The viscosity of fluids can be modified by addition of particulates that may strictly change the viscosity index. When non-interacting buoyant particles are used in these
fluids, the viscosity of the dispersion can be predicted using the Einstein relation. Examples of such rheology-modifying substances include silica gels, fumed silica, carbon black, titanium dioxide and aluminum-magnesium-stearates when used at very small concentrations. Low molecular weight polymers also fit in this category and may be preferred if a smooth or fluid like formulation is desired.

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

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