The role and the importance of
fluid inertia in different microfluidic applications has been recently
recognized. Among these we recall enhanced mixing, particle separation and bioparticle focusing. To
further develop inertial microfluidic devices, it is therefore necessary to properly understand the behaviour
of suspensions at low Reynolds numbers and in confined geometries. When considering shear
flows, the
increase of the effective viscosity of a dense suspension with the shear rate is known as shear thickening.
The effects of volume fraction and shear rate on this phenomenon have been studied both experimentally
and numerically. Nonetheless, not much is known about the impact of confinement. Here we consider a
noncolloidal suspension of rigid spheres in a simple shear
flow. We show that the effective viscosity varies
sharply by increasing the height of the channel, and that it presents a series of maxima and minima before
adjusting to the bulk behaviour. These minima and maxima occur precisely when two or three particle
layers can form, so that layering can significantly alter the
flow in confined geometries. We therefore
argue that layering can be effectively used to reduce viscosity.
