RHEOLOGY OF COSMETIC PRODUCTS 493 persistence of empirical formulation approaches, other factors, such as the possibility of at- tractive interactions between droplets, can also be important in some cases. Although much progress has been made, there is still considerable room for improvement in the develop- ment of predictive tools for the design and formulation of emulsions, and no single theory or simulation yet describes emulsion rheology over the very broad range of possibilities of compositions and fl ow histories of these interesting systems. CONCLUSION The colloidal and complex fl uid properties are critical in optimizing the performance of cosmetic products. In this review, it was described how the microstructural properties of various materials used in the formulation of cosmetic products affect and infl uence the rheological performance of the formulations. A deep understanding of rheology and fac- tors that infl uence and govern it as well as its properties and the vast applications of rheology in the cosmetic industry is key in establishing structure–property–performance linkages. This is critical to the engineering and formulation of novel cosmetic products. As the new trends in cosmetics are moving toward increased personalization of cosmetic products and sustainability (69), a profound knowledge of rheology is an important skill that should be harnessed in ensuring that consumer needs are satisfi ed at all times. The future direction of complex fl uids used in cosmetic products is the utilization of bio- ingredients such as bio-surfactants (19,70), and biopolymers (71) as well as the utiliza- tion of an automated formulation platform which makes it possible to vary the formulation composition of each sample simultaneously thereby saving time and cost (72). The mass introduction of automated formulation into the cosmetic industry (production line) will be benefi cial to formulation and postformulation processes in the industry because it en- ables its operator to perform complex formulations simultaneously as customized opera- tions for consumers. REFERENCES (1) F. Begum, L. Xu, and S. Amin, “Surfactants,” in Kirk-Othmer Encyclopaedia of Chemical Technology, Wiley Online Library, Hoboken, NJ (2000), pp. 1–34. (2) R. J. Underwood, The tribological effects of contamination in rolling element bearings (Doctoral dis- sertation, Imperial College London, 2008). (3) N. Gitis and R. Sivamani, Tribometrology of skin, Tribol. Trans., 47(4), 1–9 (2004). (4) J. K. Prall, Instrumental evaluation of the effects of cosmetic products on skin surfaces with particular reference to smoothness, J. Soc. Cosmet. Chem., 24, 693–707 (1973). (5) M. Lodén, H. Olsson, L. Skare, T. Axéll, and A. H. Ab, Instrumental and sensory evaluation of the frictional response of the skin following a single application of fi ve moisturizing creams, J. Soc. Cosmet. Chem., 43, 13–20 (1992). ( 6) E. L. Cussler, S. J. Zlotnick, and M. C. Shaw, Texture perceived with the fi ngers, Percept. Psychophys., 21(6), 504–512 (1977). ( 7) S. Nacht, J. A. Close, D. Yeung, and E. H. Gans, Skin friction coeffi cient: changes induced by skin hydration and emollient application and correlation with perceived skin feel, J. Soc. Cosmet. Chem., 32(2), 55–65 (1981). ( 8) A. R. Davies and S. Amin, Microstructure design of CTAC: FA and BTAC: FA lamellar gels for opti- mized rheological performance utilizing automated formulation platform, Int. J. Cosmet. Sci., 42(3), 259–269 (2020). ( 9) Y. Zhou, S. Harne, and S. Amin, Optimization of the surface activity of biosurfactant-surfactant mix- tures, J. Cosmet. Sci., 70(3), 127–136 (2019).

JOURNAL OF COSMETIC SCIENCE 494 ( 10) D. Xanthos and T. R. Walker, International policies to reduce plastic marine pollution from single-use plastics (plastic bags and microbeads): a review, Mar. Pollut. Bull., 118(1–2), 17–26 (2017). ( 11) C. Gallegos and J. M. Franco, Rheology of food, cosmetics and pharmaceuticals, Curr. Opin. Colloid In- terf. Sci., 4(4), 288–293 (1999). ( 12) W. Richtering, Rheology and shear induced structures in surfactant solutions, Curr. Opin. Colloid Interf. Sci., 6(5–6), 446–450 (2001). ( 13) L. L. Schramm, Fundamentals and applications in the petroleum industry, Adv. Chem., 231, 3–24 (1992). ( 14) J. N. Israelachvili, Intermolecular and Surface Forces, 2nd Ed. (Academic Press, London, United Kingdom, 1992). ( 15) T. Tadros, Encyclopedia of Colloid and Interface Science (Springer, Berlin, 2013). ( 16) S. Sinha, D. Tikariha, J. Lakra, A. K. Tiwari, S. K. Saha, and K. K. Ghosh, Effect of polar organic sol- vents on self-aggregation of some cationic monomeric and dimeric surfactants, J. Surfactants Deterg., 18(4), 629–640 (2015). ( 17) D. Myers, Surfactant Science and Technology 3rd Ed., (John Wiley & Sons, Hoboken, NJ 2005). ( 18) H. Jiang, G. Beaucage, K. Vogtt, and M. Weaver, The effect of solvent polarity on wormlike micelles using dipropylene glycol (DPG) as a cosolvent in an anionic/zwitterionic mixed surfactant system, J. Colloid Interf. Sci., 509, 25–31 (2018). ( 19) S. Amin, S. Blake, R. C. Kennel, and E. N. Lewis, Revealing new structural insights from surfactant micelles through DLS, microrheology and Raman spectroscopy, Materials, 8(6), 3754–3766 (2015). ( 20) K. N. Silva, R. Novoa-Carballal, M. Drechsler, A. H. Müller, E. K. Penott-Chang, and A. J. Müller, The infl uence of concentration and pH on the structure and rheology of cationic surfactant/hydrotrope struc- tured fl uids, Colloid Surf. Physicochem. Eng. Aspect., 489, 311–321 (2016). (21) L. Xu and S. Amin, Microrheological study of ternary surfactant-biosurfactant mixtures, Int. J. Cosmet. Sci., 41(4), 364–370 (2019). (22) S . R. Raghavan and E. W. Kaler, Highly viscoelastic wormlike micellar solutions formed by cationic surfactants with long unsaturated tails, Langmuir, 17(2), 300–306 (2001). (23) B . Lu, X. Li, J. L. Zakin, and Y. Talmon, A non-viscoelastic drag reducing cationic surfactant system, J. Non-Newtonian Fluid Mech., 71(1–2), 59–72 (1997). (24) B . Lu, Y. Zheng, H. T. Davis, L. E. Scriven, Y. Talmon, and J. L. Zakin, Effect of variations in counter- ion to surfactant ratio on rheology and microstructures of drag reducing cationic surfactant systems, Rheol. Acta, 37(6), 528–548 (1998). (25) B . C. Smith, L. C. Chou, and J. L. Zakin, Measurement of the orientational binding of counterions by nuclear magnetic resonance measurements to predict drag reduction in cationic surfactant micelle solu- tions, J. Rheology, 38(1), 73–83 (1994). (26) S . Padasala, V. Patel, K. Singh, D. Ray, V. K. Aswal, and P. Bahadur, Effect of polymers on worm-like micelles of cetyltrimethylammonium tosylate, Colloid Surf. Physicochem. Eng. Aspect., 502, 147–158 (2016). (27) M . T. Truong and L. M. Walker Controlling the shear-induced structural transition of rodlike micelles using nonionic polymer, Langmuir, 16(21), 7991–7998 (2000). (28) T . Imae and S. Ikeda, Characteristics of rodlike micelles of cetyltrimethylammonium chloride in aque- ous NaCl solutions: their fl exibility and the scaling laws in dilute and semidilute regimes, Colloid Polym. Sci., 265(12), 1090–1098 (1987). (29) R . Gamez-Corrales, J. F. Berret, L. M. Walker, and J. Oberdisse, Shear-thickening dilute surfactant so- lutions: equilibrium structure as studied by small-angle neutron scattering, Langmuir, 15(20), 6755– 6763 (1999). (30) M . Brigante and P. C. Schulz, Synthesis of mesoporous silicas in alkaline and acidic media using the systems cetyltrimethylammonium tosylate (CTAT)–Pluronic F127 triblock copolymer and CTAT– Pluronic F68 triblock copolymer as templates, J. Colloid Interf. Sci., 369(1), 71–81 (2012). (31) P . Alexandridis and T. A. Hatton, Poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) block copolymer surfactants in aqueous solutions and at interfaces: thermodynamics, structure, dynam- ics, and modelling, Colloid Surf. Physicochem. Eng. Aspect., 96(1–2), 1–46 (1995). (32) E . Miller and J. P. Rothstein, Transient evolution of shear-banding wormlike micellar solutions, J. Non- Newtonian Fluid Mech., 143(1), 22–37 (2007). (33) B . Arenas-Gómez, C. Garza, Y. Liu, and R. Castillo, Alignment of worm-like micelles at intermediate and high shear rates, J. Colloid Interf. Sci., 560, 618–625 (2020). (34) Y . Yamagata and M. Senna, Change in viscoelastic behaviors due to phase transition of the assembly com- prising cetyltrimethylammonium chloride/cetyl alcohol/water, Langmuir, 15(13), 4388–4391 (1999).