Dielectrophoresis

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Dielectrophoresis (or DEP) is a phenomenon in which a force is exerted on a dielectric particle when it is subjected to a non-uniform electric field. This force does not require the particle to be charged. All particles exhibit dielectrophoretic activity in the presence of electric fields. However, the strength of the force depends strongly on the medium and particles' electrical properties, on the particles' shape and size, as well as on the frequency of the electric field. Consequently, fields of a particular frequency can manipulate particles with great selectivity. This has allowed, for example, the separation of cells or the orientation and manipulation of nanoparticles and nanowires.

For a field-aligned prolate ellipsoid (a>b=c)of radius $a$ and half-length $b$ with dielectric constant $ε p$ in a medium with constant $ε m$ , the dielectrophoretic force is given by:

$F_{dep} = \frac{\pi a^2 b}{3}\epsilon_m \textrm{Re}\left\{\frac{\epsilon^*_p - \epsilon^*_m}{\epsilon^*_m}\right\}\nabla \left|\vec{E}\right|^2$

This is valid if the electric field does not change significantly over the particle length. The equation only takes into account the dipole formed and not higher order polarisation. Dielectrophoresis had been investigated a few decades ago (Pohl, 1978) but has recently been revived due to its potential in the manipulation of microparticles, nanoparticles and cells.

Pohl H.A.¹ wrote in his book defining dielectrophoresis as the translational motion of neutral matter caused by polarization effects in a nonuniform electric field. The phenomenological bases are catalogued below:
1.The dielectrophoresis force can be seen only when particles are in the nonuniform electric fields.
2.Since the dielectrophoresis force does not depend on the polarity of the electric field, thus the phenomenon can be observed either with AC or DC excitation.
3.Particles are attracted to regions of stronger electric field when their permittivity exceeds that of the suspension medium.
4.When permittivity of medium is greater than that of particles, this results in motion of particles to lesser electric field.
5.DEP is most readily observed for particles with diameters ranging from approximately 1 to 1000 μm.

Phenomena associated with dielectrophoresis are electrorotation and traveling wave dielectrophoresis (TWDEP).

Dielectrophoresis coupled with Field-Flow Fractionation (DEP-FFF)
The utilization of the difference between dielectrophoretic forces exerted on different particles in nonuniform electric fields is now well known as DEP separation. The exploitation of DEP forces has been classified into two groups: namely DEP migration and DEP retention. DEP migration uses opposing polarities of DEP forces exerted on different particle types, so that one type is attracted toward high-field regions by positive dielectrophoresis while the other types are repelled by negative dielectrophoresis². DEP retention uses competition between DEP and fluid-flow forces. Particles experiencing a weaker negative DEP forces are eluted by fluid flow, whereas particles experiencing strong positive DEP forces are trapped at electrode edges against the drag of the fluid flow³.
Field-Flow Fractionation, a family of chromatographic-like separation methods, is an elution technique capable of simultaneous separation and measurement, which was primarily introduced by Davis and Giddings⁴ and Giddings⁵. Thereafter, DEP forces were combined with field-flow-fractionation (FFF) for particle separation^6,7,3. The idea of using DEP-FFF is summarized in the next paragraph.
Particles are injected into a carrier flow that passes through the separation chamber, with an external separating force (a DEP force) being applied perpendicular to the flow. By means of different factors, such as diffusion and steric, hydrodynamic, dielectric and other effects, or a combination thereof, particles (<1 μm in diameter) with different dielectric or diffusive properties attain different positions away from the chamber wall, which, in turn, exhibit different characteristic concentration profile. Particles that move further away from the wall reach higher positions in the parabolic velocity profile of the liquid flowing through the chamber and will be eluted from the chamber at a faster rate.

References:
1. Pohl, H.A., 1978. Dielectrophoresis the behavior of neutral matter in nonuniform electric fields. Cambridge University Press. Cambridge.
2. Gascoyne, P.R.C., Y. Huang, R. Pethig, J. Vykoukal and F.F. Becker, 1992. “Dielectrophoretic separation of mammalian cells studied by computerized image analysis”. Meas. Sci.Technol. 3, 439-445.
3. Huang, Y., J. Yang, X.B. Wang, F.F. Becker and P.R.C. Gascoyne, 1999. “The removal of human breast cancer cells from hematopoietic CD34+ stem cells by dielectrophoretic field-flow-fractionation”. Journal of Hematotherapy & Stem Cell research. 8, 481-490.
4. Davis, J.M. and J.C. Giddings, 1986. “Feasibility study of dielectrical field-flow fractionation”. Sepa. Sci. and Tech. 21, 969-989.
5. Giddings, J.C., 1993. “Field-Flow Fractionation: Analysis of macromolecular, colloidal, and particulate materials”. Science. 260, 1456-1465.
6. Huang, Y., X.B. Wang, F.F. Becker and P.R.C. Gascoyne, 1997. “Introducing dielectrophoresis as a new force field for field-flow fractionation”. Biophys. J. 73, 1118-1129
7. Wang, X.B., J. Vykoukal, F.F. Becker and P.R.C. Gascoyne, 1998. “Separation of polystyrene microbeads using dielectrophoretic/gravitational field-flow-fractionation”. Biophysical Journal. 74, 2689-2701.