RHEOLOGY OF COSMETIC PRODUCTS 491 Jones et al. (57) examined both the fl ow behavior and the textural properties of pharma- ceutical polymer gels and interpreted textural data using rheology in terms of shear stress shear rate. Also, Lukic et al. (58,59) studied the infl uence of emollients on rheological and textural properties of water in oil cosmetic creams to predict their sensory properties. Effect of po lymers on emulsion rheological behavior. Polymers are one of the main routes of modifying the rheological properties in cosmetic products. This also holds for emulsion- based systems where the rheological properties of the system can be impacted by the concentration of the emulsion and lead to glassy behavior as discussed earlier. However, for more dilute systems, the addition of polymers to impact the rheological response is the preferred route used by formulators. A complete analysis of the various types of poly- mers used for rheological modifi cation is beyond the scope of this review. However, some of the more recent studies on polymer effects on emulsions are discussed in the following texts. The objectiv e of the research work by Gilbert et al. (60) was to evaluate the effect of various polymers on both the rheological and mechanical/textural properties of cosmetic O /W emulsions. To achieve this, eight hydrophilic polymers, natural [Ceratonia siliqua gum (carob) and xanthan gum], or natural modifi ed [hydroxypropyl (HP) guar gum, hydroxypropylmethyl (HPM) cellulose, or hydroxyethyl (HE) cellulose], or synthetic [carbomer, polyacrylamide (PA) (and) C13–C14 isoparaffi n (and) laureth-7, or ammonium acryloyldimethyltaurate/VP copolymer] were selected. Each one was incorporated in an O /W emulsion at a concentration of 1% w/w, and a formulation without any polymer was also prepared to be used as a control. The rheological and mechanical/textural properties of the equivalent dispersions were then investigated, and this assessment was carried out by analyzing continuous shear fl ow, creep recovery, and dynamic oscillatory tests. Pene- tration and compression tests were also performed using a texture analyzer. The effects of experimental parameters (probe type, speed of displacement, and diameter of the con- tainer) on the textural properties of the emulsions were examined. Rheological and tex- tural data were then statistically analyzed, thus uncovering correlations between both approaches and highlighting the effects of incorporating the different polymers. The effect of the polymer was signifi cant on most of the parameters collected from various tests. Emulsions containing synthetic polymers used as gelling agents [ammonium acryloyldi- methyltaurate/VP copolymer–co-VP (AADMT), Laureth-7, C13–C14 isoparaffi n, and PA, carbomer (PAA)] exhibited yield stress, high viscosity, high G′ and G″ values corresponding Table I Values of Viscosity (Means ± SD) Obtained at Different Shear Rates (in s-1) from the Shear F low Test for the Nine O/W Emulsions. Values of Viscosity (Pa.s) are Given in Descending Order for Each Shear Rate Cream η (0.1) η (1) η (10) η (100) η (1000) PAA 748.2 ± 35.2 162.7 ± 9.2 43.8 ± 1.3 7.7 ± 0.1 1.6 ± 0.0 AADMT–co-VP 380.4 ± 13.1 105.7 ± 1.5 25.3 ± 0.1 4.7 ± 0.1 1.0 ± 0.0 PA 136.5 ± 5.7 40.8 ± 0.2 9.3 ± 0.1 1.8 ± 0.0 0.5 ± 0.00 HP guar 108.2 ± 0.4 19.2 ± 0.3 3.6 ± 0.0 0.76 ± 0.00 0.18 ± 0.00 Xanthan 128.4 ± 9.3 20.1 ± 0.4 3.2 ± 0.0 0.44 ± 0.00 0.10 ± 0.00 HE cellulose 101.1 ± 2.5 22.2 ± 0.3 4.8 ± 0.0 1.0 ± 0.0 0.26 ± 0.00 Carob 85.9 ± 4.0 19.2 ± 0.7 4.4 ± 0.1 0.99 ± 0.01 0.25 ± 0.00 HPM cellulose 53.6 ± 2.3 10.5 ± 0.0 2.6 ± 0.0 0.75 ± 0.01 0.24 ± 0.00 Control 42.4 ± 2.3 8.6 ± 0.4 1.5 ± 0.0 0.27 ± 0.00 0.07 ± 0.00

JOURNAL OF COSMETIC SCIENCE 492 to a strong gel network, and a great resistance to penetration and compression. On the other hand, emulsions containing natural or modifi ed polysaccharides (carob, HE cellulose, HPM cellulose, HP guar, and xanthan) displayed different shear thinning characteristics, low viscosity, and G′ and G″ values and were less structured as behaving as viscous or weak gel systems. Among these polysaccharides, xanthan showed a singular behavior, in particular in terms of shear thinning and deformability it confers to emulsion. Kettler et al. (61), characterized rheological properties of emulsions containing C10–C30 acrylate polymeric thickener (close to polyacrylic acid). They found that the physical net- work built up by acrylate polymers in the range of 0.1–1 wt% was the dominant factor for rheological properties and increases both the moduli and the viscosity of emulsions. They also showed that oil droplets size and distribution do not affect emulsion elasticity when the polyacrylate concentration is higher than 0.1 wt%. Concerning emulsions con- taining polysaccharides, very few studies deal with the impact of droplet size or chemical composition on textural or rheological properties of the emulsions most results are related to much simplifi ed emulsions, with very limited number of ingredients (62,63), and not necessarily cosmetic ones. The rheological behavior of various polymers in emulsions is highlighted in Tables I and II. Nano-emulsions and p rocessing challenges. Future directions fo r emulsions include the poten- tial use of nano-emulsions which would potentially yield benefi ts in terms of optical clarity and potential enhancement of skin stratum corneum penetration of various actives. Nano- emulsions are a form of emulsion whose formation, properties, and stability are well discussed and reviewed in two publications (64,65). Although several articles on nano- emulsions indicate that nano-emulsions can be stable even by years, the small droplet size makes nano-emulsions break by the Ostwald ripening mechanism (66–68) in time peri- ods too short so that to constitute a limitation for developing applications. Emulsifi cation proce ssing can pose signifi cant challenges. Kim and Mason (55) conclusively state that it is still challenging to formulate and to tailor emulsifi cation processes for large- scale production of emulsions having desired compositions, droplet sizes distributions, and rheological properties. Formulators still regularly design emulsions and emulsifi cation pro- cesses for particular applications based on modifying a set of compositional parameters and processing conditions empirically, often by trial-and-error or iterative approaches, until the resulting emulsion composition, structure, and rheological properties are within desired ranges. Although a lack of control over droplet polydispersity is typically one reason for the Table II Viscoelasticity Data (Means ± SD) Obtained from the Strain and Time Sweep Tests for the Nine O/W Emulsions Product G′ (Pa) G″ (Pa) PAA 1,841.0 ± 15.0 335.8 ± 3.4 AADMT–co-VP 1,268.7 ± 43.3 263.1 ± 10.1 PA 668.0 ± 13.2 161.1 ± 3.3 Control 84.3 ± 1.6 31.0 ± 0.5 Xanthan 62.6 ± 0.3 17.6 ± 0.1 Carob 58.9 ± 0.0 51.3 ± 0.1 HP guar 56.5 ± 0.9 35.0 ± 0.4 HPM cellulose 48.6 ± 0.3 27.2 ± 0.4 HE cellulose 46.6 ± 0.3 36.2 ± 0.4