Rheology of diluted ferrofluid emulsions in planar extensional flows

In this talk, we present a 3D computational study on how external magnetic fields impact the rheology of dilute ferrofluid emulsions in planar extensional flows. The intensity and direction of uniform magnetic fields influence the planar extensional rheology by altering the shape, orientation, and magnetization of the ferrofluid droplets in suspension. The conventional extensional viscosities associated with the normal stresses of the bulk emulsion either remain constant or increase with the field intensity, except when the field is perpendicular to the extension plane. In that case, the planar extensional viscosity stays constant, and the second extensional viscosity slightly decreases. The droplets tilt in the flow when the external field is not aligned with one of the main flow directions, resulting in a change in the recirculation pattern and flow topology inside the droplet. At the microscopic level, the droplets experience a magnetic torque due to a misalignment between their magnetization and the external field direction. At the macroscopic level, the bulk emulsion experiences a field-induced internal torque leading to a nonsymmetric stress tensor with unexpected shear components in extension. To describe this unconventional stress-strain response, we introduce new extensional material functions, to provide a complete rheological description of ferrofluid emulsions in planar extensional flows. This study provides fresh perspectives on the practical applications involving the manipulation of ferrofluid emulsions through the assistance of magnetic fields.