Refractive index matching (RIM) using double-binary liquid\u2013liquid mixtures

Abstract

For using microscopic multiphase flows in microreactors, an exact understanding of the underlying hydrodynamic interrelations is key for successful reactor layout and reaction control. To examine the local hydrodynamic behavior, non-invasive optical measurements techniques like particle tracking velocimetry (PTV) or micro particle image velocimetry (µPIV) are the methods of choice, since they provide precise velocity measurement with excellent spatial resolution. Such optical approaches require refractive index matching (RIM) of the involved flow phases to prevent optical distortion due to light refraction and reflection at the interfaces. Established RIM approaches often provide a single degree of freedom, which is sufficient to solely match the refractive index (RI) of the flow phases. Using these approaches, the material properties (Oh number) are fixed and the relevant dimensionless numbers (Ca, Re) may only be altered hydrodynamically or geometrically. To avoid expansive geometric scaling of the microchannels, we propose an approach using two binary mixtures (double-binary mixtures) to introduce an additional degree of freedom. The approach allows examining liquid–liquid two-phase flows at a distinct velocity while being able to change the material properties (Oh number). Thus, Ca and Re can be chosen individually and the proposed RIM-approach provides undisturbed optical access. Furthermore, we present four different binary mixtures, which allow to vary the viscosity ratio of the phases. The relevant material parameters are successfully correlated to measurement data, which delivers a system of equations that determines the mass fractions and the velocities to address Re and Ca individually. A proof-of-principle for the proposed double- binary mixture RIM-approach is successfully established using µPIV raw images.Graphic abstract