On the use of rough geometries in rheometry

•We show that rough geometries induce an apparent wall slip in rheometry.•Two commercial sandpapers are used as rough geometries.•We investigate two rheometer setups: sandpaper on (i) lower plate and (ii) both plates.•We show that the fluid flows within the sandpaper roughness.•We propose two procedures to correctly use rough geometries in rheometry.

Multiphase fluids, like suspensions, often show wall slip. This is induced by depletion of particle concentration in proximity of wall surfaces. The rheometrical data can be corrected for wall slip in a post processing analysis. Alternatively, it is typically tried to suppress wall slip using modified devices, like rough geometries. We here investigate whether rough geometries themselves affect the rheological measurements. To this end, we glue sandpaper on the smooth surfaces of a plate–plate device. We use two commercial sandpapers and we measure five different Newtonian fluids proving that the fluids actually flow within the sandpaper roughness. This shows up as an apparent wall slip that we characterise. We, then, observe that the same happens also with a model suspension made of hollow glass beads in a Newtonian fluid. Finally, we propose two experimental procedures to correctly infer the rheological properties of a fluid when rough geometries are used. The first is more time consuming and accurate, the second is fast, only requires a single measurement and, in any case, is quite robust.