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Colloids, Polymers and Temperature: Melting by Cooling and Quenching by HeatingPerhaps surprisingly, at the dawn of the 21st century, we still do not fully understand many of the processes that surround us in everyday life. What exactly is it that distinguishes the glass in the window-pane from chemically identical crystalline quartz? How does ice melt? What both these examples have in common is the importance of local behaviour. Around the liquid-solid interface in melting ice, the molecules experience a change in environment. In glass, moreover, most constituent particles are almost fixed, but some exhibit correlated events of considerable motion. If we could see these constituent atoms and molecules, we might be able to move towards a better understanding of these phenomena. Beyond this fundamental interest, 'seeing' atoms and molecules is important to real-world problems, such as protein crystallisation. In order to resolve the structure of proteins, which is crucial for drug development, they must be crystallised. This has been possible with just a few proteins, and progress is only made through painful trail and error. Like melting and glasses, if we could directly see the individual particles, we should have a much better idea of what is going on.Resolving atomsIt is very hard to directly image atoms and molecules, they are simply too small. Instead, X-ray or neutron diffraction is used. Rather than direct imaging, this picks out periodic structure, such as crystal lattices and has surely contributed more than any other technique to our knowledge of the structure of matter. Although tremendously powerful in the case of regular structures, diffraction, or scattering, is less effective for amorphous materials. Enter ColloidsAlthough atoms and molecules are too small to visualise, the classroom experiment of Brownian motion in smoke particles shows that larger particles also exhibit the thermal motion which drives so much of atomic behaviour. Rather than smoke particles, we use micron-sized plastic spheres (colloids) dispersed in a solvent, which we can resolve with 3D optical microscopy. Colloids thus form a simple model of atoms and molecules, by adding an attraction between the colloids they condense to form liquids or solids. Just such an attraction results from adding polymers, the osmotic pressure of the polymers in solution 'pushes' the colloids together, to form colloidal gases, liquids and solids. Our aim is to produce a temperature-responsive colloid-polymer system, where we can study non-equilibrium behaviour, at the level of the constituent particles by changing the interactions between the particles. With this system, we shall be able to tackle a range of phenomena. In particular, we shall consider melting, self-assembly and vitrification.Industrial RelevanceThis project will have two main industrial applications, tuneable colloidal crystals and new ways to control the structure of colloidal gels.(1) 3D tuneable colloidal crystals. Colloidal crystals have interesting optical properties, due to their periodicity at optical and infra-red wavelengths, and the race is on to produce 3D tuneable colloidal crystals which will find a number of applications from tuneable filters for wavelength selection to data storage. Our temperature-responsive colloid-polymer system will deliver this 3D tuneability.(2) Controlling gelation. In many applications, colloidal and nanoparticle gels are central components of many high-value production activities, for example emerging technologies such as high-performance photovoltaic cells. The system presented here will provide a means to obtain the first direct imaging of the fundamental process of particle aggregation which leads to gelation. By controlling the quench rate, and leveraging our precise knowledge of the inter-particle forces, we shall develop a methodology by which application-specific gel structures may be optimised.
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