Rheology is the study of how materials flow and deform. It extends beyond viscosity, which is a single number, to describe how a material's behaviour changes under different conditions - how hard it is pushed, for how long, and at what temperature.
The distinction matters because many products deliberately behave differently under different conditions. Paint should be thick enough not to run on a wall but thin enough to brush out easily, so it must thin under the shear of brushing and thicken again at rest - a property called shear-thinning. Ketchup behaving the same way is the everyday illustration.
Other rheological behaviours include shear-thickening, where a material stiffens under stress, and thixotropy, where viscosity depends on how long stress has been applied rather than just how much. Some products also behave as solids until a threshold force is exceeded, which is what stops a gel flowing under its own weight.
Rheology is controlled through thickeners and rheology modifiers, and it is where formulation development often concentrates - a product can have the correct chemistry and still fail in use because it does not flow the way its market expects. Where behaviour under shear matters, specify it rather than relying on a single viscosity figure.
If your product must behave differently under shear than at rest, say so in the brief rather than specifying a single viscosity figure. Manufacturers can formulate for it, but only if they know it is required.
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Last reviewed: August 2026. Regulation and guidance change — verify anything you rely on against the primary source. Tell us if something here is out of date.
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