Rotation of cells and ion exchange beads in the MHz-frequency range

Whereas human red blood cells and protoplasts of leaves of Avena sativa show rotation only at discrete frequencies in the kHz-range of a linear alternating electric field, rotation of these cells is observed at practically every frequency in the MHz range (15 to 200 MHz). The cell rotation in the MHz-range can be explained in terms of field induced polarisation and orientation of permanent dipoles within the membrane and the cell. This interpretation is supported by the finding that Chelex beads (polystyrene cross-linked with divinylbenzene and coupled to iminodiacetic acid) do exhibit rotation in the MHz-range, but not in the kHz-range. It is most interesting that the Cu2+- and Ca2+-forms of the Chelex beads show a shift in the rotation frequency spectrum in addition to an increase in the magnitude of the rotation speed with respect to the Na+-form. At frequencies in the MHz-range at which rotation of the Chelex beads is not observed, formation of chains of beads from the electrodes occurs instead. This is due to positive polarisation of the beads leading to positive dielectrophoresis. The results support the view that rotation needs negative polarisability which is determined by the functional groups.