What is Rheology ?
Rheology is the study of the mechanical properties of condensed matter and in particular complex fluids. The name derives from the greek work "rheo" which means "to flow". A solid will usually respond to a stress force by deforming and storing energy elastically. A liquid, however, will flow and dissipate energy continuously in viscous losses. A Newtonian liquid has a linear relationship between the shear rate and stress. Complex fluids are interesting because they exhibit both an elastic and viscous responses and exhibit a non-linear relationship between the shear rate and strain.
The non-linear mechanical properties of complex fluids are attributed to changes in the organisation of the molecules that comprise the fluid as they experience a deforming force. These changes can range from fast Brownian motions (ps to ns) of smaller molecules and molecular segment rotations, to slower (ms to s) reorientation or reorganisation of macromolecules or large assemblies of molecules.