Rotational and translational diffusion near the colloidal glass transition

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In this award, funded by the Experimental Physical Chemistry Program of the Chemistry Division, Professor Weeks and his students from Emory University will study diffusion near the colloidal glass transition. In regular liquids, particles and molecules undergo Brownian motion, causing them to both translate and rotate. For a given particle (or molecule) size and shape, the rates of these motions have a constant ratio. For example, doubling the viscosity of the liquid will slow both types of motion by the same factor. However, in supercooled molecular liquids close to the glass transition, this is not the case. Indirect measurements suggest that molecules translate faster than they rotate in these samples. This then suggests that supercooled liquids are not merely simple liquids with large viscosities, but that diffusion takes place by fundamentally changed mechanisms.Professor Weeks will use a laser-scanning confocal microscope to simultaneously and directly observe the rotational and translational dynamics of rod-like particles in a colloidal "supercooled fluid". Colloidal suspensions are composed of micron-sized solid particles in a liquid, and manifest a glass transition as the particle concentration is increased beyond a critical value. Because they can be viewed directly, these are ideal systems for studying the mechanisms of this transition. Confocal microscopy will be used to observe the motion of both the rod-like particles and the surrounding colloidal particles. Thus, both translational and rotational motion can be measured simultaneously and directly, leading to an understanding of how they are related.The scientific results may help in the design new types of glassy materials, which could impact technologies such as fiber optics. Professor Weeks will conduct at least one field trip each year to his laboratory for groups of primary school students. These students will investigate the properties of "squishy" materials such as the colloidal pastes used in this project.