Experiment on Negative Magneto-Rheological Characteristics to Verify the Theoretical Prediction Based on the Orientational Distribution Function

In a suspension composed of spindle-like hematite particles (α-Fe₂O₃), the previous theoretical study based on the orientational distribution function has shown that the viscosity due to the magnetic properties of spindle-like hematite particles exhibits negative magneto-rheological characteristics in a certain situation of the orientational distribution of the particles under a certain applied magnetic field circumstance. This is mainly because the spindle-like hematite particles have a unique characteristic in that they are magnetized in a direction normal to the particle axis direction. This weak magnetic characteristic yields an advantage to the suspension in that it is relatively straightforward to synthesize a stable dispersion of such hematite particles because the electric double layer functions well for preventing the particles from aggregating. The negative magneto-rheological effect has also energetically been investigated by a simulation approach based on the Brownian dynamics and these simulation results clearly show that the negative magneto-rheological characteristics certainly arise in a multi-particle suspension system. From this background, in the previous experimental study, we synthesized a suspension composed of spindle-like hematite particles and measured the viscosity due to the magnetic properties of the particles in a simple shear flow. This first simple experiment has succeeded in verifying that the negative magneto-rheological characteristics surely arise in an actual hematite particle suspension. The present study further advances the experimental investigation of the negative magneto-rheological effect in order to obtain the more detailed data of these negative magneto-rheological characteristics. The viscosity due to the magnetic properties was measured using a cone-plate-type rheometer, located in the uniform area of the magnetic field, under various conditions of the magnetic field strength and the shear rate of a shear flow. The viscosity of hematite-glycerol-water dispersions becomes negative, attains to a minimum value, after that starts to increase, and finally becomes positive with increasing magnetic field strength. These characteristics of the negative viscosity are in good agreement with the theoretical prediction that was obtained by the orientational distribution function.