LARGE AMPLITUDE OSCILLATORY SHEAR 151 dramatic. No difference in quick break is seen between the Cushion Cream SPF-15 and Buttery Cream, where both prototypes are sampled with the fi nger and, hence, warming of the Buttery Cream occurs before and during spreading. In the Lissajous curves, the Refreshing Gel Cream and Buttery Cream own the only signifi cant changes in slope be- tween 25° and 32°C with salt data (Figure 13)—where the slope of the Lissajous is pro- portional to a net decrease in the complex viscosity (η*). The Buttery Cream shows a dramatic change in overall stress, which affects other sensorial attributes, but the change may be very rapid and closely related to how the sample is delivered to the skin or, the gradual melting, although ultimately dramatic, is not seen as a quick break. Nevertheless, the change in viscosity is clearly evident after rubbing the Buttery Cream on the skin for a few seconds. The Cushion Cream SPF-15 formulation is only slightly affected by the salt and warmer temperature, and no evidence of a quick break is noted. The Sunscreen Gel SPF-50 exhibited a slight break—this change cannot be clearly discerned by the Lis- sajous plot and could be more related to how the sample microstructure is initially sheared when delivered to the skin (via a pump chassis). Trends in perceived slip versus LAOS are as follows: Sunscreen Gel SPF-50 (7.6 ± 1.3) Refreshing Gel Cream (6.6 ± 0.5) Buttery Cream (4.8 ± 0.5) Cushion Cream SPF-15 (4.6 ± 0.9). The σmax (ambient temperature, smooth, no salt) displays a signifi cant link (R2 = 0.992) with the panel data, suggesting that the summation of viscous and residual elastic stress after shearing correlates with perceivable slip. Interestingly, but not surpris- ingly, the slip panel data also correlate well with stress data from low shear rate Lissajous analyses (R2 = 0.905 Figure 9). The correlation no doubt includes the manifestation of wall slip, which is a reality at lower shear rates for all four textures hence, wall slip effects in rheometry may relate to components of initial feel and the perception of slip. Figure 13. Lissajous plots for (A) Sunscreen Gel SPF-50, (B) Refreshing Gel Cream, (C) Buttery Cream, and (D) Cushion Cream SPF-15 with and without NaCl (500 nmol/cm2). Systems tested at 25°C and no salt are meant to mimic the product in the jar, whereas testing at 32°C with electrolyte represents a model on-skin environment.

JOURNAL OF COSMETIC SCIENCE 152 Cushion is ranked sensorially in the following order: Cushion Cream SPF-15 (6.4 ± 1.3) Sunscreen Gel SPF-50 (5.2 ± 0.5) = Buttery Cream (5.2 ± 0.5) Refreshing Gel Cream (3.6 ± 0.6). After removing Sunscreen Gel SPF-50 from the array of textures, it is evident that the cushion characteristic correlates with ZSV (R2 = 0.974), and steady torsional with rough surfaces (R2 = 0.995). From the LAOS data, the cushion data correlate well with the stress data at 32°C and salt (R2 = 0.958), but less so with the 25°C no salt data (R2 = 0.783). In low shear rate LAOS testing (maximum 4 s-1) with no salt, the shapes of the Lissajous plots were very different, and so were the stress maxima. The stress maxima data correlated well (R2 = 0.994) with the cushion data for Refreshing Gel Cream, Buttery Cream, and Cushion Cream SPF-15. In the fi nal analysis, the TPA and rheology data did not correlate well with the expert panel’s cushion data for the Sunscreen Gel SPF-50 formulation. Light feel appears to be a more complex parameter as it is confounded by mid-to-late rub- out parameters markers such as dry time, play time, and tribology could have an infl uence on the perceived texture. However, although correlation of ratings with LAOS and stan- dard rheology was scattered, it appears that the impact of the formulation with the in-vivo skin environment infl uenced the light-feel ratings for the Refreshing Cream. More intriguing is the perception of the Cushion Cream SPF-15, which has a high resistance to spreading at fi rst, but becomes powdery and dry as the emulsion breaks and components vaporize and wick into the stratum corneum. All said, the overall sensorial rating for light feel appears to point to perceived mid-to-late feel properties of the Cushion Cream SPF-15. Finally, the Sunscreen Gel SPF-50 and Buttery Cream are perceived as having the same light feel properties—evidently these are rated in the mid-to-late spreadability zone, after the Buttery Cream melts and interacts with skin surface electrolyte. The Sunscreen Gel SPF-50 is oily and has a long play time, giving ample time to rate its average resistance to spreading. In summary, the sensorial data correlate with changes that are observed in the Lissajous plots as well as in the magnitudes of other rheological parameters. Rheological analyses, while very instructive for examining the perceived intensities of textures, do not allow differentiation in emotional or hedonic ratings, such as acceptability, color, odor, shine, cooling/warming, moistness, or matte effects further, fi ne morphological properties of texture, including grittiness, smoothness, and graininess, are not well suited to rheologi- cal discrimination. Instead, rheology best correlates with the kinesthetic sensitivity of the muscles in the skin as the microstructure and/or viscosity of a product changes as a func- tion of applied shear (23). Additionally, LAOS testing brings us much closer to the picto- rial evaluation of a number of textural attributes that traditionally could only be monitored by expert panels, or estimated by a bevy of instrumental outcomes. For ex- ample, overlaying Lissajous plots of the four prototypes and qualitatively observing rela- tive stress trends in the loop trajectories facilitate a cursory appraisal of textural properties, including cushion, spreadability, elasticity, stress dissipation, and viscosity (Figure 14). Ultimately, most traditional rheological data confer abstruse information about a single point in rheological time in contrast, the Lissajous curve can be imagined as an unpretentious textural fi ngerprint that in one glance conveys a rheological snapshot of a formulation as its microstructure viscoelastically adapts to the infl uence of increasing shear strains. NEXT STEPS: TRIBO-RHEOMETRY Although outside the scope of this work, which focuses on correlating elastoviscoplastic rheological transitions, Kavehpour and McKinley (24) describe a novel triborheometry