Non-uniform AC Electric Field Research Articles

Fluid flow induced by nonuniform ac electric fields in electrolytes on microelectrodes. I. Experimental measurements

Under the influence of an ac electric field, electrolytes on planar microelectrodes exhibit fluid flow. The nonuniform electric field generated by the electrodes interacts with the suspending fluid through a number of mechanisms, giving rise to body forces and fluid flow. This paper presents the detailed experimental measurements of the velocity of fluid flow on microelectrodes at frequencies below the charge relaxation frequency of the electrolyte. The velocity of latex tracer particles was measured as a function of applied signal frequency and potential, electrolyte conductivity, and position on the electrode surface. The data are discussed in terms of a linear model of ac electroosmosis: the interaction of the nonuniform ac field and the induced electrical double layer.

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.

Dielectrophoretic investigations of sub-micrometre latex spheres

A non-uniform AC electric field induces a motion in polarizable particles, called dielectrophoresis. The force responsible for this motion is governed by the dielectric properties both of the suspending medium and of the particles, as well as the geometry of the field. The dielectrophoretic properties of sub-micrometre latex spheres have been studied using micro-fabricated electrode structures. The electric field geometry for electrodes used in the measurements has been solved using numerical analysis. Measurements of the dielectrophoretic properties of the spheres have been made over a range of medium conductivities and applied field frequencies and strengths. Comparisons between the observed behaviour and that expected from theory are presented.

Effects of interparticle electric interactions on dielectrophoresis in colloidal suspensions.

Using our recently published statistical theory concerning the phase diagram of a low-conducting colloidal suspension subject to uniform electric fields [Phys. Rev. E 52, 1669 (1995)], we examine how the long-range electric-field-induced interparticle interactions affect the spatial arrangement of particles in such a suspension under the action of a nonuniform ac electric field. We find the conditions under which the resulting dielectrophoresis in nonuniform electric fields is accompanied by an electric-field-induced phase transition in the suspension. Moreover we predict that, in the case of positive dielectrophoresis, the particles will form chainlike aggregates aligned parallel to the electric field lines and attracted towards the higher electric-field region; whereas, for negative dielectrophoresis, the particles will form disklike aggregates aligned perpendicularly to the electric-field lines and repelled from the higher electric-field region. The theory also provides some insight regarding the dependence of the particle aggregation on the frequency of the applied nonuniform ac electric field. The predictions of the theory are consistent with the characteristic patterns of cell aggregation observed previously in high-gradient electric fields generated in microelectrode systems. \textcopyright{} 1996 The American Physical Society.

Localized multi-alternating-current modulation Kerr measurement of nonuniform three-dimensional electric fields

One of the electric-field measurement methods without intrusiveness uses the Kerr electro-optic effect. This method can be widely applied to gas, liquid, and solid materials, which are transparent and electrically birefringent. However, this method has not been adequately developed as yet for practical measurement of nonuniform three-dimensional electric fields. We describe a Kerr electro-optic measurement method for nonuniform three-dimensional dc and ac electric fields that employs localized multi-ac modulation, taking into account the effect of the observing (laser-beam) direction on the apparent electric field.

Non-uniform spatial distributions of both the magnitude and phase of AC electric fields determine dielectrophoretic forces

It is well known that the conventional dielectrophoretic force acting on a polarised particle in a non-uniform AC electric field is proportional to the in-phase component of the induced dipole moment and the non-uniformity of the field strength. In contrast, the travelling-wave-dielectrophoretic force that acts on a particle subjected to a travelling electric field is proportional to the out-of-phase component of the induced dipole moment. We derive a theory that unifies the description and interpretation of conventional dielectrophoretic and travelling-wave-dielectrophoretic forces. We show that a particle in a non-uniform AC electric field experiences a dielectrophoretic force due to spatial non-uniformities of the magnitude and the phase of the field interacting, respectively, with the in-phase and out-of-phase components of the induced dipole moment. The theory is used to explain the translational effects observed for particles in the presence of standing, travelling and rotating fields in several experimental electrode configurations. The good agreement found between the experimental observations and the theoretical predictions validate the theory.

A unified theory of dielectrophoresis and travelling wave dielectrophoresis

We show for the first time that a particle in a non-uniform AC electric field experiences a dielectrophoretic force arising from spatial non-uniformities of the magnitude and phase of the field interacting, respectively, with the in-phase and out-of-phase components of the induced dipole moment.

Dielectrophoretic characterization and separation of micro-organisms

The effective electrical conductivity values for a variety of Gram-negative and positive bacteria have been determined in the frequency range 10 kHz to 100 kHz. This information enabled experimental conditions to be selected where micro-organisms of different species can be separated using dielectrophoresis - the movement of particles induced by non-uniform AC electric fields. Mixtures of micro-organisms of different species were separated locally on a microscope slide using micro-electrodes of polynomial geometry, and their physical isolation into two separate suspensions was accomplished using a dielectrophoresis chamber that incorporated interdigitated, castellated micro-electrodes.

Separation of viable and non-viable yeast using dielectrophoresis

Dielectrophoresis, the movement of particles in non-uniform AC electric fields, was used to rapidly separate viable and non-viable yeast cells with good efficiency. Known mixtures of viable and heat-treated cells of Saccharomyces cerevisiae were separated and selectively isolated using positive and negative dielectrophoretic forces generated by microelectrodes in a small chamber. Good correlations with the initial known relative compositions were obtained by direct microscopic counting of cells at the electrodes after initial dielectrophoretic separation ( r = 0.995), from methylene blue staining ( r = 0.992) and by optical absorption measurements ( r = 0.980) of the effluent after selectively flushing out the viable and non-viable cells from the chamber. Through measurement of cell viability by staining with methylene blue and plate counts, for an initial suspension of approx. 1.4 × 10 7 cells per ml containing 60% non-viable cells, the dielectrophoretically separated non-viable fraction contained 3% viable cells and the viable fraction 8% dead cells. The separation efficiency is increased by dilution of the initial suspension or by repeat operation(s). Cell viability was not affected by the separation procedure.

Dielectrophoretic precipitation of silver bromide suspensions

Spatially nonuniform ac electric fields will exert a unidirectional force upon either charged or neutral polarizable bodies suspended in liquid dielectric media. The motion resulting from this force, called dielectrophoresis, has been studied in aqueous suspensions of silver bromide particles. The rate of dielectrophoretic precipitation of the particles onto parallel wire electrodes was measured as a function of frequency at various fixed values of solution pAg. Additional measurements were made using dioxane and water/dioxane mixtures as the suspending medium. The observed frequency dependence of the precipitation rate or “yield” shows a maximum at frequencies between 10 5 and 10 6 Hz, which correlates well with published data on the frequency dependence of the dielectric dispersion of silver bromide emulsions. The results imply that the major contribution to polarization in the system studied is provided by the migration of point defects in a diffuse space charge layer located in the subsurface of the silver bromide particles. The type and density of defects is dependent upon the solution pAg, as evidenced by observed shifts of the frequency of peak yield with pAg. This frequency is, however, insensitive to variation of the macroscopic dielectric constant of the suspending medium. These observations are attributed to a modified Maxwell-Wagner effect.