A comparison between flow-induced structures in electrorheological and magnetorheological fluids

We present the patterns observed in suspensions placed in an oscillating shear flow. We show that, in the presence of an electric field, above a critical field inducing a phase separation, and above a critical strain, γc = 0.15, different patterns are found depending on the strain amplitude and on the field intensity. The ultimate structure at high field and strain is formed of thick stripes aligned perpendicularly to the flow. In certain conditions an instability with sawteeth formation is found to develop on these stripes. For magnetic suspensions, above the same critical strain, we only observe the formation of a regular pattern of stripes aligned with the flow. This difference between the patterns formed under electric or magnetic field is explained by a difference in boundary conditions. It is also proved that the stripes perpendicular to the flow are due to inertial effects.