Percolation transition in nanowire magnetorheological fluids

We measure the yield stress of magnetorheological (MR) fluids that employ cobalt nanowires as the ferromagnetic component and observe a percolation transition in the yield stress at a critical value of the cobalt-nanowire volume fraction, $p_c $. The critical volume fraction depends not only on the particle size and aspect ratio (as expected) but also on the external magnetic field applied to the MR-fluid sample. We fit the yield-stress data using McLachlan's generalized effective medium (GEM) model to determine $p_c $ and the percolation exponents $s$ and $t$ that describe the transition behavior below and above $p_c $, respectively. The phase transition from low- to high-yield stress at low magnetic-particle volume fraction ($<1\% )$ has potential application to the development of precision magnetic sensors and actuators.