Using a birefringent probe particle in variable polarisation optical tweezers to probe properties of biological materials.

Probe particles held in force measuring optical tweezers (OT) are being widely used to probe biological fluids, membranes, and living cells. In some cases, the properties of a discrete molecule or microstructure to which the probe is attached are probed. In other cases, as here, it is the viscoelastic properties of the material surrounding the probe which are of interest. These can be deduced from measurements of the response of the probe to a known applied force. Alternatively, measurements of Brownian motion of a trapped particle can give information on the surrounding material.