  Multiphase shear flows often show banded structures that affect the global
behavior of complex fluids e.g. in microdevices. Here we investigate
numerically the banding of emulsions, i.e. the formation of regions of high and
low volume fraction, alternated in the vorticity direction and aligned with the
flow (shear bands). These bands are associated with a decrease of the effective
viscosity of the system. To understand the mechanism of banding experimentally
observed we have performed interface resolved simulations of the two-fluid
system. The experiments were perfomed starting with a random distribution of
droplets which, under the applied shear, evolves in time resulting in a phase
separation. To numerically reproduce this process, the banded structures are
initialized in a narrow channel confined by two walls moving in opposite
direction. We find that the initial banded distribution is stable when droplets
are free to merge and unstable when coalescence is prevented. In this case,
additionally, the effective viscosity of the system increases, resembling the
rheological behavior of suspensions of deformable particles. Droplets
coalescence, on the other hand, allows emulsions to reduce the total surface of
the system and hence the energy dissipation associated to the deformation,
which in turn reduces the effective viscosity.
