Serums and lotions are a popular self-care products to keep skin healthy. Formulation ingredients can vary dramatically. Some use plant-derived ingredients while others use synthetic oils depending on consumer preferences and needs. Thus, development of these products is a dynamic process requiring a plethora of characterization experiments to differentiate between formulations.
When developing skin care products, manufacturers must carefully choose ingredients that must meet both consumer tastes and regulatory standards. Skin care products are comprised of multiple materials including polymers, oils, waxes, gels, and silicones (Sharma et al. 2018). These ingredients can dramatically alter the product viscosity, which can make measuring viscosity difficult, messy, and variable. The combination of these ingredients can even cause products to behave as thixotropic materials.
Thixotropy is the time dependent shear thinning property of non-Newtonian fluids (Mewis 1979). Thixotropic materials will slowly return to their high viscosity state once shearing has ceased. The thixotropic behavior of lotions can result in inconsistent data. The same shear force and/or shear rate can be applied to a material multiple times in quick succession and the measured viscosity will vary after each measurement because the microstructure (one of the driving forces of thixotropy) of the material has not recovered to its original state.
In our Viscosity of Cosmetics - Thixotropic Samples application note we present a ten-step protocol to measure high viscosity, complex lotion samples using Viscometer-Rheometer-on-a-chip (VROC®) technology. This protocol will describe the steps to prime the VROC chip with glycerol, run a lotion sample, and clean the VROC chip with glycerol, using minimal material, quickly and efficiently, resulting in repeatable data. The lotion tested in this protocol had an average viscosity of 424.40 mPa-s, a standard deviation of 1.68 mPa-s, and a %RSD (relative standard deviation) of 0.40%.