Probing the textures of composite skin care formulations using large amplitude oscillatory shear

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 film is further thinned while spreading on the skin surface, shear rates during flow 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 fluid 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 fluid. 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 profiles are amassed with smooth and rough surfaces data utilizing standard rheological techniques, including oscillatory SAOS, stress ramps, Brookfield viscometry, and the manifestation of interfacial or complex flow properties, such as wall-slip and shear-banding phenomena. Practical influences on the human stratum corneum, including thermal softening and electrostatic shielding, are also considered. Additionally, outcomes from texture profile 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.