= Solution
For ordinary lossless dielectrics with positive real permittivities,
$$
K=\frac{\epsilon_p-\epsilon_m}{\epsilon_p+2\epsilon_m}.
$$
The denominator is positive, so
$$
\boxed{\text{positive dielectrophoresis: }\epsilon_p>\epsilon_m.}
$$
The particle is more polarizable than the displaced medium and is driven towards the region where its polarization lowers the field-plus-source energy most effectively. If the two permittivities match, this ideal dipole contrast vanishes.
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