How Stokes Law Relates to Cosmetic Stability

Stokes Law and Emulsion Stability

There are several mechanisms of emulsion instability including creaming/sedimentation, phase inversion, coalescence, Ostwald ripening, and flocculation. Creaming (oil phase rises to the surface) and sedimentation are driven by density differences between oil and water phases. Stoke’s Law, which describes spherical particle movement in a fluid, provides insight into options for reducing this instability phenomenon.

Stokes Law

What can a formulator do?

Examining the equation in detail, what parameters can a formulator affect?

Gravity, good luck!

Density differences, not likely.

That leaves particle size and viscosity of the continuous phase. The viscosity of the continuous phase can be modified by adding gums, thickeners, or polymers in O/W systems. In W/O systems there are also polymeric options in addition to waxes, soaps, and some emulsifiers like glyceryl oleate. Particle size can be easily minimized with high-shear homogenization and use of emulsifiers that bring the interfacial tension to a minimum.

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