Thus, by measuring, if the particle size and temperature of the fluid are known, the fluid viscosity can be determined. In that equation, k is the Boltzmann constant and pi=3.14159. The diffusion constant is related to the radius of the tracer particles a, the temperature of the fluid, and the viscosity of the fluid eta this is given by the second equation at right. The mean square displacement of the batch of tracer particles grows linearly in time dt, with a growth rate given by the diffusion constant D (this is the first equation at the right). Viscosity and diffusionĪ simple experiment is to put small tracer particles in a viscous fluid, and study their motion. Additionally, this technique increases our microscopic understanding of these complicated materials. Only small amounts of material are needed for this technique. In the Weitz lab we have developed the technique of microrheology which looks at the thermal motion of small particles embedded in a material in order to extract the bulk rheological properties. However, conventional rheology requires several milliliters of the substance to be measured. For a viscoelastic system, rheology can describe the combined viscous and elastic properties of a material. Conventional rheology is a way to measure the squishiness of materials: how viscous is a liquid, how elastic is a solid. To some degree, these materials can act both like fluids and like solids, depending on how you put forces on them.

Complex fluids like mayonaise, complex materials like plastics, and biomaterials like cells are all somewhat squishy.

Small bead viscosity test