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where ε p * = ε p p / w and ε m * = ε m m / w , ε p and ε m are the absolute
dielectric permittivity ( ε r ε o ) of the particle and surrounding medium, respec-
tively, σ p and σ m are the electrical conductivity of the particle and surround-
ing medium, respectively, ω is the angular frequency ( ω = 2π f ) of the applied
ac field, r is the particle radius, E is the root mean square amplitude of the
electric field, represents the gradient operator and CM( ω ) is the Clausius-
Mossotti (CM) factor related to the effective polarizability of the particle.
Indeed, dielectrophoresis relies upon the interaction of the electric field with
an induced dipole in the particle. The direction of the resulting force depends
upon whether the particle is more or less polarizable than the medium, as
expressed by the CM factor ( Eqn (6.4) ). If the particle is more polarizable it
will be attracted to areas of high electric field strength and vice versa.
Related techniques include traveling wave dielectrophoresis 24 and elec-
trorotation, 25 both of which exploit phase-shifted electric fields to achieve
translational or rotational particle movement. Traveling wave dielectropho-
resis can be formed using either a linear or a spiral set of electrodes to
move particles, perpendicular to the electrode array ( Fig. 6.6 ). The direction
Figure 6.6 Outline of the main electrokinetic phenomena exhibited by particles in
electric fields. Electrophoresis occurs in DC electric fields (A), whereas AC electric fields
can be used to observe the other effects (B-E). To observe electrorotation and particle
movement in traveling wave devices (D-E), phase-shifted electric signals are applied to
the electrodes. Source: Adapted from Ref. 26 .
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