Translational tracer diffusion of spherical macromolecules in crowded suspensions of rodlike colloids is investigated. Experiments are done using several kinds of spherical tracers in fd-virus suspensions. A wide range of size ratios L/2a of the length L of the rods and the diameter 2a of the tracer sphere is covered by combining several experimental methods: fluorescence correlation spectroscopy for small tracer spheres, dynamic light scattering for intermediate sized spheres, and video microscopy for large spheres. Fluorescence correlation spectroscopy is shown to measure long-time diffusion only for relatively small tracer spheres. Scaling of diffusion coefficients with a/ , predicted for static networks, is not found for our dynamical network of rods ͑with the mesh size of the network͒. Self-diffusion of tracer spheres in the dynamical network of freely suspended rods is thus fundamentally different as compared to cross-linked networks. A theory is developed for the rod-concentration dependence of the translational diffusion coefficient at low rod concentrations for freely suspended rods. The proposed theory is based on a variational solution of the appropriate Smoluchowski equation without hydrodynamic interactions. The theory can, in principle, be further developed to describe diffusion through dynamical networks at higher rod concentrations with the inclusion of hydrodynamic interactions. Quantitative agreement with the experiments is found for large tracer spheres, and qualitative agreement for smaller spheres. This is probably due to the increasing importance of hydrodynamic interactions as compared to direct interactions as the size of the tracer sphere decreases.

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We studied the thermal diffusion behavior of octadecyl coated silica particles (R h = 27 nm) in toluene between 15.0 • C and 50.0 • C in a volume fraction range of 1% to 30% by means of thermal diffusion forced Rayleigh scattering. The colloidal particles behave like hard spheres at high temperatures and as sticky spheres at low temperatures. With increasing temperature, the obtained Soret coefficient S T of the silica particles changed sign from negative to positive, which implies that the colloidal particles move to the warm side at low temperatures, whereas they move to the cold side at high temperatures. Additionally, we observed also a sign change of the Soret coefficient from positive to negative with increasing volume fraction. This is the first colloidal system for which a sign change with temperature and volume fraction has been observed. The concentration dependence of the thermal diffusion coefficient of the colloidal spheres is related to the colloid-colloid interactions, and will be compared with an existing theoretical description for interacting spherical particles. To characterize the particle-particle interaction parameters, we performed static and dynamic light scattering experiments. The temperature dependence of the thermal diffusion coefficient is predominantly determined by single colloidal particle properties, which are related to colloid-solvent molecule interactions.

We address the fundamental question: how are pair correlations and structure factors of hard-sphere fluids affected by confinement between hard planar walls at close distance? For this purpose, we combine x-ray scattering from colloid-filled nanofluidic channel arrays and first-principles inhomogeneous liquidstate theory within the anisotropic Percus-Yevick approximation. The experimental and theoretical data are in remarkable agreement at the pair-correlation level, providing the first quantitative experimental verification of the theoretically predicted confinement-induced anisotropy of the pair-correlation functions for the fluid. The description of confined fluids at this level provides, in the general case, important insights into the mechanisms of particle-particle interactions in dense fluids under confinement.

Block copolymer micelles in the size range between 10 and 100 nm are investigated as model systems for soft spheres. The zero shear viscosity 0 and complex modulus G* of micellar solutions are studied via dynamic mechanical spectroscopy and shear viscosity measurements over a wide range of concentrations. Depending on their structure, block copolymer micelles exhibit the characteristic rheological behavior of hard spheres, soft spheres, or polymers. With increasing concentration, hard-sphere and most soft-sphere samples exhibit a sharp liquid-solid transition as apparent by a divergence of zero shear viscosity 0 and the development of a frequency-independent elastic modulus. The transition occurs at a certain volume fraction which can be related to the softness of the particles. In the solid regime the elastic modulus GЈ exhibits a characteristic concentration dependence which is related to the spatial variation of the soft sphere repulsive potential. We observe a GЈϰZ 1.48 r Ϫ2.46 relation between modulus, aggregation number Z and intermicellar distance r which is close to the theoretical prediction GЈϳZ 3/2 r Ϫ2 of Witten and Pincus derived for polymerically stabilized colloidal particles. © 1997 American Institute of Physics. ͓S0021-9606͑97͒50825-7͔

We investigate the phase behavior of surface-functionalized silica colloids at both the molecular and macroscopic levels. This investigation allows us to relate collective properties such as aggregation, gelation, and aging directly to molecular interfacial behavior. By using surface-specific vibrational spectroscopy, we reveal dramatic changes in the conformation of alkyl chains terminating submicrometer silica particles. In fluid suspension at high temperatures, the interfacial molecules are in a liquid-like state of conformational disorder. As the temperature is lowered, the onset of gelation is identified by macroscopic phenomena, including changes in turbidity, heat release, and diverging viscosity. At the molecular level, the onset of this transition coincides with straightening of the carbon-carbon backbones of the interfacial molecules. In later stages, their intermolecular crystalline packing improves. It is the increased density of this ordered boundary layer that increases the van der Waals attraction between particles, causing the colloidal gas to aggregate. The approach presented here can provide insights into phase transitions that occur through surface modifications in a variety of colloidal systems.sum frequency generation ͉ surface spectroscopy ͉ transition ͉ nonlinear optical scattering ͉ calorimetry C olloidal dispersions are stable because intimate contact between the dispersed particles is physically barred. Given that the particles remain independent, they can exist in distinct states of aggregation analogous to the phases of molecular matter as follows: isolated as gas particles, condensed as an amorphous liquid, or ordered as a crystal. The macroscopic phase of a colloidal dispersion expresses the balance between interparticle attraction at relatively large separations, interparticle repulsion on close contact, and the energy available as thermal fluctuations. Because of their composite nature, such mixtures offer a unique opportunity to manipulate the interparticle potential energy: Attractive forces can be screened more or less by the dielectric properties of the solvent, and repulsive forces depend on the chosen chemical modification of the particle surface. In addition to their interest as models of phase behavior, surface-functionalized colloids are increasingly used to probe biomolecular interactions (1-3), specifically at (model) membranes (4). In such cases, the extreme sensitivity of a colloidal phase to surface modification can be exploited as a detection method, with dramatic changes in the collective properties (such as phase separation and gelation) indicating, for example, molecular adsorption at the interface.Robust dispersions can be created by attaching a bulky molecular layer to the outer surface of the dispersed particles. Such coatings resist the interparticle van der Waals attraction at close range and prohibit or delay irreversible coalescence (5). Particles that have been sterically stabilized in this way show a rich variety of phase behavior in response to external...

We present an experimental study of short-time diffusion properties in fluidlike suspensions of monodisperse charge-stabilized silica spheres suspended in dimethylformamide. The static structure factor S͑q͒, the short-time diffusion function D͑q͒, and the hydrodynamic function H͑q͒ have been probed by combining x-ray photon correlation spectroscopy experiments with static small-angle x-ray scattering. Our experiments cover the full liquid-state part of the phase diagram, including de-ionized systems right at the liquid-solid phase boundary. We show that the dynamic data can be consistently described by the renormalized density fluctuation expansion theory of Beenakker and Mazur over a wide range of concentrations and ionic strengths. In accordance with this theory and Stokesian dynamics computer simulations, the measured short-time properties cross over monotonically, with increasing salt content, from the bounding values of salt-free suspensions to those of neutral hard spheres. Moreover, we discuss an upper bound for the hydrodynamic function peak height of fluid systems based on the Hansen-Verlet freezing criterion.

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