J. Cosmet. Sci., 67, 121–159 (May/June 2016) 121 Probing the textures of composite skin care formulations using large amplitude oscillatory shear TIMOTHY GILLECE, ROGER L. MCMULLEN, HANI FARES, LARRY SENAK, SEHER OZKAN, and LINDA FOLTIS, Ashland, Inc., Bridgewater, NJ 08807. Accepted for publication May 30, 2016. Synopsis Identifying meaningful and measurable rheological parameters that shadow the dynamic shear stresses sustained in the initial application and subsequent spreading of structured cosmetic formulations onto the skin is quite challenging. When applied to non-Newtonian soft solids, traditional oscillatory rheological testing tends to best correlate with the “at-rest” state, or, more fundamentally, with the initial and thermodynamically reversible perturbations in the physiochemical networking that binds components of the amalgamated microstructure. In addition, after yielding, as an applied fi lm is further thinned while spreading on the skin surface, shear rates during fl ow processes may rapidly and dynamically increase to 104 s-1, which is a magnitude that is not practically simulated with a standard rotational rheometer. Realistically speaking, it is rare that a single rheological measurement or resultant parameter predicts the sensorial appeal of a complex fl uid during the entire scope of a spreading process. Large amplitude oscillatory shear (LAOS) methodology is an augmentation of standard oscillatory rheology, or small amplitude oscillatory shear (SAOS), and delivers a means to dynamically probe the deforming microstructure of a soft solid as it rheologically transitioned from a viscoplastic material to a structured fl uid. LAOS rheology was performed on four different prototypes having different skincare textures to produce Bowditch–Lissajous plots (henceforth truncated to Lissajous in the remainder of the document) for subsequent association with previously measured sensorial properties. Insights into the shapes of the curves and their relation to paralleled sensorial analyses are primarily based on the performance of the composite prototypes rather than speculating on the individual contribution of each constituent to the dynamics of the adapting microstructure. Therefore, transitions in the Lissajous trajectories may be used to visually describe changes in the bulk rheology as the physical components of the local viscoelastic environment are controllably sheared. In this work, Lissajous profi les are amassed with smooth and rough surfaces data utilizing standard rheological techniques, including oscillatory SAOS, stress ramps, Brookfi eld viscometry, and the manifestation of interfacial or complex fl ow properties, such as wall-slip and shear-banding phenomena. Practical infl uences on the human stratum corneum, including thermal softening and electrostatic shielding, are also considered. Additionally, outcomes from texture profi le analysis are reported and contrasted with the accompanying results. Ultimately, the objective is to make meaningful connections between trends in Lissajous trajectories and paralleled sensorial analyses conducted by a trained expert panel. For the reader, a basic level of rheological knowledge is assumed. Address all correspondence to rmcmullen@ashland.com.

JOURNAL OF COSMETIC SCIENCE 122 INTRODUCTION The tactile perception of substances is a personal physiological experience that varies from observer to observer and is dependent on the local sensory/nerve structures of the individual (1). Moreover, other perceptions, such as vision and smell or wishful thinking, may subconsciously bias the tactile experience. For this reason, panels of experts are often used to objectify tactile observations. Even the most expert panelist, however, cannot cleanly sort out the rheological contributions to the tactile experience. How these experi- ences add up to the observer’s impressions will vary between panelists no matter what precautions are taken to ensure objectivity. When rheological techniques are applied to complex fl uids, they most often provide a collection of individual parameters that feed into discerning neat material properties, rather than expressing the full spectrum of physiological challenges from the in vivo tactile experience. For this reason, rheological outcomes are often not intuitive, or do not fully describe palpable differences in the sensorial experience. We propose to leverage specifi c rheological techniques that close the gap in understanding measurable contributions infl uencing textural properties, while implicitly arguing that perhaps a less complex, “one-pot” visualization of an array of rheological properties more succinctly elucidates distinctions in the complex textural properties of substances. Successfully correlating textural properties with rheological properties involves a thor- ough understanding of the strengths and weaknesses of the measurement, as well as an appreciation for the tactile perception limits and complexity of the human somatosensory system—where signals from pressure, skin stretch, vibration, itch, and/or temperature are translated from touch via specialized sensory organs in the skin to the spinal cord and, fi nally, to the brain for processing (2). Further, and to complicate matters, applying cer- tain topical products to the skin may alter specifi c surface properties, thereby altering the mechanical stimulation of the skin (3). Although the biology of the individual is essen- tially fi xed, training the human to duplicate the restricted motions of a rheometer or texture analyzer, or confi guring the instrumentation to mimic the methodology of the sensorial challenge, is a valid option. Even more complex, each person’s biology and in- terpretation of perception is unique, leading to a natural bias in the panel data. Further, the unmistakable reality is that not all instrumental outputs relate to discernible senso- rial properties and, understandably, no single result resolutely defi nes the complex gamut of end-use textural appeal. Malcolm Bourne summarizes this emblematically by noting that there are many measurable wavelengths of electromagnetic radiation, but only select wavelengths between 400 and 700 nm (i.e., visible light) are perceptible basically, many instrumental techniques detect physical properties in materials that are not necessarily discernible by the somatic senses (4). Although daunting, many researchers have attempted sensorial correlations with the am- bition of developing predictive measurement models to mitigate the potential subjectiv- ity, sizeable cost, and time demands of using trained professional panelists for sizeable sensorial studies on cosmetic formulations (5–8). Many have focused on using parameters from fl ow models, such as the power law, or the yield stress and consistency parameters of the viscoplastic Herschel–Bulkley or Casson relationships (9). Some have focused on SAOS parameters to gauge the initial spreadability, cushion, or body of a structured sys- tem, while the 2010 work by Greenaway found the correlation of fi rmness, thickness, and the resistance and diffi culty of spreading with trends in the elastic modulus at high strain (10) others have focused on combining rheological and texture analyses to make paral-