  We report that the average rotation speed of optically trapped crenated
erythrocytes is direct signature of their membrane deformability. When placed
in hypertonic buffer, discocytic erythrocytes are subjected to crenation. The
deformation of cells brings in chirality and asymmetry in shape that make them
rotate under the scattering force of a linearly polarized optical trap. A
change in the deformability of the erythrocytes, due to any internal or
environmental factor, affects the rotation speed of the trapped crenated cells.
Here we show how the increment in erythrocyte membrane rigidity with adsorption
of $Ca^{++}$ ions can be exhibited through this approach.
