Lifestyle

Shaving comfort is linked to friction, and thus lubrication, in the razor cartridge/skin interface. It is a distinguishing factor when a user evaluates the perceived quality of a shave. A novel Laser Induced Fluorescence (LIF) technique was developed to study lubrication in a razor cartridge/skin mimic contact under simulated shaving conditions. The experiments were conducted with a 5-blade cartridge, loaded and sliding against a transparent silicone elastomer. The lubricant was distilled water marked with fluorescent dye. The LIF technique measured in-contact film thickness at different sliding speeds (50, 150 mm s⁻¹) and showed differences in lubricant distribution for each component (lubricant strip, blades, guard) interface. The measured calibration film thickness range was 2–135 μm. The guard and lubricant strip maintain relatively thick and consistent lubricant films across the speed range. Minimum films were measured at the blade cutting edges, although at sliding speeds of 150 mm s⁻¹ the blade cutting edge film thickness increases towards the back of the cartridge; and film thickness under blades 1 and 2 decreases at high speeds, suggesting starvation of fluid flow. The new technique allows study of lubrication behaviour in complex geometries, multiphase fluids and hard/soft material combinations for example oral processing, contact lens/eyelid and personal care product application. These systems are not easily studied by conventional test methods or classical lubrication analysis.

Water swellable crosslinked polymers are widely used in oil-in-water emulsions for the healthcare and cosmetic industries due to their thickening properties. In this study, we investigate the rheological and lubrication behavior of a microgel-forming polymer, a lightly-crosslinked hydrophobically modified polyacrylic acid (HMPAA), in an aqueous medium and in an emulsion. Hydrogenated phosphatidylcholine, a class of phospholipids, is used as a surfactant in the emulsions composed of different oil content. Rheological behavior is probed both in the linear and non-linear regimes using small strain amplitude and large amplitude oscillatory shear (LAOS) experiments, respectively. We observe all systems to exhibit gel-like behavior with the elastic modulus (G’) dominating and being frequency independent. Lissajous-Bowditch plots and nonlinear parameters obtained under large deformation show that the emulsions can resist greater deformations with smaller increase in the viscous dissipation when compared to a HMPAA gel. For tribology experiments, friction curves in a range of entrainment speeds are examined using substrates to mimic the skin surface (PDMS and Bioskin®). The role of polymer hydrophobicity on the different substrates are also explored by comparing the behavior of HMPAA to that of its hydrophilic analog, a polyacrylic acid highly crosslinked. We find the friction coefficient to be dependent on the hydrophobicity of the substrate and the polymer as well as the substrate roughness. These results taken together provide insights in the formulation of skincare products with efficient lubrication properties for different skin characteristics.