Viscoelastic normal stresses can generate propulsion in swirlers – axisymmetric swimmers rotating about their axis of symmetry – even in the absence of propulsion in a Newtonian fluid at low Reynolds numbers. This introduces the exciting possibility of an in situ “swimming rheometer”, which measures non-Newtonian fluid properties through propulsion. In this presentation, we will demonstrate how a tethered, torque-free, two-body swirler can perform sensitive measurements of the steady primary normal stress coefficient over a range of low shear rates inaccessible on conventional benchtop rheometers, simply via measurement of its propulsive force. Results will show that the combined application of micro-hydrodynamic theory and numerical simulations enables the experimental measurement of viscometric fluid properties without a viscometric flow. Consequently, such a “swimming rheometer” opens up a large design space where swimmer geometry and confinement can be designed to enhance measurement sensitivity and range. Finally, we will demonstrate how dynamic measurements of linear viscoelastic properties can also be obtained using the same tethered swimmer.
