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In numerous industrial processes involving fluids, viscosity is a determinantfactor for reaction rates, flows, drying, mixing, etc. Its importance is evenmore determinant for phenomena observed are at the micro- and nano- scales asin nanopores or in micro and nanochannels for instance. However, despitenotable progresses of the techniques used in microrheology in recent years, thequantification, mapping and study of viscosity at small scales remainschallenging. Fluorescent molecular rotors are molecules whose fluorescenceproperties are sensitive to local viscosity: they thus allow to obtainviscosity maps by using fluorescence microscopes. While they are well-known ascontrast agents in bioimaging, their use for quantitative measurements remainsscarce. This paper is devoted to the use of such molecules to performquantitative, \textit{in situ} and local measurements of viscosity inheterogeneous microfluidic flows. The technique is first validated in thewell-controlled situation of a microfluidic co-flow, where two streams mixthrough transverse diffusion. Then, a more complex situation of mixing inpassive micromixers is considered and mixing efficiency is characterized andquantified. The methodology developed in this study thus opens a new path forflow characterization in confined, heterogeneous and complex systems.hemethodology developed in this study thus opens a new path for flowcharacterization in confined, heterogeneous micro- and nano- systems.

Molecular rotors for in situ local viscosity mapping in microfluidic chips