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Electrorotation
Electrorotation is a technique for applying controlled torque to dielectric objects. Basically, here's how it works...1. Microelectrodes are used to apply a rotating electric field. The electrodes are arranged in a cross, with a gap of about 40 microns in the middle. Sin(wt) is applied to one opposite pair, cos(wt) to the other, so that the resulting field rotates. The frequency is about 2 MHz.
2. The field polarizes the object, say a tethered bacterial cell.
3. Because the field is rotating fast (ie. its period is comparable to the relaxation time of the cell and surrounding solution) the polarization dipole lags behind the instantaneous field.
4. The dipole tries to line up with the field, so the object feels a torque.
In practice, tethered cells can be rotated at several hundred Hz using this technique. The movie shows a tethered cell being electrorotated. Initially, the cell rotates on its own driven by the flagellar motor. Next it is speeded up by electrorotation. The electrorotation is turned down steadily until it is zero, then cranked up the other way - driving the cell backwards. At the end electrorotation is turned off, and the cell spins on its own again.
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The movie shows a tethered cell subjected to electrorotation using
lithographically drawn microelectrodes with a 40 micron gap. At
the beginning and end the cell is free-spinning, in the middle it is
electrorotated. First it is pushed forwards, then gradually less
and less, through zero, then finally backwards before it is let
go. (click on image for movie. 1.7MB .mov) |
Data from this sort of experiment are shown below. The speed of the cell is plotted against the amount of electrorotation torque. This cell has a motor which spontaneously switches direction - so there are two lines symmetrically disposed about the origin. The plot was made by putting down data from many runs, in each run the electrorotation was smoothly varied and the motor switched direction occasionally, jumping from one line to the other. The bottom plot shows the amount of torque contributed by the motor as a function of speed. The step at about +50 Hz is because the motor was damaged by electrorotation during one run. This is a problem we're working on. This work was published a while ago ( Biophys. J. 69:280-286. Biophys. J. 71:3501-3510. Biophys J. 76:580-587).
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last updated: 21 November 2011