Tribology of simple and complex (food) systems
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Tribology is the study of friction, lubrication and wear of interacting surfaces in relative movement. Since the oral cavity is effectively a lubricated system, tribology is seen as a valuable means of relating instrumentally measured friction with oral lubrication and accompanying textural sensory attributes of food. In this work, the effects of several sample, material and operational parameters on the tribological characterization with UMT TriboLab are investigated. Simple glycerol/water mixtures confirm the effect of increasing viscosity on the lubricated friction, shifting the lubrication from the boundary to the hydrodynamic regime. Measurements of these mixtures with steel-glass, steel-rubber and PDMS-glass tribopairs in addition to the standard PDMS-rubber tribopairs show that the stiffer and smoother tribopairs generate lowered friction coefficients and a less extended mixed regime. Steel-glass tribopairs generally weaken the dependence of the friction coefficient on the entrainment speed and sample viscosity. Contact areas between PDMS probes and rubber substrates could be estimated from wear marks on PDMS probes after a measurement, and showed a linear dependence on the applied normal load. With the current tribological setup, no significant differences in friction between milks of different fat content (with saliva) could be measured. The experiments with milk and with saliva and astringent mixtures suggest that the choice of the type of tribopairs and the sliding speed are vital for a proper functionality of the UMT TriboLab for oral tribology.