A Physical Basis for Stability in Bimodal Dispersions Including Micrometer-sized Particles and Nanoparticles using Both Linear and Non-linear Models to Describe Yield

This work investigates the settling response of bi-disperse silicone oil-based magnetorheological fluids with varying solids loading (content) of Fe magnetic particles at three ratios of nanoparticles to micron size particles. The solid loading (content) of Fe magnetic particles in silicone oil was 50 or 60% by weight with nanoparticle fractions of 0, 10, or 15% by weight of the total solids loading. Sedimentation experiments are conducted using Z-axis translating laser light scattering at ambient temperature. Sedimentation velocities are determined from clear layer separation and functional particulate transport by creeping flow. Settling rates of 1—3 μm/s have been observed in the dispersions without nanoparticles. Viscosity testing shows lower yield stress with nanoparticle inclusion and the typical rise in shear stress with increasing shear rate and applied magnetic field using both the linear Bingham and the non-linear Hershel—Bulkley models. Adding nanoparticles also reduced sedimentation velocity by roughly two orders of magnitude from those without nanoparticles. The nanoparticles act as plugs or otherwise interact in forming soft gel particulates leading to a denser sedimentation front yielding fewer percolation holes through which the fluid can permeate. Either way, the addition of nanoparticles leads to a more effective stabilization mechanism for the dispersion.