Article by: Kelly Dobos

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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3 comments

  1. Michael Fields

    Particle size has been found to be a most useful way of enhancing stability of inherently unstable emulsions at HBS, now HPC. We have a sunscreen product that requires brutal homogenization to form small and uniform sized oil particles required for this O/W emulsion to achieve stability. We have also used small dispersion particle size to increase viscosity in some of our cleansing gel products. I greatly agree that it is a critical property to manage.

  2. Colin

    The theory is lovely, but whenever I have looked at it I have found that the viscosity it predicts you need is really really high. It would give you a product so thick you would be able to play tennis with it.

    1. Kelly Dobos
      Kelly Dobos

      Good point Colin! I tend to think of the particle size as the most important parameter, but maybe I didn’t emphasize it enough in my writing. I also must figure out how to display equations better.

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