The topic of calcite and aragonite polymorphism attracts enormous interest from fields including biomineralization and paleogeochemistry. While aragonite is only slightly less thermodynamically stable than calcite under ambient conditions, it typically only forms as a minor product in additive-free solutions at room temperature. However, aragonite is an abundant biomineral, and certain organisms can selectively generate calcite and aragonite. This fascinating behavior has been the focus of decades of research, where this has been driven by a search for specific organic macromolecules that can generate these polymorphs. However, despite these efforts, we still have a poor understanding of how organisms achieve such selectivity. In this work, we consider an alternative possibility and explore whether the confined volumes in which all biomineralization occurs could also influence polymorph. Calcium carbonate was precipitated within the cylindrical pores of track-etched membranes, where these enabled us to systematically investigate the relationship between the membrane pore diameter and polymorph formation. Aragonite was obtained in increasing quantities as the pore size was reduced, such that oriented single crystals of aragonite were the sole product from additive-free solutions in 25-nm pores and significant quantities of aragonite formed in pores as large as 200 nm in the presence of low concentrations of magnesium and sulfate ions. This effect can be attributed to the effect of the pore size on the ion distribution, which becomes of increasing importance in small pores. These intriguing results suggest that organisms may exploit confinement effects to gain control over crystal polymorph.

The ability to control crystallization reactions is required in a vast range of processes including the production of functional inorganic materials and pharmaceuticals and the prevention of scale. However, it is currently limited by a lack of understanding of the mechanisms underlying crystal nucleation and growth. To address this challenge, it is necessary to carry out crystallization reactions in well-defined environments, and ideally to perform in situ measurements. Here, a versatile microfluidic synchrotron-based technique is presented to meet these demands. Droplet microfluidic-coupled X-ray diffraction (DMC-XRD) enables the collection of time-resolved, serial diffraction patterns from a stream of flowing droplets containing growing crystals. The droplets offer reproducible reaction environments, and radiation damage is effectively eliminated by the short residence time of each droplet in the beam. DMC-XRD is then used to identify effective particulate nucleating agents for calcium carbonate and to study their influence on the crystallization pathway. Bioactive glasses and a model material for mineral dust are shown to significantly lower the induction time, highlighting the importance of both surface chemistry and topography on the nucleating efficiency of a surface. This technology is also extremely versatile, and could be used to study dynamic reactions with a wide range of synchrotron-based techniques.

As crystallization processes are often rapid, it can be difficult to monitor their growth mechanisms. In this study, we made use of the fact that crystallization proceeds more slowly in small volumes than in bulk solution to investigate the effects of the soluble additives Mg2+ and poly(styrene sulfonate) (PSS) on the early stages of growth of calcite crystals. Using a “Crystal Hotel” microfluidic device to provide well‐defined, nanoliter volumes, we observed that calcite crystals form via an amorphous precursor phase. Surprisingly, the first calcite crystals formed are perfect rhombohedra, and the soluble additives have no influence on the morphology until the crystals reach sizes of 0.1–0.5 μm for Mg2+ and 1–2 μm for PSS. The crystals then continue to grow to develop morphologies characteristic of these additives. These results can be rationalized by considering additive binding to kink sites, which is consistent with crystal growth by a classical mechanism.

Within the field of crystal growth it is recognized that secondary species can sometimes be occluded within a growing crystal according to the crystallization conditions and pairing of the additive and host crystal. This article takes inspiration from this phenomenon to create multifunctional inorganic nanocomposites with unique structures: inorganic single crystals containing embedded inorganic nanoparticles. Using magnetite (Fe 3 O 4 )/ZnO as a suitable test system, ZnO crystals are precipitated from aqueous solution at 90 °C and atmospheric pressure in the presence of Fe 3 O 4 nanoparticles functionalized with anionic diblock copolymers. Analysis of product nanocomposite crystals using atomic absorption spectroscopy shows that the Fe 3 O 4 nanoparticles are embedded within the ZnO single crystal hosts at levels of approximately 10 wt %, and TEM analysis shows that there is no apparent discontinuity between the nanoparticles and host crystal matrix. Importantly, we then demonstrate that this occlusion approach can also be employed under hydrothermal conditions at 160 °C, without a loss in incorporation efficiency. This offers an important advance on our previous occlusion studies, which were all conducted at room temperature, and vastly increases the range of target materials that can be generated using our synthesis approach. Finally, measurement of the magnetic properties of these nanocomposites shows that they retain the attractive features of the wide band gap semiconductor ZnO while benefiting from added magnetism.

This article presents a simple and highly reliable method for preparing PDMS microfluidic double emulsion devices that employs a single-step oxidative plasma treatment to both pattern the wettability of microchannels and to bond the chips. As a key component of our strategy we use epoxy glue to define the required hydrophobic zones and then remove this after plasma treatment, but prior to bonding. This novel approach achieves surface modification and device sealing in a single process, which reduces chip preparation times to minutes and eliminates the need for unreliable coating processes. The second key element of our procedure is the maintenance of the patterned surfaces, where we demonstrate that immediate filling of the prepared device with water or the use of solventextracted PDMS vastly extends the operational lifetimes of the chips. The reliability of this technique is confirmed by generating water-in-oil-in-water (W/O/W) double emulsion droplets with controlled core/shell structures and volumes, while its versatility is demonstrated by simply using a different placement of the epoxy glue on the same chip design to create oil-in-water-in-oil (O/W/O) double emulsion droplets. Both W/O/W and O/W/O double emulsion droplets can therefore be created from the same soft-lithography mould. This simple method overcomes one of the key problems limiting the wider use of double emulsions lack of reliability while its speed and simplicity will facilitate the high-throughput production of monodisperse double emulsions. Our method is demonstrated to produce double emulsion down to 55 µm in diameter and could be readily extended to produce microfluidic chips with more complex hydrophilic and hydrophobic patterns.