The following excerpt is adapted from an article by Kelly Dobos of Kao Corp., titled, "Fluid Viscosity and the Formulation Chemist," which was featured on the Chemist's Corner. The article discusses the fluid viscosity of cosmetic products, specifically types of flow behavior, that can be used by the formulator to determine the rheological properties of formulations.
Cosmetic Viscosity Science
Viscosity is the measure of a fluid’s internal friction (resistance to flow) when one layer of fluid is forced to move over another layer. A fluid may be comprised of a single type of molecule or molecules that vary in size, shape and cohesiveness. As these molecules are forced to move or flow past one another, the molecular properties determine how much force is required to move them past each other. The force required to cause movement is referred to as shear. A few examples of shear forces in cosmetics include: spraying hair products, pumping products into packaging during manufacture, spreading of lotion on the skin, and pouring shampoo from a bottle.
Viscosity typically is 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 a 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, which are the easiest to measure but fairly uncommon since far more complex fluid behaviors are involved in cosmetics. There are many types of non-Newtonian behaviors and this term basically refers to any fluid that exhibits changes in viscosity with variations in shear rate.
Shear thinning: Most cosmetic products like emulsions and suspensions 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 the opposite of shear thinning, as viscosity increases with increasing shear. This type of flow behavior is rare but examples include quicksand and slurries of cornstarch.
Thixotrophic materials: These products thin with a constant shear rate but recover their structure and thus increase in viscosity over time once the shear force is removed. Occasionally, shear thinning behavior is misinterpreted as thixotropy, so it is important to remember the distinction: thixotropy is that thinning occurs at a constant shear rate over time, as opposed to thinning at an increasing shear rate in shear thinning behavior.
Viscosity can also increase with shear forces like shaking or mixing and then lower to the original value in what is described as rheopectic behavior, but this type of behavior also is rare. Both thixoptrophy and rheopexy can occur 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 liquids 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 considering these types of fluid behavior, the cosmetic formulator can design the desired rheological properties of cosmetic formulations through choice of ingredients and processing conditions, as well as learning to troubleshoot product failures.