RHEOLOGY OF COSMETIC PRODUCTS 487 and link to the microstructure. As the shear rate increases and the solution is no longer at rest, there exists a shear banding region. At very high shear rates (shear rate 100 s-1), a rate where the micellar solution is supposed to be aligned, it presents two regions: a re- gion where the orientation parameter decays and a second region where this orientation parameter linearly increases again. The origin of these regions is not clear, but the pre- sented possible explanations of why they have been observed are stated. At high shear rate values of 100 s-1 and above, where no shear banding is observed and there are highly oriented micelles (paranematic region), the breaking rate increases because of friction, the contour length decreases, the fl uid slightly thins, or viscosity is almost constant. In this region, micellar breaking is important, and there is an immediate micelle shortening due to the entropic forces that instantaneously forces a coil structure that impacts the ordered fl ow prevalent before the rupture of the wormlike micelles. As a result of this, the orienta- tion parameter decreases, the shortened micelles equilibrate to the local conditions of the surrounding media, elongate again, and realign properly when the shear rate is above approximately 500 s-1, and the system presents shear thinning of the unclear origin. Lamell ar gel network: Hair conditioners. Hair conditioners primarily use a mixture of cat- ionic surfactants and fatty alcohols in an aqueous medium to form a structured mesophase that gives rise to good lubrication properties and performance. These conditioner emul- sions usually have high viscosity, and understanding the viscosity buildup and meso- structure and how it breaks down on dilution is essential for controlling the conditioning performance. The gel network theory of emulsion stability gives a coherent explanation for the manner in which fatty amphiphiles and surfactants combined as mixed emulsifi ers not only stabilize multiphase oil-in-water (O/W) lotions and creams but also control their viscosities. Although most of the early work was performed using long-chain (C16–C18) fatty alcohols, the theory is general, and the same broad principles apply to whichever amphiphile or surfactant (ionic or nonionic) is used. The theory relates the stabilities and physicochemical properties of multiphase O /W emulsions to the fact that the lamellar gel network is mainly an extended, highly interconnected lamellar structure of surfactant bilayers and interlamellar water layers, which is called the lamellar gel phase (Lβ) (34–37). Davies a nd Amin (8) reported that high yield stress values were engineered through formulation variation and observed in the surfactant–fatty alcohol systems with higher ratios of fatty alcohol, that is, at an increased fatty alcohol concentration of 10% w/w and an abundance of surfactant in the system, an increased swelling rate was observed and reported in the aqueous phase. This ultimately dictates the viscosity of the formulation, causing a signifi cant increase in viscosity which also impacts the overall yield stress of the system. This is in accordance with the literature that states that an excess or increase in fatty alcohol in an aqueous phase with surfactants in solution controls and predicts the overall consistency or viscosity of the formulation as the gel phase is formed (38), at a temperature high enough to melt the fatty amphiphile by the swelling of the fatty amphiphile and its ability to incorporate signifi cant quantities of water in the interlamel- lar space. This high yield stress values impacted wet lubrication as a result of higher structural integrity of the microstructure during the dilution process, resulting in both higher viscosity and deposition of larger more interconnected meso-structures on the hair surface, giving rise to a more continuous fi lm. An overall reduced combing force is observed in hair treated with the systems with highest yield stress values, further validating this effect.

JOURNAL OF COSMETIC SCIENCE 488 FOAM RHE OLOGY Foams ar e generated during in-use conditions for shampoos, body washes, and face cleansers. Ordinary foam is polydisperse and isotropic, and its elastic properties are characterized by its bulk modulus and its shear modulus. There is a range of strain over which the foam is elastic. Beyond the elastic range of strain, the foam is plastic. Under continuous shear, polydisperse foams show a tendency to separate into regions of smaller and larger bubbles (39). The structure and dynamics of foams and emulsions strongly depend on the particle size and on the dispersed phase volume fraction. The structure of foams and emulsions is metastable. It evolves with time because of drainage, coarsening (also referred to as Ostwald ripening in dilute emulsions), or coalescence (40–42). Foam ins tability mechanisms. Two majo r foam instability mechanisms are foam drainage and foam coarsening. Coarsening in foams and emulsions arises from diffusion of the chemical species of the dispersed phase between neighboring particles, driven by Laplace pressure differences (43). Foam drainage is a complex physicochemical hydrodynamic process gov- erned by many simultaneous factors, which are not fully understood. The rate of foam drainage depends not only on the hydrodynamic parameters of the foam system but also on the rate of internal foam destruction by the bubble coalescence. The foam drainage plays a critical role in the foam stability. Foam drainage causes both a decrease in the liquid volume fraction and an increase in the capillary pressure, which are related through the size of the foam bubbles and the height of the foam column. The mechanism of the foam decay depends not only on the liquid volume fraction but also on the types of the surfactant and the foam fi lms separating the foam cells (44). In foams, drainage can be eliminated in microgravity (45) or by using a continuous phase which has a yield stress. Coarseni ng in foams and emulsions arises from diffusion of the chemical species of the dispersed phase between neighboring particles, driven by Laplace pressure differences. When foam dries out, its structure becomes very fragile as the liquid fi lm becomes thinner and more susceptible to breakage, which means the foam collapses. Coarsening leads to a growth of the average particle diameter with time, accompanied by intermittent struc- tural rearrangements (42). For similar droplet and bubble sizes, coarsening in emulsions is generally much slower than that in foams because of differences of internal phase den- sity and solubility in the continuous phase. Foam drainage and foam coarsening are inter- connected and tend to impact on the rheological response of the system as highlighted in the fi gure. Material s for impacting foam performance. Foam qua lity is largely controlled by the properties of the surfactant monolayers that protect the air–water and oil–water interfaces. It is therefore critical that they are both optimized in cosmetic formulations to deliver optimal foaming during cleansing and positive sensorial attributes to consumers. The knowledge of surface tension alone is not suffi cient to understand the foam and emulsion properties. The surface viscoelasticity and compression viscoelasticity, in particular, play an important role in a variety of dynamic processes (46). Marinova et al. (47) studied surfactant mixtures for fi ne foams with slowed drainage. In this study, systematic investigation of foams stabilized by a triple surfactant mixture containing a nonionic alkyl polyglucoside, an ionic sodium lauryl dioxyethylene sulfate, and zwitterionic CAPB was performed. The foaming of the triple surfactant mixture was found to be comparable with that of the single components and with the binary mixture without alkyl polyglucoside at alkaline pH. It was reported that fatty alcohol and/or