Investigation of the jamming transition using confocal microscopy of dense colloidal suspensions and complex biomaterials

Jamming is the process by which a liquid stops flowing and assumes some properties commonly associated with solids. Everyday examples include the setting of a bowl of Jell-O(TM) or the vitrification of a molten mixture of silica, lime and soda at the end of a glassblower's blowpipe. In these examples, the system resists flow and can even support some pressure just like a crystalline solid. At the same time its structure is not crystalline and is in fact hardly distinguishable from that of a liquid. Here we focus our attention on two separate but related questions presented by the jamming (or glass) transition.;The dynamical properties of a glass (e.g. viscosity) depend on the time elapsed since vitrification. This slow evolution is called aging and is currently poorly understood. For example a static, or structural, measure of the age of a glass has not yet been identified and aging can only be detected by following the system's dynamics. We consider the aging of the microscopic structure in out-of-equilibrium glasses by measuring local tetrahedral geometry in two systems: a monodisperse colloidal suspension observed by confocal microscopy and a binary Lennard-Jones glass simulated by molecular dynamics. While these models differ in some details they are both widely used in the study of glasses. Tetrahedral structure is found to be a poor indicator of age in the experiments though it weakly correlates with the slowing dynamics. In contrast, the structure of the simulated glass shows clear signs of aging. In both cases, tetrahedral structure samples geometry in a non-trivial way.;Similarly, a static measure of a liquid's proximity to the glass transition has remained elusive. We present preliminary results from a magnetic tweezers experiment aimed at dynamically exciting a dormant length scale in a binary "supercooled" colloidal liquid. We observe the sample's local response to a point disturbance from a magnetic probe. We find that the disturbance decays exponentially from the probe's position with a characteristic length scale of 2-3 particle diameters. We do not observe appreciable variation with packing fraction or applied force.;Finally we present some microrheological data from a genetically designed Ca2+ sensitive biomaterial. We find that upon addition of Ca 2+ the viscosity of the protein solution can be varied by three orders of magnitude. We found this increase to be highly specific to the nature of the added divalent cation and fully reversible upon addition of a calcium chelator. This work lays the foundation for developing a material which might exhibit elasticity allowing us to test the universality of the jamming transition.