Abstract
The clean and reproducible conditions provided by microfluidic devices are ideal sample environments for in situ analyses of chemical and biochemical reactions and assembly processes. However, the small size of microchannels makes investigating the crystallization of poorly soluble materials on-chip challenging due to crystal nucleation and growth that result in channel fouling and blockage. Here, we demonstrate a reusable insert-based microfluidic platform for serial X-ray diffraction analysis and examine scale formation in response to continuous and segmented flow configurations across a range of temperatures. Under continuous flow, scale formation on the reactor walls begins almost immediately on mixing of the crystallizing species, which over time results in occlusion of the channel. Depletion of ions at the start of the channel results in reduced crystallization towards the end of the channel. Conversely, segmented flow can control crystallization, so it occurs entirely within the droplet. Consequently, the spatial location within the channel represents a temporal point in the crystallization process. Whilst each method can provide useful crystallographic information, time-resolved information is lost when reactor fouling occurs and changes the solution conditions with time. The flow within a single device can be manipulated to give a broad range of information addressing surface interaction or solution crystallization.
Highlights
The clean and reproducible conditions provided by microfluidic devices are ideal sample environments for in situ analyses of chemical and biochemical reactions and assembly processes
In the T-junction, the input solutions begin to mix immediately adjacent to their interface. This interfacial region grows as the solutions travel downstream, until the solutions are fully mixed. This occurs at approximately position 1 (9.7 s) with a flow rate of 14 μL min−1 (Fig. S5‡), which demonstrates that mixing is rapid as compared with the total device residence time of 5.44 min
While many industrial processes are performed at larger length scales, our results provide broad guidance on fouling mitigation and are directly applicable to recent developments in miniaturised industrial platforms including oscillatory baffled reactors (OBRs),[61] continuous stirred tank-reactors (CSTRs),[62] and automated “plug-and-play” systems.[63]
Summary
This can be attributed to the challenges associated with characterizing the molecular-scale processes that govern these phenomena.[11,12,13] Further, the majority of crystallization experiments are performed in large reaction volumes (≥1 mL), which experience non-uniform mixing during the early stages of the reaction and inevitably contain impurities.[13,14,15] Thanks to the ever-improving accessibility of microfabrication techniques, microfluidic devices are drawing increasing attention as a means of performing crystallization, where these offer clean and controllable reaction environments.[16,17,18,19,20]. An additional benefit of utilizing microfluidics is the ability of lab-on-a-chip devices to be coupled to characterization techniques, such as X-ray scattering and diffraction, which can facilitate in situ analysis of crystallization and other assembly processes Performing these types of analyses require highly controlled and low X-ray absorbing sample environments, where microfluidic devices seem like a natural choice, since their small channel size provides efficient and welldefined heat and mass transport and minimises the required beam path through a sample. Previous work in developing microfluidic sample environments for in situ X-ray analysis has been focused on small-angle X-ray scattering (SAXS) Many of these devices have been proof-of-concept and limited in the observable residence time, i.e. the device window allows only a few seconds of the flow to be analysed and/or the short channel length limits onchip reaction times. The results presented here demonstrate that foreign material which promotes heterogeneous nucleation in solution can reduce scale build-up on reactor surfaces
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