The use of rheological additives such as clays, plant exudates, and natural polymers, to formulate personal-care products dates back to ancient times. These rheological additives are used to thicken the fluid, suspend dispersions of additives in the fluid, and improve the stability of the ensuing dispersion or emulsion as a function of temperature and shear history. An attempt will be made in this chapter to classify the wide array of rheological additives with respect to the actual function they serve in the final product.

Water and oils form the base fluids in which most personal-care and cosmetic products are formulated. These base fluids are generally classed as viscous or Newtonian fluids in that they possess a characteristic viscosity that is independent of the imposed rate of deformation. Newtonian fluids are also viewed as ideal fluids, in that they flow readily when subjected to very low deformations.
Non-Newtonian fluids on the other hand possess viscosities that are dependent on the rate of deformation and may exhibit other properties such as elasticity, yield stress, and thixotropy not seen in Newtonian fluids.

Source: Handbook of Cosmetic Science and Technology - André O. Barel, Marc Paye, Howard I. Maibach

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