Sensing of flow and shear stress using fluorescent molecular rotors

Molecular rotors are fluorescent molecules with two competing pathways of deexcitation: They return from the excited singlet state to the ground state either through fluorescence or through nonradiative intramolecular rotation. Molecular rotors are known as viscosity sensors, because intramolecular rotation rate depends on the viscosity of the solvent. In this study, we describe a new observation that the emission intensity of certain molecular rotors with hydrophilic head groups is elevated in fluids under shear. This intensity increase is dependent on both fluid velocity and viscosity. Statistically significant intensity increase was observed at fluid velocities as low as 0.6 mm/s. Using fiberoptics, local flow profiles could be probed. Measuring emission intensity of molecular rotors in sheared fluids may lead to the development of new shear field sensors, allowing real-time measurement of shear and flow without disturbing the fluid.