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.

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.

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