Molecular Rotors for In Situ Local Viscosity Mapping in Microfluidic Chips

Abstract

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 at the micro- and nanoscale such as in nanopores or in micro and nanochannels, for instance. However, despite notable progress in the techniques used in microrheology in recent years, the quantification, mapping, and study of viscosity at small scales remain challenging. Fluorescent molecular rotors are molecules whose fluorescence properties are sensitive to local viscosity; thus, they allow us 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, in situ, and local measurements of viscosity in heterogeneous microfluidic flows. The technique is first validated in a 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 the mixing efficiency is characterized and quantified. The methodology developed in this study thus opens a new path for viscosity characterization in confined, heterogeneous, and complex systems.

Molecular Rotors for In Situ Viscosity Mapping during Evaporation of Confined Fluid Mixtures

“…The FMR used in this study was a BODIPY-based compound (see the structure in Figure ) which was synthesized and characterized in a previous work . As its solubility in aqueous solutions was limited, the FMR was initially dissolved in glycerol (fresh glycerol of purity ≥99%, Sigma-Aldrich G5516-1L) and then diluted in the fructose/water solutions in order to reach a final mole concentration of FMR of 10 μmol L –1 in every solution.…”

“…Structure of the BODIPY-based FMR of formula C 18 H 17 BF 2 N 2 O 2 and molar mass 342.15 g mol –1 , as synthesized and characterized in ref . Reprinted from Nalatamby et al Copyright 2023, Industrial & Engineering Chemistry Research, American Chemical Society.…”

“…FMRs have initially been introduced in the context of bioimaging and used to image semiquantitatively viscosity changes in biofluids and inside cells. − More recently, they have become an interesting tool to monitor physicochemical transformations , and for the observation of nanoscopic contacts . However, it has been shown that they can also be used as quantitative viscosity probes in bulk fluids, in microfluidic flow, − or in confined lubricated flows …”

“…More recently, they have became an interesting tool to monitor physicochemical transformations [80,81] and for the observation of nanoscopic contacts [82][83][84][85]. However, it has been shown that they can also be used as quantitative viscosity probes in bulk fluids [86], in microfluidic flow [87][88][89] or in confined lubricated flows [90].…”

Visualization of the Sol–Gel Transition in Porous Networks Using Fluorescent Viscosity-Sensitive Probes

“…Fluorescent molecular rotors have previously been used as viscosity sensors for biological samples, − to probe at different scales the structure and properties of fluids such as polymer solutions − and polymer glasses , and to study the aging of gels. − However, in these previous investigations, a correlation was sought between the macroscopic mechanical properties and the fluorescence of the molecular rotor. To the best of our knowledge, such viscosity-sensitive probes have not yet been used to monitor the spatial development of a gel at the microscale in a porous network.…”