Fast micrometer resolution magneto-optical viscosity measurements with hard magnetic nanoplatelets

The paper reports on the application of hard magnetic nanoplatelets for direct local dynamic viscosity evaluation in liquids through their drag-hindered rotation after a magnetic field. Strontium hexaferrite, SrFe 12 O 19 nanoplatelets with a diameter of 50 nm and a thickness of 5 nm were employed as nanoscale probes. The dynamics of nanoplatelets is investigated with AC magnetometry, small-angle X-ray scattering, and optical absorption/reflectance spectroscopy. Those reveal well-defined relation of drag force and phase lag tangent for platelets rotation under AC magnetization with the viscosity of the medium. The analytical quantitative description for magneto-optical response is provided and utilized successfully for independent viscosity evaluation of various liquids within the viscosity range of 0.5–1000 mPa∙s. The nanoscale size of the probes also enabled viscosity visualization with micrometer resolution in microchannels. • Hexaferrite nanoplate rotors were applied as nanoprobes for viscosity evaluation. • Solvents within a wide range of η = 0.7–1000 mPa∗s were tested. • Micrometer range locality and possibility for in-situ measurements were demonstrated. • The theoretical background for nanoplates dynamics is provided.