Structure and dynamics of two-dimensional binary colloidal hard spheres

Lead Research Organisation: University of Oxford
Department Name: Oxford Chemistry

Abstract

Glasses and metals have near universal use throughout technology and construction. A greater understanding of their structure and dynamics on an atomic scale can lead to the development of superior materials that are tailored to their application from the microscopic level. Colloidal systems are a valuable model for the microscopic structure of materials due to the versatility of their interactions via tuning of chemical properties, and similar phase behaviour to atoms as a result of Brownian motion. Their use as a model is important as the atomic scale is too small with fast timescales to accurately examine in as great detail, and computational simulations cannot equal the complexity and scale of colloidal systems. In this project we intend to use a versatile two-dimensional hard sphere model system to investigate the atomic structure and dynamics of these materials. This will be achieved through the use of spherical colloidal particles on the scale of micrometres, with interactions finely tuned such that they behave as hard spheres. These particles will be confined by gravity to a two-dimensional plane, and then examined using optical microscopy. By introducing two different particle sizes and varying the relative concentrations of the colloids, systems that model doped metals/alloys and glasses can be developed. Specifically, a small amount of larger particles doping a colloidal crystal made of smaller particles is a direct model of alloys. Of particular interest in these systems is the impact of doping atoms on the formation of grains within the crystal. The mechanical properties of polycrystalline metals are dependent upon the average grain size. The growth of grains is a type of coarsening, where multiple grains join together. We intend to examine the dynamics of grain growth in relation to the size of and number of doped impurities. By using this, the time scale upon which this occurs, and nature of the grain boundaries can be tuned. Therefore using this knowledge, the mechanical properties of the alloy could be designed. This investigation could be furthered by the use of magnetic active particles to investigate microrheology at grain boundaries of these crystals. With closer to equal concentrations of the two particles, an amorphous structure can form; similar to the structure of glass. The mechanism behind the fluid-solid glass transition is currently unknown. We intend to experimentally investigate the possibility of a thermodynamic mechanism via the use of a custom built microscope with an angle adjustable base. This will allow for samples to be tilted, introducing a gravitational field, resulting in sedimentation-diffusion equilibrium and a concentration gradient. By matching the solvent relative masses of the two colloids, their particle density distribution will be identical leading to mixing throughout the sample, preventing phase separation. Since particle density is an inverse analogue of temperature, the system will have effectively a temperature gradient. This can be probed to find a height dependent equation of state, and height-resolved structural and dynamic properties, which would indicate the presence of a thermodynamic or kinetic mechanism for vitrification. This could then be furthered through the use of optical tweezing to manipulate the system and particle to study additional local phenomena sensitive to glass formation such as elastic properties. This project falls within the EPSRC Physical Sciences research area, specifically within Biophysics and Soft Matter Physics.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/R512333/1 01/10/2017 30/09/2021
1949698 Studentship EP/R512333/1 01/10/2017 30/09/2021 Joseph David Hutchinson
 
Description We have investigated the mechanisms by which solids containing two atoms form, from a liquid. This work was done as a means to better understand how materials such as alloys and metallic glasses can their atomic structures controlled, for tailoring their macroscopic properties for specific uses.

We have examined and modelled how the structure of alloys develops after recrystallisation, with regards to the concentration of dopant atoms.
We also compared at the two mechanisms with which grains (regions of similarly aligned atoms) evolve after recrystallisation: expansion/shrinkage and rotation.
We measured the forces with which the boundaries of grains are pinned in place by larger dopant atoms through microrheological measurements.
We also examined how glass forming systems are affected when in equilibrium under a gradient of density.
Exploitation Route Our insights into the mechanisms by which alloy grain structure forms, and how dopant atoms affect crystal formation could be used to design new alloys for specific uses.
Sectors Aerospace, Defence and Marine,Construction