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

## TL;DR

This paper introduces a novel refractive index matching method using double-binary liquid mixtures, enabling independent control of flow parameters for optical measurements in multiphase flows.

## Contribution

It presents a new RIM approach with two binary mixtures, allowing independent adjustment of flow properties and improved optical access in microfluidic experiments.

## Key findings

- Successfully correlated material parameters with measurement data.
- Developed a system of equations for mixture composition and flow velocities.
- Proof-of-principle demonstrated with micro-PIV images.

## Abstract

Within the last decade microscopic multiphase flows have gained broad interest. An exact understanding of the underlying hydrodynamic interrelations is the 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 ((\textmu)PIV) are the method 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 one degree of freedom which is sufficient to match the RI of the flow phases solely. With that, 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 combination ($Oh$-Number). Therefore $Ca$ and $Re$ can be chosen individually and the RIM provides undisturbed optical access. We present 4 different binary mixtures to be used, e.g. with Taylor droplets. 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 \textmu PIV raw images.