JOURNAL OF COSMETIC SCIENCE 134 Temperature has a large impact on the fi rmness, fl ow, thixotropy, and subsequent sensorial properties of the Buttery Cream. At 32°C, τ0 drops to 76 ± 4 Pa, which is a lower magni- tude drop than expected, suggesting that the consistency and viscoelasticity of the slip layer dominates the apparent stress response of the rheometer at each isotherm. The tan δ value at 32°C, 50 rad/s, and 100% strain (strain sweep data), which is outside the LVR, but directionally pertinent to in vivo applications, is 2.37 ± 0.20, and expresses a fl uid-like state (i.e., tan δ 1). Compared to other samples, at 32°C, the length of the LVR plateau is only slightly affected by frequency, indicating that temperature, rather than solely deformation time or dynamic shear rate, greatly dictates the physical state of the butter. At 25°C, 1% strain frequency sweep (i.e., LVR), no modulus crossover is seen for the Buttery Cream, where G’ G” for the entire frequency range tan δ 1 infers the stability of the Buttery Cream in the chassis, as well as the cohesiveness of the internal network within the LVR. An additional frequency sweep was performed on the Buttery Cream outside the LVR, at 50% strain and 32°C although referring to dynamic moduli produced well outside the LVR is tenuous at best, the trends are being used to render a rough, directional comparison be- tween microstructures. The frequency sweep with higher-magnitude strain was used to mimic reality, and clearly demonstrated that warming the formulation affects the rheology control of the waxy matrix—tan δ varies from 6.0 at 0.1 rad/s to 2.0 at 100 rad/s, indicating that the Buttery Cream is disposed to a much softer and fl owing state on the surface of warm skin. Relative to 25°C, the I2/I1 (0.03) slightly increases at 32°C this may be related to the onset of heterogeneous melting transitions in the waxy matrix. The Cushion Cream SPF-15. It has a mousse-like texture and is packaged in a jar. The standard rheology data show that the apparent yield stress, as judged by standard stress sweeps, is τ0 = 76 ± 4 Pa, and that the ZSV for the Cushion Cream SPF-15 is higher than the other textures, including the Buttery Cream, indicating that the Cushion Cream SPF-15 has more apparent viscoelasticity as the material builds stress and initially fl ows at near-zero shear rate. The τ0 drops to 49 ± 2 Pa at 32°C, suggesting that thermal energy impacts the plate-sample interface during the lengthy application of very small stresses the stress ramp to produce the yield stress data (Figure 3) is a slow test and shows that—given time—the Cushion Cream SPF-15 will slip at the smooth interface to offset the applied stress this is also noted in the 32°C LVR plateau data, where, at lower frequency (1 rad/s), the plateau is similar to that of Buttery Cream. However, at higher frequency, the LVR plateau extends to 26%, indicating that the plate-formulation adhesion and the response and strength of the cohesive microstructure have a time dependency. In addition, the tan δ at higher strain and frequency is 0.95, indicating the texture of a gel-like state, rather than a fl uid (in contrast to the Buttery Cream). The stress growth experiment shows an overshoot at 926 Pa for the sandpaper and 862 Pa for the smooth plates again, at high shear rates, the Cushion Cream SPF-15 has a stronger structure, and the difference be- tween smooth and rough surfaces is probably related to wall slip due to a more dominant inherent sample cohesiveness. As determined by preshear and recovery work, the level of thixotropy is insignifi cant, meaning that a very slight drop in the microstructural sturdiness fully recovers in less than 20 s. The LVR frequency sweep showed G’ G” (G’:G”~10:1) across the entire frequency range. In the 50% strain, 50 rad/s strain sweep at 32°C, which was also performed on the Buttery Cream, G’ crosses G” at 12 rad/s (~6.0 s-1), indicating that the microstructural rigidity, and perhaps the interfacial response of the Cushion Cream SPF- 15, are time dependent at 32°C the internal structure is relatively less temperature depen- dent than the Buttery Cream and, hence, its rheological response may be more expected to

LARGE AMPLITUDE OSCILLATORY SHEAR 135 rival the rheological trends seen in the jar. Finally, at higher shear rates in situ, the Cushion Cream SPF-15 does peel from the parallel discs of the rheometer—evidently concurring that the formulation exhibits properties of cohesiveness. Note that, in vivo, interfacial phenomena and natural asperities aid in overcoming deleterious peeling or pilling of the formulation on the skin surface. At 25° and 32°C, the I2/I1 harmonic ratio suggests some asymmetric fl ow at oscillatory shear rates less than 1 s-1 (0.02 and 0.05, respectively) however, plug fl ow or slip was not seen with the application of a marker (25°C). At the end of the LVR, there were large and steady rises in I3/I1 and I5/I1 (0.08 and 0.02 at 100 s-1), signifying nonlinear rheo- logical changes. Finally, correlation of technology with the sensorial panel may be tricky for this sample as components of the initially thick-feeling Cushion Cream SPF-15 sorb fairly quickly into the skin, leaving behind a fi lm with a nontacky, dry feel. BROOKFIELD VISCOMETRY As a reference point, Table I provides apparent viscosity and pH data for the four textural formulations. Generally, the pH values of skincare formulations are formulated as closely as possible to the skin surface pH in order to avoid incompatibility with skin fl ora. The viscosity data provide a general reference point for the formulations since Brookfi eld vis- cometry is employed almost universally in development and quality control laboratories. Removing the Buttery Cream result, the T-spindle Brookfi eld viscometry data correlated well (R2 = 0.988 and 0.999, respectively) with the steady torsional and ZSV data for the Sunscreen Gel SPF-50, Cushion Cream SPF-15, and Refreshing Gel Cream. As the rheo- logical data for the Buttery Cream were observed below the steady torsional and ZSV versus apparent viscosity correlation lines, this could indicate that the T-spindle results are less sensitive than rheometry to the generation of a signifi cant interfacial slip layer for a sample structured with a waxy matrix. SUMMARY OF NON-LINEAR RHEOLOGY DATA (LISSAJOUS REPRESENTATIONS) Traditionally, dynamic oscillatory shear tests are conducted in the SAOS mode, and have been the preferred method to characterize the viscoelastic properties of soft solids. In re- cent years, however, much interest has focused on collecting data in the LAOS mode since many engineering, processing, and application operations often require impinging large deformation and higher shear rates on samples. As mentioned above, this certainly ap- plies to preparations that are applied to the skin where the shear rates in spreading and Table I Brookfi eld Viscometry and pH Results (T Spindle, 25°C) Formulation T-spindle Apparent viscosity (cP) pH Sunscreen Gel SPF-50 C 27,500 ± 2,500 NA Cushion Cream SPF-15 D 95,000 ± 25,000 5.3 ± 0.2 Refreshing Gel Cream B 42,500 ± 7,500 5.6 ± 0.2 Buttery Cream D 185,000 ± 25,000 5.2 ± 0.2