Dynamic characterization of bimodal particle mixtures in silicone rubber magnetorheological materials

Samples of a magnetorheological composite comprised of a silicone elastomer containing varying mixtures of 40- and 10-μm iron particles were tested in dynamic shearing experiments. These bimodal mixtures were used in order to determine whether such particle combinations might influence the composite's behavior as evidenced by changes in the relative magnetorheological (MR) effect, ΔG. Field-dependent results are consistent with ΔG ∝ M², M being the magnetization, and allow one to extrapolate a maximum relative MR effect. The extrapolated maximal ΔG values were effectively independent of particle-size ratio for fixed 30% [v/v%] Fe, suggesting that volume fraction was the important parameter and that particle positions were disordered. However, for 1% [v/v%] of the 10-μm particles to 30% [v/v%] 40-μm particles, there was enhanced response, perhaps due to the smaller particles sitting in beneficial interstitial positions. Further addition of 10-μm particles resulted in decreased performance.