  Microfluidic technology provides a solution to the challenge of continuous
CaCO$_3$ particle synthesis. In this study, we utilized a 3D-printed
microfluidic chip to synthesize CaCO$_3$ micro and nanoparticles in vaterite
form. Our primary focus was on investigating a continuous one-phase synthesis
method tailored for the crystallization of these particles. By employing a
combination of confocal and scanning electron microscopy, along with Raman
spectroscopy, we were able to thoroughly evaluate the synthesis efficiency.
This evaluation included aspects such as particle size distribution,
morphology, and polymorph composition. The results unveiled the existence of
two distinct synthesis regimes within the 3D-printed microfluidic chips,
featuring a channel cross-section of 2 mm$^2$. In the first regime,
characterized by turbulence, particles with an average diameter of around 2
$\mu$m were produced, displaying a broad size distribution. Conversely, the
second regime, marked by laminar flow, led to the synthesis of submicron
particles (approximately 800-900 nm in diameter) and even nanosized particles
(70-80 nm). This research significantly contributes valuable insights to both
the understanding and optimization of microfluidic synthesis processes,
particularly in achieving controlled production of submicron and nanoscale
particles.
