On the Hamaker constant of the metallic iron, the retardation effect and their relevance in DLVO theory applied to oil-based magnetorheological fluid with 1-octylamine

To explain the redispersibility results of a poly(alpha-olefine) oil-based magnetorheological fluid with carbonyl iron powder (80 vol%) and 1-octylamine (0.5 wt% on iron powder), as a dispersant, we apply the DLVO theory. We reviewed and discussed values of the Hamaker constant for the iron metal and adopted A(Fe) similar to 460 100 zJ. For the MRF studied, therefore, the Hamaker constant Fe / PAO oil / Fe can be estimated as A(131) similar to 2.4 0.5 x 10(-19) J. We have analysed the equations for the retardation factor and as such factor changes the total potential curves in the DLVO approach. We conclude that the equation proposed by Ho and Higuchi can be applied to correct the van der Waals attractive potential curve. The electrokinetic potential (or zeta potential) of MR fluid was estimated from electroacoustic measurements, resulting zeta = 158 15 mV. For A(131) = 288 zJ, with zeta = 60 mV and including the retardation factor, the total potential curve resulted in a barrier height of 11 kT, without presenting a secondary minimum. The results of redispersibility of the MR fluid after one year at rest showed a normal force peak of 0.15 N and work of 1.12 mJ at 80 mm depth penetration.