Microrheology is a technique to measure the viscoelasticity of a material by oberving the motion of a suspension of micron size particles. At the micron scale particles suspended in any complex liquid will undergo small thermal fluctuations. The size of the fluctuations are directly related to the size of the suspended particles, temperature, and the properties of the material, and hence by measuring the fluctuations one is actually measuring the rheology. To learn more about microrheology take a look at Introduction to microrheology by Eric Weeks & Dave Weitz at Harvard University