Reorientation dynamics and micromanipulation of natural microscopic soft matter in an optical trap with varying polarization of the laser

We report here on the reorientation dynamics of the avian Red Blood Cell (aRBC), a structure ellipsoidal in shape and with a non-uniform distribution of birefringence across its diameter. We find that in linearly polarized light, an aRBC shows a dual reorientation behavior with the first reorientation about the major axis and the second about the minor axis so as to align its major axis along the laser propagation direction. We are able to explain the observed sequence of reorientation as a consequence of the minimization of work done in rotating the avian cell first about the major axis and then about the minor axis. These calculations are also used to also show why a single reorientation about the minor axis to reach the final equilibrium position is not seen experimentally. Further, in the case of elliptically polarized light, we see that the second reorientation process about the minor axis of the cell can be controlled by the ellipticity of the polarization of light. We explain this as a consequence of balance between the torques due to birefringence and reorientation.