3D rotational diffusion microrheology using 2D video microscopy

We propose a simple way to perform three-dimensional (3D) rotational microrheology using two-dimensional (2D) video microscopy. The 3D rotational brownian motion of micrometric wires in a viscous fluid is deduced from their projection on the focal plane of an optical microscope objective. The rotational diffusion coefficient of the wires of length between 1-100 \mu m is extracted, as well as their diameter distribution in good agreement with electron microscopy measurements. This is a promising way to characterize soft visco-elastic materials, and probe the dimensions of anisotropic objects.