Steady shear magnetorheology of inverse ferrofluids

Abstract

Silica-based inverse ferrofluids (IFFs) are synthesized and their pre-yield and post-yield rheological properties are investigated as a function of magnetic field strength (8.8–276 kA/m), volume fraction (12.6–26.1 vol %), silica particle size (104–378 nm radius), and ferrofluid Newtonian viscosity (44–559 mPa s). The Mason number (Mn) provides a good scaling of the data in the steady simple shear flow regime. Special emphasis is made on the low and moderate Mason number region. At low Mn values, two different behaviors are observed depending on the IFF formulation and magnetic field strength applied: (i) either the viscosity monotonically increases with decreasing shear rate suggesting the existence of a yield stress (ii) or a low-shear plateau is reached. At medium Mn values, a power law behavior is found η/η∞∝MnΔ with −1<Δ<−2/3. Yielding behavior is modeled by using both macroscopic and microscopic approaches under the assumption of spheroidal, cylindrical, and single-width particle chain models.