Electrothermal flows generated by alternating and rotating electric fields in microsystems

Electrothermal motion in an aqueous solution arises from the action of an electric field on inhomogeneities in the liquid induced by temperature gradients. The temperature field can be produced by the applied electric field through Joule heating, or caused by external sources, such as strong illumination. Electrothermal flows in microsystems are usually observed at applied signal frequencies around 1 MHz and voltages around 10 V. In this work, we present self-similar solutions for the motion of an aqueous solution in a constant temperature gradient placed on top of: (a) two coplanar electrodes subjected to an a.c. potential difference, and (b) four coplanar electrodes subjected to a four-phase a.c. signal, generating a rotating field. The first case produces two-dimensional rolls whereas the second case produces a liquid whirl. Finally, we present experimental results of electrothermal liquid flows generated by alternating and rotating electric fields under strong illumination, and these experiments are compared to the analytical solutions. The induced rotating flow could be used in the mixing of analytes and of liquids in microsystems.

Electric field induced fluid flow on microelectrodes: the effect of illumination

The electrokinetic manipulation of particles suspended in a fluid medium is accomplished using microelectrodes that generate non-uniform fields of significant strength from low applied potentials. The high strength fields produce not only forces on the particles but also on the fluid medium used for suspension. This paper presents qualitative and semi-quantitative observations of the movement of the fluid at applied field frequencies of the order of 1MHz and higher. The importance of the illumination in generating the fluid flow is described, the flow depending on both the intensity of illumination and the applied electric field. The theory of electrothermally induced fluid flow is briefly described and compared with the experimental observations. Reasonable agreement is found between the experiments and the theory, with the light generating temperature gradients, and therefore gradients in fluid permittivity and conductivity, and the electric field responsible for the motive force.

The tensile strength of cohesive powders and its relationship to consolidation, free volume and cohesivity

The tensile strength of a powder is related to the interparticle force and to the free volume, which, in turn, are related to consolidation stress. The relationship between stress and free volume is described by the state diagram that has been measured at zero shear for a set of cohesive powders (xerographic toners) with a range of concentrations of a flow control additive. The toners are 12.7 μm diameter particles of styrene/ butadiene copolymer, and the surface additive is a submicron fumed silica that is used to control the interparticle forces. To overcome problems of sample non-uniformity, powder samples are initially fluidized and then allowed to settle under gravity. The tensile strenghts, σt, of these powders have been measured by means of a powder bed technique in which gas flow through the bed is increased until the bed fractures due to the tensile stress produced by the gas flow. The overpressure required to fracture the bed then provides a measure of σ1. The consolidation stress in the bed, σe, can be altered by varying the weight of the powder per unit area. Tensile strength is found to be linearly related to the consolidation stress in the limited range of stresses we have investigated, and the slope of this relationship is the same for all additive concentrations below 0.1%; above this concentration the slope decreases, consistent with a change from polymer-dominated to silica-dominated contacts between the particles. From the ratio σt/σe, we show that the contacts are fully plastic event at zero load, and that hardness of the contacts increases with increasing additive concentration.