Effective viscoelasticity of non-Newtonian fluids modulated by large-spherical particles aligned under unsteady shear

Abstract

The effective viscoelasticity of non-Newtonian fluids with spherical particles has been examined by ultrasonic spinning rheometry [Yoshida et al., “Efficacy assessments in ultrasonic spinning rheometry: Linear viscoelastic analysis on non-Newtonian fluids,” J. Rheol. 63, 503 (2019)]. Under unsteady shear flows, the dispersed particles make alignments in the sheared direction if the relaxation time of the fluid media is sufficiently long. The alignments modulate the effective rheological properties, and the effective viscosity does not reach the value estimated by Einstein’s law; the effective elasticity increases significantly with increasing volume fraction in the bulk of the measurement volume. To establish further details of factors influencing the aligned particles under unsteady shear flows/deformations, numerical tests using a simple toy model assuming dispersed particles combined by spring forces considering yield stresses were conducted, and the model identified the importance of the relaxation process on the orientation of the particles. Finally, considering the experimental findings, local and macrorheological characteristics are strongly modulated by the particle alignment when the test fluid media have long relaxation times (or high Weissenberg numbers).