(Link) “The science of how things flow.” (RM). So rheology modifiers are ingredients that can change how fluids flow, altering the viscosity of a surfactant based product, like bubble bath, body wash, shampoo, or cleanser, or an emulsified product, like a lotion, cream, or moisturizer. You can also alter the viscosity of an anhydrous product, like those made with silicones, like a primer, anti-frizz spray, or facial serum, or oils, like a body oil, facial serum, or balm.