Velocity-profiling-assisted rheometry for evaluating complex multiphase fluids

Velocity-profiling-assisted rheometry (VPAR) based on particle image velocimetry (PIV) has been developed for the rheological characterization of complex multiphase fluids. In contrast to the conventional method based on ultrasonic velocity profiling, whose applicability is often hindered by attenuation or multiple reflections of ultrasonic waves in suspensions containing bubbles, droplets, or solid particles, the PIV-based VPAR offers high adaptability to complex multiphase fluids by measuring the surface flow between the concentric cylinders. As a demonstration, the effective viscosity of a bubble suspension in a xanthan gum-based non-Newtonian fluid was exemplarily investigated. A conventional torque-based interpretation misleadingly indicates that the bubbles decrease the effective viscosity at low shear rates, whereas at high shear rates they increase the effective viscosity relative to the single-phase condition. However, VPAR analysis revealed a consistent increase in the effective viscosity with bubble dispersion, aligning with established theoretical predictions. These findings highlight the risk of misinterpretation when using conventional rheometry for complex fluids and underscore the robustness of VPAR as a complementary methodology.