LARGE AMPLITUDE OSCILLATORY SHEAR 133 onset of the apparent yield stress (peak in elastic stress), I3 and I5 steadily rise. There is also evidence of very minor thixotropy, with a slight downward trend in stress as a func- tion of time in steady torsional experiments (with and without sandpaper). Figure 3 also shows that the intrinsic cohesiveness of the Refreshing Gel Cream might infl uence the tan δ values and LVR plateau, where both values suggest the presence of a stable micro- structure hence, considering all minor effects, the rheological changes affecting sensorial appeal for the Refreshing Gel Cream are almost certainly related to direct changes in the dynamically shearing microstructure. Evidence of the microstructural stability in the LVR is visible in the frequency sweep data, which show that G’:G” 10:1 and that G’ and G” are essentially parallel across the tested frequency range. The Buttery Cream. It is provided in a jar, meaning that the user retrieves the product, more than likely, with the fi ngers. As the surface of fi ngers and skin are typically warmer than ambient conditions, the impact of temperature on the sensorial experience is critical. The apparent yield stress, as judged by stress sweep methodology, was τ0 = 92 ± 6 Pa. Standard rheological methods for evaluating thixotropy (preshear and recovery, steady torsional) showed a decrease in viscosity as a function of steady shear rate, taking more than 5 min to recover from preshearing. By using a marker, it was shown that the sample plug fl ows at 25°C, meaning that the sample slips at both walls of the discs instead of uniformly deforming hence, the total stress plateaus because the energy of the motor is no longer fully translated to the entirety of the sample. The transient stress growth over- shoot maximum at 25°C was 1560 Pa (highest magnitude of all four prototypes) for rough and 1006 Pa for smooth discs (±10 Pa), corroborating the manifestation of plug fl ow. Steady torsional data from smooth and rough surfaces also showed that wall slip is present at low shear rates. Because low shear rates correspond to very initial spreadabil- ity, cushion evaluation, etc., slip layers are important factors in correlating rheology with sensory perception. At 25°C, the I2/I1 harmonic ratio also suggests some small asymmetric fl ow at oscillatory shear rates less than 4 s-1. Below yield, the small asym- metric contributions from the even harmonics may be indicative of the formation of a smooth and consistent slip layer. Figure 3. Summary of selected standard rheological results (smooth surface). SR = stress ramp result End LVR (%) = approximated with intersecting lines method.

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