Studies on shape anisotropy in red blood cells

Asymmetric particles, such as biological cells, often experience torque under optical tweezers. The cause is believed to be either birefringence or unbalanced scattering forces. The estimate of torque relies on the accurate measurement of rotational motion. Here we present a new technique to quantify the asymmetry of trapped particles relying upon the cross coupling between rotational and translational Brownian motion. We observe that RBC does indeed show cross coupling indicating asymmetry of the shape. Further we also show by polarimetry that the retardance of the RBC is not sufficient to make it rotate since the scattering torque is much higher.