Shear stress characteristics of microtextured surfaces in gap-controlled hydrodynamic lubrication

•A custom precision-aligned setup for gap-controlled tribo-rheometry is described, based on a rotational rheometer.•The instrument is validated for gap offset error, the primary imprecision responsible for potential misinterpretation of friction reduction.•Measurements span the dimensionless parameter space of Reynolds number and dimensionless gap.•We demonstrate in gap-controlled conditions that microtextured surfaces reduce friction.

Microtextured surfaces can reduce friction in lubricated dynamic contact. However, no prior experimental study has explored the dimensionless parameter space of Reynolds number and dimensionless gap, since normal force is easier to control than the fluid film thickness. Here, we develop a custom precision-aligned setup for gap-controlled tribo-rheometry based on a rotational rheometer. The novel experimental setup allows for measurement of full film lubrication of parallel disks down to 20 μm gaps with gap precision ±3 μm, over a range of Reynolds numbers. We show for the first time in gap-controlled conditions that microtextured surfaces reduce friction. The reduction in dimensionless shear stress is nearly independent of velocity for Re<10 which matches computational results in this work.