In biochemistry, flow birefringence is a hydrodynamic technique for measuring the rotational diffusion constants (or, equivalently, the

rotational drag coefficients Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...

). The

birefringence Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive). The birefring ...

of a solution sandwiched between two concentric cylinders is measured as a function of the difference in rotational speed between the inner and outer cylinders. The flow tends to orient an ellipsoidal particle (typically, a protein, virus, etc.) in one direction, whereas rotational diffusion (tumbling) causes the molecule to become disoriented. The equilibrium between these two processes as a function of the flow provides a measure of the axial ratio of the ellipsoidal particle.

See also