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Nano-Particle-Resolved Studies

Final Report Summary - NANOPRS (Nano-Particle-Resolved Studies)

The ERC Consolidator grant NANOPRS was concieved to extend real space analysis with optical microscopy of Soft Materials to lengthscales below the diffraction “limit”. This was made possible by the development of “super-resolution” imaging technqiues. STimulated Emission via Depletion (STED) nanoscopy was chosen, as it has a much higher frame rate than other methods. Because colloidal systems follow the same laws of statistical mechanics as do atoms and molecules, they can be regarded as representative of simple materials in general.

In particle-resolved studies of colloidal systems, the coordinates of each particle are found in three dimensions. This is ideally suited to study phenomena where local heterogeneities are important, such as the formation of a crystal nucleus, or the dynamic heterogeneities of supercooled colloidal liquids, such behaviour can be hard to access with techniques which take an average measurement over a larger volume, such as scattering.

The ultra-high resolution of the STED nanoscope makes it possible to track the coordinates of much smaller colloids than had been studied previously, indeed one can work with nanoparticles in the 100 nm size rage, rather than colloids whose diameter is typically 3 microns. Because the timescale of colloidal systems scales with the cube of the particle diameter, by reducing the diameter by a factor of ten, the passage of time is effectively accelerated by a factor of 1000. Thus for a given measurement time, one can access very much longer timescales — in other words, time is “rescaled” for the system. This is important in the slow dynamics of the glass transition and rare events like crystal nucleation. The results of both are summarised below.

Two key longstanding challenges in matter physics were chosen to demonstrate the new avenues of discovery made possible by using STED to image soft materials. Crucially, both benefitted from the increased dynamic range of the nano-particle resolved studies of colloidal particles, due to the “rescaling of time” in the system. The two lines of research investigated were the glass transition and crystal nucleation of colloidal systems. The glass transition is a longstanding scientific challenge, where the basic mechanism of dynamic arrest is not understood, with competing theories taking dynamic — or thermodynamic — standpoints which are thought to be incompatible with one another. Crystal nucleation in colloids has been described as the “second-biggest discrepancy in physics”, with experimental measures of nucleation rates which are 10 orders of magnitude higher than the predictions from computer simulation.

Glass transition. Using the nano-particle resolved studies method, we performed an experimental study of a model colloidal system over a dynamic window a thousand times larger than previous measurements, revealing structural ordering more strongly linked to dynamics than previously found. Furthermore we found that immobile regions and domains of local structure grow concurrently with volume fraction (approaching the glass transition), and that these regions have low configurational entropy. We thus showed that local structure plays an important role at deep supercooling, consistent with a thermodynamic interpretation of the glass transition rather than a principally dynamic description.

Crystal Nucleation in Colloids. We developed a novel method for obtaining and analysing super-resolution data of crystallising systems. Our real space-reciprocal space technique allows for far larger systems to be analysed than previously, and therefore much rarer events can be studied than was previously possible. We also adapted the earlier methods for establishing precise volume fractions used this technique to measure volume fractions within the coexistence region with an error of around 0.25%. This is much more precise than was previously believed possible. These techniques have been applied to studying the nucleation barriers of a colloidal system at a variety of supercooled volume fractions. We find similar results to previous scattering and real-space measurements, with the height of the nucleation barrier depending far more weakly on the supersaturation than is measured in simulations. In other words, the discrepancy between experiment and simulation predictions persists.