Rheology modifiers impart viscosity, stability and sensorial properties to final cosmetic products, and very few formulations are made without them.
Rheology and viscosity are related, but there are technical differences: rheology is the study of the flow of matter, whereas viscosity is a measure of its resistance to deformation.
To measure rheology, shear stresses are recorded graphically at different shear rates. If the result is a straight line the product is described as a Newtonian fluid; if shear stress declines in proportion to shear rate it is pseudoplastic. If a composition flows easily on the application of shear, then regains its original viscosity, it is termed thixotropic.
Thixotropy enables the easy application of a ‘solid’ cream by a user; it will thin as it is rubbed on the skin then regain viscosity once application ceases. Thixotropic behaviour is built into a toothpaste formulation to enable it to be squeezed from its tube, but to regain sufficient viscosity to stay on the brush. Stress of thixotropic materials to enable short term thinning may also be caused by agitation or shaking.
The opposite effect is termed rheopectic or dilatant and this can prevent the easy pumping of a product as its resistance causes it to thicken substantially.