The properties of magneto-rheological (MR) materials are temperature dependent. Compared to MR fluids, MR greases (MRGs) are more sensitive to temperature due to their inherent behavior of carrier materials. In this study, MRGs are studied to examine the temperature effect on their yield stress and apparent viscosity. Experimental data are obtained for magnetic fields ranging from 0.14 T to 0.53 T and temperatures ranging from 10°C to 70 °C. It is observed that temperature has a significant effect on the field-induced yield stress of MRGs. A new yield stress model, based on an extended Herschel—Bulkley constitutive relation, in which the shear yield stress is a function of magnetic field and temperature, is proposed. Excellent agreement between the theoretical results and experimental data is obtained.

We present a study of powder agglomeration and thermal conductivity in copper-based nanofluids. Synthesis of the copper powders was achieved by the use of three different surfactants (polyvinylpyrrolidone, oleic acid, and cetyl trimethylammonium bromide). After careful determination of morphology and purity, we systematically and rigorously compared all three of the surfactants for the production of viable copper-based nanofluids using dynamic light scattering. Our results show that the use of surfactants during synthesis of copper nanopowders has important consequences on the dispersion of the powders in a base fluid. The oleic-acid-prepared powders consisted of small particles of ∼100 nm that did not change with the addition of dispersant. The CTAB-prepared powders exhibited the best dispersion characteristics, as they formed small particles of approximately 80 nm in the presence of SDBS. The thermal conductivity enhancement in our nanofluids exhibited a linear relationship with powder loading for an average particle size of ∼100 nm and similar particle size distributions that range from ∼50 to 650 nm, but independent of crystallite size and with all other factors maintained constant (surface area, surface additives, levels of oxidation) such that a 0.55 vol % loading results in a thermal conductivity enhancement of 22% over water and a 1.0 vol % loading results in a thermal conductivity enhancement of 48% over water. This study is the first to decouple the effect of a carefully characterized particle size distribution using dynamic light scattering versus crystallite size from X-ray line broadening on the thermal conductivity enhancement of a nanofluid.

We present an analysis of the dispersion characteristics and thermal conductivity performance of copper-based nanofluids. The copper nanoparticles were prepared using a chemical reduction methodology in the presence of a stabilizing surfactant, oleic acid or cetyl trimethylammonium bromide (CTAB). Nanofluids were prepared using water as the base fluid with copper nanoparticle concentrations of 0.55 and 1.0 vol.%. A dispersing agent, sodium dodecylbenzene sulfonate (SDBS), and subsequent ultrasonication was used to ensure homogenous dispersion of the copper nanopowders in water. Particle size distribution of the copper nanoparticles in the base fluid was determined by dynamic light scattering. We found that the 0.55 vol.% Cu nanofluids exhibited excellent dispersion in the presence of SDBS. In addition, a dynamic thermal conductivity setup was developed and used to measure the thermal conductivity performance of the nanofluids. The 0.55 vol.% Cu nanofluids exhibited a thermal conductivity enhancement of approximately 22%. In the case of the nanofluids prepared from the powders synthesized in the presence of CTAB, the enhancement was approximately 48% over the base fluid for the 1.0 vol.% Cu nanofluids, which is higher than the enhancement values found in the literature. These results can be directly related to the particle/agglomerate size of the copper nanoparticles in water, as determined from dynamic light scattering.

In this study, the response times of magnetorheological fluids and magnetorheological fluid valves are studied under various flow configurations. Two types of valving geometries, annular flow and radial flow, are considered in the magnetorheological fluid valve designs. The transient pressure responses of magnetorheological fluid valves are evaluated using a diaphragm pump with a constant volume flow rate. The performance of each magnetorheological valve is characterized using a voltage step input as well as a current step input while recording the activation electric voltage/current, magnetic flux density, and pressure drop as a function of time. The variation of the response time of the magnetorheological valves under constant volume flow rate is experimentally investigated. The Maxwell model with a time constant is employed to describe the field-induced pressure behavior of magnetorheological fluid under a steady flow. The results demonstrate that the pressure response times of the magnetorheological fluid and the magnetorheological valves depend on the designs of the electric parameters and the valve geometry. Magnetorheological valves with annular flow geometry have a slower falling response time compared to their rising response time. Magnetorheological valves with radial flow geometry demonstrate faster pressure response times both in rising and in falling states.

A surface grafting technique for poly(2-fluorostyrene) onto iron particles via atom transfer radical polymerization (ATRP) is described. Grafted poly(2-fluorostyrene)-iron particles were synthesized by immobilizing 2-4(-chlorosulfonylphenyl)-ethyltrichlorosilane to the iron particles through the covalent bond of a silanol group, followed by the polymerization of 2-fluorostyrene monomer. The grafted polymer-iron particles display a higher thermal transition temperature compared to bulk polymer because the covalent bond between the polymer backbone and the surface of the iron particles restricts the molecular mobility. The molecular weight of the synthesized poly(2-fluorostyrene) has been measured and it has a narrow molecular weight distribution (M w /M n < 1.1). From thermogravimetric analysis, the thermal stability of poly(2-fluorostyrene) is superior to polystyrene. Also, the high viscosity magnetorheological fluid (HVMRF) prepared from surface coated iron particles has excellent thermo-oxidative stability, having nearly constant viscosity. These materials exhibit a large increase in shear yield stress for the off-and on-state as compared to a benchmark high viscosity magnetorheological fluid (HVMRF) and -coated iron particle HVMRF. In addition, this type of fluid eliminates iron particle settling which is a common problem found in traditional magnetorheological fluids (MRFs).