In numerous industrial processes involving fluids, viscosity is a determinant
factor for reaction rates, flows, drying, mixing, etc. Its importance is even
more determinant for phenomena observed are at the micro- and nano- scales as
in nanopores or in micro and nanochannels for instance. However, despite
notable progresses of the techniques used in microrheology in recent years, the
quantification, mapping and study of viscosity at small scales remains
challenging. Fluorescent molecular rotors are molecules whose fluorescence
properties are sensitive to local viscosity: they thus allow to obtain
viscosity maps by using fluorescence microscopes. While they are well-known as
contrast agents in bioimaging, their use for quantitative measurements remains
scarce. This paper is devoted to the use of such molecules to perform
quantitative, \textit{in situ} and local measurements of viscosity in
heterogeneous microfluidic flows. The technique is first validated in the
well-controlled situation of a microfluidic co-flow, where two streams mix
through transverse diffusion. Then, a more complex situation of mixing in
passive micromixers is considered and mixing efficiency is characterized and
quantified. The methodology developed in this study thus opens a new path for
flow characterization in confined, heterogeneous and complex systems.he
methodology developed in this study thus opens a new path for flow
characterization in confined, heterogeneous micro- and nano- systems