  We used scattering dichroism to study the combined effects of viscous and
magnetic forces on the dynamics of dipolar chains induced in magnetorheological
suspensions under rotating magnetic fields. We found that the chains adjust
their size to rotate synchronously with the field but with a constant phase
lag. Two different behaviors for the dichroism (proportional to the total
number of aggregated particles) and the phase lag are found below or above a
critical frequency. We obtained a linear dependence of the critical frequency
with the square of the magnetization and with the inverse of the viscosity. The
Mason number (ratio of viscous to magnetic forces) governs the dynamics.
Therefore there is a critical Mason number below which, the dichroism remains
almost constant and above which, the rotation of the field prevents the
particle aggregation process from taken place being this the mechanism
responsible for the decrease of dichroism. Our experimental results have been
corroborated with particle dynamics simulations showing good agreement.
