LOW-FREQUENCY MICROMOTIONS OF DEP-LEVITATED PLANT-PROTOPLASTS .1.

Precision dielectrophoretic levitation measurements performed on certain biological cells and other colloidal particles reveal an anomalous response at low frequencies inconsistent with the predictions of conventional DEP theory, One hypothesis for this anomalous behavior is that a time-varying electrophoretic force on the naturally charged cells induces small-scale motion (micromotion) of the particles about their equilibrium position. This motion contributes a time-average force component that mimics dielectrophoresis and complicates the interpretation of DEP spectra, Both levitation measurements and direct observation of the particle motions provide support for the micromotion hypothesis, At modest voltage values, the motion is dominantly at the frequency of the applied field and its amplitude is directly proportional to the applied levitator voltage. Furthermore, the motion exhibits the frequency dependence expected of a damped, second-order system, Were the motion dielectrophoretic in origin, its dominant frequency would be twice the applied frequency and the amplitude would depend on the square of the voltage. (C) 1995 academic Press, Inc.