= Solution
Under the same positive, real-permittivity assumptions,
$$
\boxed{\text{negative dielectrophoresis: }\epsilon_p<\epsilon_m.}
$$
The particle is less polarizable than the surrounding medium and moves towards weaker-field regions. The sign describes the \b[polarizability contrast], not a negative net electric charge. Conducting or lossy media require the real part of the full complex <Clausius-Mossotti factor>; a comparison of only the real dielectric constants is then insufficient.
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