Microviscosity is an important parameter for the flow of complex fluids however, its measurement is difficult with traditional rheometers especially in confined systems. An emerging tool that can be used to probe viscosity at a microscopic scale is molecular rotors. They allow accurate quantitative measurement of microviscosity in confined zones because of their simple calibration and fast signal acquisition.<br/>   In this work, 9-(dicyano-vinyl)-julolidine (DCVJ), a fluorescent molecular rotor, was used for in-situ measurement of microviscosity in a microfluidic co-flow. DCVJ’s sensitivity to viscosity is due to its twisted intramolecular charge transfer state upon photoexcitation at a specific wavelength. The conformational change is hampered by higher viscosity which leads to an increase in fluorescence intensity. This property of molecular rotors combined with a suitable microfluidic device and fluorescence microscope, allowed us to conduct real-time measurements of microviscosity in a complex system.<br/>   During a co-flow of dimethylsulfoxide (DMSO) and glycerol inside a Y-junction microfluidic chip, viscosity mapping was carried out along the mixing channel with a good spatial resolution. A calibration curve based on the Forster-Hoffman equation, established beforehand, allowed us to retrieve the local viscosity from the fluorescence response of DCVJ and to perform the microviscosity mapping. These viscosity values were converted to volume fractions of glycerol and the concentration maps along the microchannel were fitted. The parameters of the fitting enabled us to calculate the effective interdiffusion coefficient of glycerol.

In-situ microviscosity measurements in microfluidic chip using fluorescent molecular rotors

ACS Spring 2022

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