  In this article, we present experimental results of chaotic mixing of
Newtonian uids and yield stress fluids using rod-stirring protocol with
rotating vessel. We show how the mixing of yield stress fluids by chaotic
advection is reduced compared to the mixing of Newtonian fluids and explain our
results bringing to light the relevant mechanisms: the presence of fluid that
only flows intermittently, a phenomenon enhanced by the yield stress, and the
importance of the peripheral region. This finding is confirmed via numerical
simulations. Anomalously slow mixing is observed when the synchronization of
different stirring elements leads to the repetition of slow stretching for the
same fluid particles.
