Microfluidic devices, allowing superior control over the spatial and temporal distribution of chemical substances and high process reproducibility, are nowadays essential in various research areas and industrial fields where the traditional "macroscopic" approach was no longer able to keep up with the increasing demands of high-end applications. In the present work, internal mixing of droplets formed by a flow-focusing X-junction at constant flow rates of both phases for three different channel heights (i.e. 20, 40 and 60 μm) was investigated and characterised. Both experimental methods and 3D CFD simulations were employed in order to resolve governing factors having an impact on internal mixing and homogenization time of model tracers inside of droplet reactors. Additionally, the influence of channel height on internal mixing was experimentally studied for continuous preparation of iron oxide nanoparticles by co-precipitation reaction. Since the initial nucleation phase is strongly affected by mixing and spatial distribution of all reactants, the final particle size and particle size distribution (PSD) can be used as direct indicators of mixing performance. It has been demonstrated that the smallest 20 μm channels provided narrower PSD and smaller particle mean size compared to higher channels.
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