Effect of polar interactions on the magnetorheology of silica-coated magnetite suspensions in oil media

• Published in: Journal of colloid and interface science 2009-09, Vol.337 (1), p.254-259
• Main Authors: Pacull, Julien, Gonçalves, Stephanie, Delgado, Ángel V., Durán, Juan D.G., Jiménez, Marı´a L.
• Format: Article
• Language: English
• Subjects: Chains; Chemistry; Colloidal state and disperse state; Exact sciences and technology; Field strength; General and physical chemistry; Hydrophobicity; Magnetic fields; Magnetite; Magnetorheological fluids; Polar interactions; Pseudoplasticity; Surface physical chemistry; Yield stress
• ISSN: 0021-9797; 1095-7103
• DOI: 10.1016/j.jcis.2009.04.083

Nonmagnetic forces are also important for the magnetorheological effect: silica-coated magnetite particles exhibit different magnetorheological behaviors, depending on their hydrophilic or hydrophobic character. This work deals with the role of nonmagnetic interactions on the magnetorheological (MR) response of suspensions of magnetic particles in nonaqueous carriers (MR fluids). Although electrostatic interactions between particles are negligible, van der Waals and, eventually, polar forces might be present. Nevertheless, they are typically neglected when compared to magnetic or hydrodynamic ones. In order to perform a comparative evaluation of the contributions of these interactions, we carried out an MR investigation on two samples of silica-coated magnetite: one (MagSilica 50-H8) is hydrophobic and the other (MagSilica 50-85) is hydrophilic. We describe a careful surface thermodynamic characterization of the two kinds of silicas, confirming their very different hydrophobicities. MR measurements show that only the suspensions of 50-85 particles change from Newtonian to pseudoplastic when a magnetic field is applied, with a yield stress increasing with field strength H, and saturating when H ≅ 100 kA/m. The experimental values of yield stress are compared to theoretical predictions based on the chain model, and it is found that the theory overestimates the experimental values. It is suggested that the nonnegligible interfacial interactions are responsible for both the absence of MR effect in 50-H8 samples and the low yield stress in 50-85 suspensions.