Stable Magnetorheological Fluids Containing Bidisperse Fillers with Compact/Mesoporous Silica Coatings

A drawback of magnetorheological fluids is low kinetic stability, which severely limits their practical utilization. This paper describes the suppression of sedimentation through a combination of bidispersal and coating techniques. A magnetic, sub-micro additive was fabricated and sequentially coated with organosilanes. The first layer was represented by compact silica, while the outer layer consisted of mesoporous silica, obtained with the oil–water biphase stratification method. The success of the modification technique was evidenced with transmission electron microscopy, scanning electron microscopy/energy-dispersive X-ray spectroscopy and Fourier-transform infrared spectroscopy. The coating exceptionally increased the specific surface area, from 47 m(2)/g (neat particles) up to 312 m(2)/g, which when combined with lower density, resulted in remarkable improvement in the sedimentation profile. At this expense, the compact/mesoporous silica slightly diminished the magnetization of the particles, while the magnetorheological performance remained at an acceptable level, as evaluated with a modified version of the Cross model. Sedimentation curves were, for the first time in magnetorheology, modelled via a novel five-parameter equation (S-model) that showed a robust fitting capability. The sub-micro additive prevented the primary carbonyl iron particles from aggregation, which was projected into the improved sedimentation behavior (up to a six-fold reduction in the sedimentation rate). Detailed focus was also given to analyze the implications of the sub-micro additives and their surface texture on the overall behavior of the bidisperse magnetorheological fluids.