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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.