Wed, 09 Jul 2008 13:31:28 BST CiteULike: dchens Meyer http://www.citeulike.org/user/dchen/author/Meyer CiteULike.org en-gb Copyright © 2004-2008 citeulike.org http://www.citeulike.org/user/dchen/article/2730851 <i>Journal of Rheology, Vol. 50, No. 1. (2006), pp. 77-92.</i><br /><br />The microrheology of a colloidal suspension is measured using laser tweezers. Suspensions of refractive index-matched fluorinated ethylene propylene (FEP) particles are seeded with index-mismatched polystyrene or silica probe particles. Laser trapped probes are then subjected to steady uniform flows, enabling measurements of the suspension microviscosity as a function of FEP volume fraction and flow velocity. The microrheology results agree with bulk rheology, and both exhibit the same volume fraction dependence of the Krieger-Dougherty relationship for hard spheres. As volume fraction increases, the microrheology more closely agrees with the infinite shear bulk viscosity. In this regime, measurements using small probes exhibit additional shear thinning. Using confocal microscopy and fluorescent poly(methylmethacrylate) dispersions, we demonstrate that the nonlinear microrheology is consistent with the development of an anisotropic nonequilibrium pair distribution function between the probe and bath particles, with a denser region at the leading surface of the probe and a wake trailing it. The nonlinear response and underlying microstructure are in qualitative agreement with recent theory [T. M. Squires and J. F. Brady, Phys. Fluids 17, 073101 (2005)]. ©2006 The Society of Rheology Laser tweezer microrheology of a colloidal suspension Alexander Meyer Andrew Marshall Brian Bush Eric Furst doi:10.1122/1.2139098 Journal of Rheology, Vol. 50, No. 1. (2006), pp. 77-92. 2008-04-28T19:22:56-00:00 2006 Journal of Rheology 50 1 77 92 SOR colloids microrheology opticaltweezer technique http://www.citeulike.org/user/dchen/article/797340 <i></i><br /><br />The structure of, and transitions between, liquids, crystals and glasses have commonly been studied with the hard-sphere model(1-5), in which the atoms are modelled as spheres that interact only through an infinite repulsion on contact. Suspensions of uniform colloidal polymer particles are good approximations to hard spheres(6-11), and so provide an experimental model system for investigating hard-sphere phases. They display a crystallization transition driven by entropy alone. Because the particles are much larger than atoms, and the crystals are weakly bound, gravity plays a significant role in the formation and structure of these colloidal crystals. Here we report the results of microgravity experiments performed on the Space Shuttle Columbia to elucidate the effects of gravity on colloidal crystallization. Whereas in normal gravity colloidal crystals grown just above the volume fraction at melting show a mixture of random stacking of hexagonally close-packed planes (r.h.c.p.) and face-centred cubic (f.c.c.) packing if allowed time to settle(7,8), those in microgravity exhibit the r.h.c.p. structure alone, suggesting that the f.c.c. component may be induced by gravity-induced stresses. We also see dendritic growth instabilities that are not evident in normal gravity, presumably because they are disrupted by shear-induced stresses as the crystals settle under gravity. Finally, glassy samples at high volume fraction which fail to crystallize after more than a year on Earth crystallize fully in less than two weeks in microgravity. Clearly gravity masks or alters some of the intrinsic aspects of colloidal crystallization. Crystallization of hard-sphere colloids in microgravity Zhu Jx M Li R Rogers W Meyer Ottewill Rh Russell Wb Chaikin Pm 2006-08-11T17:46:58-00:00 glass gravity