Dynamic Spectral Analysis of the Stick-Slip Effects in “Oral” Tribology Measurements

“Oral” tribology, associated with oral surface movement and friction, is a new tool to understand complex texture attributes such as “creamy, oily and astringent” which could not be successfully characterized by bulk texture analysis or by rheology. Smoothness, slipperiness and roughness of food boluses acting as lubricants between tongue and palate could be responsible for the sensation of those complex attributes. In fact, surface characteristics such as asperities combined with elastic and plastic deformation produce intermittent sliding motion. These so-called stick-slip effects could constitute important influencing factors of sensory perception in the mouth.
It was hypothesized that the observed jagged behavior of most force-displacement curves in tribology measurements are due to stick-slip effects and depend on normal load, entertainment speed, surface roughness, lubricant type and its viscosity. Hence the influence of low versus high loads (0.57 and 2.53 N), different test velocities from 0.01 to 40 mm/s, smooth versus textured surfaces and different model systems was evaluated. Model foods such as oil-in-water emulsions, fat-free yoghurt, milk powder and nanocrystalline cellulose water suspensions were compared with sunflower oil and water as control lubricants. “Machine noise” without sample nor probe contact and “dry contact” of the stainless steel ball-on-disc geometry against a silicone elastomer sheet (PDMS, polydimethylsiloxane used by Chen et al. 2014) were also measured. Dynamic spectral analysis (Sanahuja and Briesen 2015) was applied to the force-displacement data. The extracted stick-slip magnitude-frequency distributions were divided into characteristic machine noise, typical dry contact effects and effects depending on the type of lubricant and surface roughness. The characteristic frequency bands of the stick-slip events could clarify a part of the interaction behavior of the PDMS surface with specific lubricants, mimicking the tongue sliding against the palate with food in-between.
This is the first study where dynamic spectral analysis was used in “oral” tribology. The spectrograms of the stick-slip effects provide a new insight into the use of “oral” tribology in food psychophysics studies. The methodology developed in this work can be applied, in addition to friction coefficient measurements and classical Stribeck curve analysis, to food oral behavior analysis and more importantly sensory interpretation.