J. Cosmet. Sci., 71, 481–496 (November/December 2020) 481 Rheology of Cosmetic Products: Surfactant Mesophases, Foams and Emulsions AINA DAVIES and SAMIUL AMIN , Department of Chemical Engineering, Manhattan College, Bronx, NY 10471 (A.D., S.A.) Accepted for publication July 24, 2020 . Synopsis Cosmetic products such as shampoos, body washes, mascaras, and foundations can all be classifi ed essentially as complex fl uids. Depending on the product format, the specifi c complex fl uid used in a formulation can range from self-assembled surfactant systems found in most cleansing products to oil-in-water and water-in- oil emulsions found in makeup, color cosmetics, and skin care. These complex fl uids play a critical role in giving rise to rheological and tribological properties necessary for both sensory and functional performance. Critical functional properties such as product stability and wet hair conditioning are impacted signifi cantly through any changes in the product rheology. Positive sensorial properties of products are always desired by consumers, and as such, it is critical to always consider how to optimize formulation rheology to adequately deliver desirable sensory performance and properties. This review will focus on the importance of understanding and characterizing the rheology of complex fl uids used in cosmetic products. A review and analysis of the recent literature in this area is presented. INTRODUCTION Many formats of cosmetic products such as foundations, mascaras, cleansers, creams, and lotions need to be optimized for their rheological performance to yield stable and func- tional products. Understanding and engineering the rheology of the underlying complex fl uids play a critical role in formulation and engineering high-performance products. Cosmetic products are usually formulated to be non-Newtonian complex fl uids and exhibit shear-thinning properties (1). Rheology plays a signifi cant role in dictating product stabil- ity, and as such, it is important to engineer certain rheological properties such as viscosity and yield stress into cosmetic products to enhance product stability. Engineering rheo- logical performance can additionally impact the tribological or lubrication performance in these complex fl uid-based cosmetic products. Certain cosmetic products such as hair conditioners require maintenance of high viscosity values to ensure that adequate lubrica- tion is achieved. The Stribeck curve perfectly describes the relationship between viscosity and friction coeffi cient at three different regimes, that is, the boundary regime, mixed regime, and hydrodynamic regime. Hydrodynamic lubrication is important for initial Address all correspondence to Samiul Amin at samin01@manhattan.edu.

JOURNAL OF COSMETIC SCIENCE 482 sensory response, whereas mixed regime lubrication performance is important for in-use sensory performance (2). Viscosity plays an important role in both of these. Particulate lubricants, such as mineral oils containing dispersed solids are extensively used in indus- trial applications. It has also been shown that the tribological properties of cosmetic products affect its performance (3,4) and skin feel (5–7). An understanding of the rheology is critical also for product processing where a knowledge of the product rheology dictates certain processing parameters such as pumping, pouring, and mixing. Product application and spreadability are also highly governed by product rheology (8). In addition to the importance of the rheology of surfactant mesophases and emulsions found in liquid and semisolid cosmetic products, the rheology of the foams gen- erated during in-use conditions plays a critical role in sensory performance of products such as facial cleansers, body washes, and shampoos. Foams are a manifestation of the surface activity of the surfactants (9), which is itself related to the rheology of the foam itself. It is also important to note that the demands of the present-day consumers have shifted dramatically from high-performing, aesthetically pleasing products to products that are eco-friendly, use ingredients from sustainable sources, and are tailored to individual needs. Another factor that is driving the cosmetic industry to a more sustainable path is laws and regulations placed by certain governments (10). This new trend requires an under- standing of the rheology of sustainable ingredients. The utilization of automated formulation platforms across the cosmetic industry enables one to perform complex formulation workfl ows in a fully automated manner. Its advantages over manual formulation include, but are not limited to, continuous real-time monitor- ing and control of all formulation parameters, including internal and jacket temperature, shear rate and viscosity, scraping speed, refl ux and pH in an inert atmosphere, if desired, and automatic logging of all data. Its overall effi ciency brings about a decrease in cost per sample up to 90% (8). Automated formulation platforms provide the fl exible mode of operation and production necessary to meet the growing demands of individualization, personalization, and mass customization. These platforms can be used heavily in estab- lishing an understanding of rheology and controlling formulation design rules in a much faster and effective manner. APPLICATIONS OF RHEOLOGY IN COSMETIC PRODUCTS SURFACTANT MESOPHASE: BODY WASHES, SHAMPOOS, AND HAIR CONDITIONERS Surfactant mesophases have a huge impact on the rheology of products such as body washes, shampoos, and hair conditioners (8,11). Surfactants possess self-assembly properties in solution when the critical micelle concentration (CMC) is surpassed. Surfactants self- assemble to form various meso-structures that dictate the formulations’ microstructure and various rheological properties (12). The formation of micelles in aqueous solution is generally viewed as a compromise between the tendency for alkyl chains to avoid ener- getically unfavorable contacts with water, and the desire for the hydrophilic parts to maintain contact with the aqueous environment. A thermodynamic description of the process of micelle formation includes both electrostatic and hydrophobic contributions to the overall Gibbs energy of the system (13). There are multiple mesophases that can be formed by surfactant systems, such as wormlike micelles and lamellar gel phases.