  This work deals with the evaluation of the flow curve of colloidal systems
that develop fluid phases with different mechanical properties, namely
shear-banding fluids. The problem involved is that, as different fluid phases
coexist in the flow domain of the rheometric cell, measured data cannot be
directly converted into rheometric functions. In order to handle this problem,
a shear stress vs. shear rate constitutive relation is introduced to interpret
the steady state flow curves. The relation derives from a phenomenological
description of structural changes, and involves the possibility of multivalued
shear rates under a given shear stress. Numerical predictions satisfactorily
match up to experimental data of wormlike micellar solutions. A crucial aspect
is the adequate computation of the shear rate function from raw data measured
in the rheometric cell.
