  In situ observation of precipitation or phase separation induced by solvent
addition is important in studying its dynamics. Combined with optical and
fluorescence microscopy, microfluidic devices have been leveraged in studying
the phase separation in various materials including biominerals, nanoparticles,
and inorganic crystals. However, strong scattering from the subphases in the
mixture is problematic for in situ study of phase separation with high temporal
and spatial resolution. In this work, we present a quasi-2D microfluidic device
combined with total internal reflection microscopy as an approach for in situ
observation of phase separation. The quasi-2D microfluidic device comprises of
a shallow main channel and a deep side channel. Mixing between a solution in
the main channel (solution A) and another solution (solution B) in the side
channel is predominantly driven by diffusion due to high fluid resistance from
the shallow height of the main channel, which is confirmed using fluorescence
microscopy. Moreover, relying on diffusive mixing, we can control the
composition of the mixture in the main channel by tuning the composition of
solution B. We demonstrate the application of our method for in situ
observation of asphaltene precipitation and beta-alanine crystallization.
