For simple fluids, viscosity is typically measured as a function of resistance to rotation of a spindle in a large (say, 600 cc) container of solution. To characterize rheology, the viscosity must be measured at different shear rates. A small amount of solution is placed between a rotating plate and a fixed surface at a defined distance or a rotating cylinder and a fixed well at a defined distance. The shear rate is a function of the distance between surfaces and the speed of the moving surface.
Polymers are large molecules built from repeating subunits of smaller molecules, like a chain. Polymers are flexible molecules and come in all sorts of shapes and sizes. A polymer shaped like a rod will have a higher viscosity than a polymer of the same mass shaped like a ball. Think of pouring a bunch of bobby pins and a bunch of BBs in a funnel. The round BBs will go through faster because the bobby pins will all point in different directions and entangle. However, if the bobby pins can all be made to point straight down, they may go through the funnel just as fast as the BBs.
Typically, when polymers are under low shear, the viscosity is high. As the shear rate increases, the viscosity decreases because the molecules are aligned in rows like the bobby pins by movement of the fluid. A decrease of viscosity with shear will be more noticeable for the rod- or plate-shaped molecules than the round molecules. Generally the viscosity will increase again as the shear force is reduced. This behavior is called pseudoplasticity.
There are other profiles of rheological behavior. Some polymers, such as cornstarch, increase in viscosity when shear is applied. Such a solution is called dilatant. If a polymer decreases in viscosity the longer it is stirred under constant shear, it is said to be thixotropic. Some polymer solutions act a little bit like a liquid rubber band. They store the energy placed in them from the rheometer. These solutions are called viscoelastic solutions.
Rheometers measure viscosity and can be programmed to apply increasing and/or decreasing shear over time. The amount of time required for polymers to return to the viscosity at a previous speed is a measure of the rate of return to their associated states or random orientations.
Studying the rheology of polymer solutions under different shear profiles can provide some insight into the molecular weight and shape of the polymer. Associations among polymer molecules can also be studied.
In industry, rheological behaviors are exploited to give products desired characteristics. A paint may go on smooth (viscosity lowers when brushed on), but won't run (viscosity rapidly increases when the brushing motion is stopped).