This paper reports experiments on the shear transient response of
concentrated non-Brownian suspensions. The shear viscosity of the suspensions
is measured using a wide-gap Couette rheometer equipped with a Particle Image
Velocimetry (PIV) device that allows measuring the velocity field. The
suspensions made of PMMA particles (31$\mu$m in diameter) suspended in a
Newtonian index- and density-matched liquid are transparent enough to allow an
accurate measurement of the local velocity for particle concentrations as high
as 50%. In the wide-gap Couette cell, the shear induced particle migration is
evidenced by the measurement of the time evolution of the flow profile. A
peculiar radial zone in the gap is identified where the viscosity remains
constant. At this special location, the local particle volume fraction is taken
to be the mean particle concentration. The local shear transient response of
the suspensions when the shear flow is reversed is measured at this point where
the particle volume fraction is well defined. The local rheological
measurements presented here confirm the macroscopic measurements of
Gadala-Maria and Acrivos (1980). After shear reversal, the viscosity undergoes
a step-like reduction, decreases slower and passes through a minimum before
increasing again to reach a plateau. Upon varying the particle concentration,
we have been able to show that the minimum and the plateau viscosities do not
obey the same scaling law with respect to the particle volume fraction. These
experimental results are consistent with the scaling predicted by Mills and
Snabre (2009) and with the results of numerical simulation performed on random
suspensions [Sierou and Brady (2001)]. The minimum seems to be associated with
the viscosity of an isotropic suspension, or at least of a suspension whose
particles do not interact through non-hydrodynamic forces, while the plateau
value would correspond to the viscosity of a suspension structured by the shear
where the non-hydrodynamic forces play a crucial role