Signal analysis to study the impact of tongue roughness on oral friction mechanisms with a custom-built tribometer

A custom-built tribometer was employed to investigate the impact of the roughness of deformable tongue mimicking surfaces (TMS) on friction mechanisms occurring under the effect of lubrication with Newtonian solutions of glycerol. TMSs with modulated roughness (range of asperity heights Ra: 20–140 μm) were manufactured from gels of polyvinyl alcohol (PVA). Newtonian aqueous solutions of glycerol covering a wide range of viscosity (1–1400 mPa.s) were used as simple food models spread on the TMSs. The tribological behavior of the system was studied during shear back and forth movements. The ratio between tangential and normal forces was analyzed both in terms of average values and of fluctuations, over specific time periods set at the end of motion and rest steps. The average values of friction level were reported to increase when (i) the roughness of the TMSs increased and when (ii) the viscosity of glycerol solutions decreased. These trends could be consistent with mixed lubrication. The fluctuations of friction level during motion steps were for their part generally of higher amplitude as the roughness of the surface increased, with main frequencies ranging from 10 to 20 Hz. The study demonstrates the importance (i) of the biological relevance of tongue properties (contact areas, rigidity, and asperity heights) and (ii) of the thorough analysis of tangential to normal force ratio to better understand the complex mechanisms of friction occurring in the mouth during food consumption.