Viscosity is typically measured with a device like a Brookfield Viscometer. Just a few examples of shear forces in cosmetics: spraying hair products, pumping products into packaging during manufacture, spreading of lotion on the skin, and pouring shampoo from a bottle. The force required to cause movement is referred to as shear. As these molecules are forced to move or flow past each other, the molecular properties will determine just how much force is required to move them past each other. A fluid may be made up of molecules that vary in size, shape, and cohesiveness or a single type of molecule. Viscosity is a measure of a fluid’s internal friction (resistance to flow) when one layer of fluid is forced to move in over another layer. Single point measurements are often used on specifications for release of products and raw materials in manufacturing, toothpastes need to flow when squeezed from the tube but not drip off a toothbrush, and body washes that contain beads need yield value to prevent them from settling out. You’ll be hard pressed to find a cosmetic formulation that doesn’t involve some understanding of rheology. This article originally appeared on my other website (the one that Randy always accuses me of cheating on him with.) Even though it’s intended for formulation chemists, I thought the Beauty Brains audience might appreciate an explanation of what is often referred to as “thickness.” Cosmetic viscosity

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