I remember the first time I learned about rheology, or the study of how materials deform and flow, at my grade school science fair. One of my classmates had a poster on the thixotropy of ketchup. My project was cryogenics, I was clearly born to be a scientist but that’s a story for some other time. It wasn’t until I started my career as a formulation chemist that I became reacquainted with the topic.
You’ll be hard pressed to find a cosmetic formulation that doesn’t involve some understanding of rheology. 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.
Cosmetic Viscosity Science
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. A fluid may be made up of molecules that vary in size, shape, and cohesiveness or a single type of molecule. 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. The force required to cause movement is referred to as shear. 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.
Viscosity is typically measured with a device like a Brookfield Viscometer. This instrument has a rotating spindle attached to a force measuring meter. When the spindle is submerged in the sample, it gives a viscosity reading based on the force required to maintain a specific rotating speed.
Types of Flow Behavior
For Newtonian fluids viscosity is constant at varying shear rates. Water and thin oils are examples of Newtonian liquids. This type of fluid is easiest to measure, but fairly uncommon, as you will encounter far more complex fluid behaviors in cosmetics.
There are many types of non-Newtonian behavior, and the term basically defines any fluid that exhibits changes in viscosity with variations in shear rate.
Shear Thinning – Most cosmetic products like emulsions and suspension are shear thinning, meaning that viscosity decreases with increasing shear rate. This behavior is also referred to as pseudoplastic and is the result of structural breakdown within the fluid.
Dilatant flow — This is just the opposite. Viscosity increases with increasing shear. This type of flow behavior is pretty rare but examples include quicksand and slurries of cornstarch.
Thixotrophic materials — These products thin with constant shear rate but recover their structure and thus increase in viscosity over time once the shear force is removed. Occasionally sheer thinning behavior is misinterpreted as thixotropy, so it is important to remember the distinction for thixotropy is that thinning occurs at a constant shear rate over time as opposed to thinning at an increasing shear rate in sheer thinning behavior. Viscosity can also increase with shear forces like shaking or mixing and then lower to the original value in what’s described as rheopectic behavior, but this type of behavior is also rare. Both thixoptrophy and rheopexy can occur in with other types of flow behavior and the initial viscosity may not be fully recovered.
Another important rheological value is the yield point. Some fluids behave like solids at rest but flow like liquid and decrease in viscosity once the yield value or yield point is exceeded. Fluids with high yield points can easily suspend particles like mica or pigments in cosmetic preparations.
By taking into consideration these types of fluid behavior the cosmetic formulator can select for the rheological properties of cosmetic formulations through choice of ingredients and processing conditions, as well as learning to trouble shoot product failures.
And by the way, ketchup is not thixotropic. See if you can reason why.