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Computational study of assisted assembly of colloids

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Colloidal crystal assembly with a twist

Nanoparticles are synthesised in a wide variety of sizes and shapes with different properties. Turning them into colloidal crystals or photonic glasses requires packing them into ordered assemblies to manifest specific electronic and optical properties.

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The EU-funded COSAAC (Computational study of assisted assembly of colloids) project was initiated to examine the possibility to aid the formation of such ordered structure. Scientists experimented with the use of external electric fields and changing the thermodynamic conditions under which nanoparticles' are assembled. Among the possibilities explored was the assembly of colloidal crystals with the use of seeds of the target structure. By assisting the formation of ice with seeds, scientists could measure the ice nucleation rate at temperatures ranging between -35 and -40 o Celsius. The COSAAC team combined experimental data with simulation results to predict the ice nucleation rate within a wider range of temperatures. Their predictions provide valuable input to climate change models that reproduce ice formation in tropospheric clouds. Scientists also explored the use of optical tweezers to confine particles and assist the formation of crystalline regions. In computer simulations of the crystal nucleation growth, they were able to estimate the crystal-fluid interfacial energy both accurately and efficiently. By assisting the formation of crystals with these techniques, COSAAC unveiled the physics behind the decelerating effect of pressure on ice nucleation. This effect allows the cryopreservation of biological samples that can be enhanced with the application of pressure and electric fields. Lastly, the assembly of disordered structures by changing the thermodynamic conditions enabling their formation was investigated. Experiments centred around colloidal glasses composed of sphere-shaped particles. Just a rapid change of pressure proved sufficient to modify the resistance of glass to crystallisation. Crystallisation is widely known as devitrification and needs to be avoided to ensure the desired mechanical properties for glasses. To minimise their crystal content, scientists defined specific compression protocols for pressure-assisted formation. Understanding devitrification was the first step towards avoiding this. Unlike self-assembly of crystalline structures, the methodology developed for assisted assembly holds potential for bottom-up fabrication of functional materials with a broad range of applications. The many, important results of COSAAC have been described in 15 papers published in peer-reviewed international journals.


Colloidal crystal, photonic glass, COSAAC, self-assembly, assisted assembly

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