A Comparison of Measuring Devices Used to Prevent Wall Slip in Viscosity Measurements of Metallic Suspensions

When analysing semi-solid metal alloys or any other suspension in rheometers a major problem is related to wall slip effects which lead to an underestimation of the suspensions viscosity. Using smooth rotating rods in rotational rheometry for analysing suspensions a thin particle free layer of liquid is formed at the surface of the rod due to near-wall segregation. Thus, mainly the viscosity of this layer is measured which is much lower than the one of the bulk phase. Kiljanski [1] proposed a method for rotational rheometers to recalculate such measurements to get viscosity values without slip effects. A device-related change to minimize wall slip is the use of rotating rods with grooved surfaces. Therewith near-wall mixing is produced to destroy the particle free layer. Alternatively, the use of multiple blade vanes is proposed in the literature to avoid wall slip. The differences of the two measuring devices (grooved, vane) are discussed at the example of measurements for a low melting tin-lead alloy in the semi-solid state. It is figured out whether a vane geometry is also suitable to be applied for rheological measurements on metallic suspensions.