  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.
