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Dynamic Control of Mineralisation

Periodic Reporting for period 2 - DYNAMIN (Dynamic Control of Mineralisation)

Reporting period: 2020-03-01 to 2021-08-31

Crystallisation underpins a vast range of processes as diverse as the production of nanomaterials, ceramics, and pharmaceuticals, the generation of bones, teeth and seashells, weathering and frost-heave, ice and rock formation in our environment and the generation of scale of kettles and oil-wells. Indeed, it is hard to think of an area of science in which crystallisation is not important. Understanding the fundamental mechanisms which govern crystallisation therefore promises the ability to inhibit or promote crystallisation as desired, and to tailor the properties of crystalline materials towards a huge range of applications.

DYNAMIN takes inspiration from biomineralisation processes - where nature achieves remarkable control over the formation of structures such as bones, teeth and seashells - to achieve exceptional control over crystallisation processes. In biomineralization organisms employ amorphous precursors and control their transformation to single crystals with complex morphologies. They select crystal polymorphs with perfect reproducibility and can switch from one polymorph to another. They can separately control nucleation and growth to create hierarchical arrays of crystals. All of these feats are achieved because biominerals form within confined volumes, where the organism interacts with the nascent mineral in order to direct crystallisation. Thanks to recent advances in microfabrication and analytical techniques, we are now able to mimic this control in the laboratory.

In our project we are using microfluidic and small volume systems – coupled to advanced analytical methods – to study and interact with crystallisation processes, and thus achieve control over crystal nucleation and growth by design. DYNAMIN will therefore provide a new framework for observing and directing crystallisation processes, which will have applications in a range of sectors, including the environment, chemical industries, development of advanced materials and healthcare.
Controlling the Early Stages of Crystallisation in Droplets
Many of the most important features of a crystal – such as its structure or orientation on a substrate – are defined at nucleation. Learning how to direct nucleation processes, such that we can select one polymorph over another, or induce nucleation when and where we want, is therefore one of the ultimate challenges in crystallisation. We are exploiting the unique capabilities of droplet systems to achieve this goal. Crystallisation is being characterised in droplets in our lab and at synchrotron beam lines. Droplet Microfluidics-Coupled X-ray Diffraction (DMC-XRD) has been established to enable the collection of time-resolved, 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 (Figure 1). The value of segmented flow devices in reproducibly studying crystallization processes was also demonstrated by comparing continuous and segmented flow configurations. Under continuous flow, scale formation on the reactor walls begins almost immediately on mixing of the crystallizing species, which over time results in blocking of the channel (Figure 2).

Crystallisation within Defined Micro-environments: The Crystal Hotel
Biomineralisation invariably occurs in privileged environments within an organism. This affords an organism exceptional control over mineralisation, dictating what polymorph nucleates, where it forms, and how it grows to its target size and morphology. We are constructing microfluidic devices that allow us to mimic such biogenic control and visualize the growth of crystals in situ within the microfluidic. We call these "Crystal Hotel" devices, where they comprise a series of inter-connected rooms into which we can flow fresh reagents (Figure 3). We can functionalise the rooms to define the orientation of the product crystals and introduce soluble additives to control their polymorph.

Material Synthesis Triggered by the Transformation of Amorphous Precursors
Biomineralisation often occurs via the precipitation of amorphous or poorly ordered precursor phases which are deposited and moulded before the final mineral is formed. This allows the organism to control nucleation and growth processes with precision, to shape the mineral phase into a desired morphology and to control the crystallographic orientation. We have developed a two-step method for controlling the crystallization of amorphous calcium carbonate (ACC) thin films, where nucleation was first triggered at a single site using a heated probe and then growth was sustained by incubating the film at a lower temperature. By independently controlling nucleation and growth we can readily generate millimeter-scale calcite crystals when and where we want, create morphologies ranging from discs, to squares to serpentine strips, and create patterned arrays of crystals. By controlling the magnesium content of the ACC films we can also tune the structure of the annealed films from calcite, to pure aragonite, through low and high magnesium calcite, and ultimately dolomite by increasing the magnesium content of the ACC.
While there is enormous potential for using microfluidic devices for synchrotron-based studies, this field is still in its infancy. We have developed new devices and data analysis methods that can be employed in the field of crystallisation and beyond. We are also developing methods for introducing additives to droplets at different times. This will ultimately deliver a new understanding of the effects of additives on crystallisation, allowing us to select additives – and create tailor-made addition protocols – to give specific products.

At larger length-scales we have developed “Crystal Hotel” microfluidic devices that offer confined environments into which the reaction solution can be flowed. These provide a unique way of exploring biomineralisation strategies, giving access to true spatio-temporal control. We have also developed a strategy in which we use heat to trigger and control the transformation of amorphous phases.

Finally, based on the advances made in DYNAMIN, we have established a new UK facility (Flow-Xl) for the in situ analysis of crystallisation processes. Using X-ray diffraction and Raman spectrometry to study crystallisation in flowing plugs or continuous flow, Flow-Xl enables researchers to conduct in situ studies of crystallisation mechanisms and pathways.
Figure 3: Crystallisation in a Hotel device which is composed of circular rooms
Figure 2: Crystallization in segmented and continuous flow
Figure 1: Microfluidic device for studying crystallization in flowing droplets using XRD