The impact of tribometer motion and surface roughness on the frictional regimes of model foods

Assessing the tribological behavior of soft materials is often challenging due to the deformative nature of the surfaces which influences the lubricant entrainment and the contact area. The surface roughness and the tribological motion (e.g. sliding, rotational) of the tribometer may also influence the contact area, surface deformation and fluid entrainment. In the present work, we uncover how the above-mentioned parameters influence the lubrication behavior of Newtonian lubricants and of hydrogel microparticle suspensions. Unique to this study is that we use four different tribometers to obtain a more complete picture of the effect of tribometer characteristics on the frictional behavior. We present a side-by-side analysis of four tribological devices: three commercially available tribometers (Anton Paar, Bruker, PCS Instruments) and a custom-made 3D-printed tribometer. With this qualitative analysis, we demonstrate how the different Stribeck regimes are influenced by tribometer motions. The friction coefficients in the boundary regime and the length of the regime are strongly influenced the tribometer used while the slope in the mixed regime only shows minor differences for different tribometers. In the EHL regime, the slopes found correspond with the theoretically predicted value of 0.5 for the rotational tribometers. The linearly oscillating tribometer, however, displays an additional increase beyond the EHL regime with a much steeper slope than expected, which is caused by the specific motion of the tribometer. We further confirm the influence of tribometer motion by introducing hydrogel particle suspensions as model foods; each tribometer shows different lubrication regimes that are specific for each of the devices, highlighting the system dependency of tribological measurements.