# Transport properties

## Static and Dynamic Scattering from Charge-stabilized Colloidal Suspensions

Using dynamic light scattering (DLS) and x-ray photon correlation spectroscopy (XPCS) techniques for suspensions of charge-stabilized colloidal particles in polar solvents, we have explored the concentration and ionic strength dependence of

static and dynamic properties including the static structure factor S(q) and hydrodynamic function H(q), and the wave- number-dependent short-time and long-time collective diffusion functions DS(q) and DL(q). We have shown that all experimental data can be quantitatively explained and described by our many-body theory methods and accelerated Stokesian dyna- mics simulations results. We have shown, in particular, the absence of hydrodynamic screening in fluid-structured suspensions, and the approximate validity of a time-wavenumber factorization scaling relation relating short-time to long-time diffusion properties (see figure).

(G. Nägele and P. Holmqvist)

## Dynamics of permeable particles systems

A variety of particles dispersions of practical interest are solvent permeable to some extent. Examples are microgel and hydrogel particles suspensions, core-shell composite particles consisting of a dry core and a solvent-permeable polymer brush, and certain protein solutions.

In concentrated particle systems, the solvent permeability has a pronounced effect on diffusion and viscoelastic properties, most notably through the changed hydrodynamic interactions. Using a powerful hydrodynamic force-multipoles simulation method encoded in the HYDROMULTIPOLE program in conjunction with analytic calculations, we made a comprehensive study of short-time dynamic properties of suspensions of solvent-permeable spheres of uniform permeability, and of an internal core-shell structure.

(G. Nägele )

## Self-diffusion of rods at high concentrations

Dispersions of colloidal viruses, in our case fd virus, show transitions from isotropic (I) to the nematic (N) and to the smectic (S) with increasing concentration. In order to study the self-diffusion of rods in the different phases we use fluorescence video microscopy, monitoring the position and orientation of single tracer viruses labelled with a fluorescent dye, dispersed in a background of unlabelled rods. The diffusion constant can be extracted from the trajectories. This research showed a direct proof of the increasing of positional entropy after the I-N phase transition. It was also shown that diffusion in the smectic phase can be effectively described as a 1 D diffusion in a periodic potential, where the rod jumps from layer to layer.

( P. Lettinga )

## Diffusion of spherical colloids through isotropic- and nematic-rod networks: Screened hydrodynamics

Tracer diffusion of spherical colloids of various sizes through rod-works (of fd-virus particles) is studied by means of Fluorescence Correlation Spectroscopy, Dynamic Light Scattering and video microscopy. For lager tracer spheres, diffusion is mainly affected by direct interactions of the sphere and rods, where the network must be distorted during displacement of the sphere. For a small sphere, the rod-network is not distorted, and diffusion is mainly hindered by hydrodynamic interactions with the network. For crowded networks, where the rods are entangled, hydrodynamic interactions are screened. We developed a theory that connects the diffusion coefficient to the "hydrodynamic screening length". This allows to determine the hydrodynamic screening length of rod networks through the measurement of diffusion coefficients for spheres that are much smaller than the mesh size of the network.

## Thermal diffusion in colloidal and polymer dispersions

A temperature gradient applied to a liquid mixture not only causes a heat flux but also gives rise to a diffusion current of the constituent components. The resulting separation of the components causes a concentration gradient parallel or antiparallel with respect to the temperature gradient. This cross-effect between temperature and concentration is known as thermal diffusion or Ludwig-Soret-effect. Since its discovery by Ludwig (1856) and the first systematic investigations in liquid mixtures by Soret (1879), the effect has been subject to experimental and theoretical studies. Furthermore, the effect is relevant in technical applications as polymer characterization, the analysis of petrol reservoir characterization and for accumulation of uranium.

( S. Wiegand )

## Transverse waves in complex systems

In glass forming liquids formed of anisotropic molecules or polymers , which possess segments with anisotropic units, the striking feature is the observation of propagating transverse waves in the fluid phase provided the frequency of the probe is high enough, usually some GHz. We have investigated the continuous transformation of these transverse modes from a diffusive to a propagative behaviour upon cooling, or increasing pressure, using depolarised Rayleigh-Brilloin spectroscopy.

J. Chem. Phys. 128, 014507 (2008)

## Interdiffusion in Colloidal Mixtures

In colloidal mixtures, additional diffusion mechanisms are operative that do not occur in monodisperse suspensions. A prominent example is interdiffusion, i.e. the relaxation of thermal fluctuations in the composition of two components. We have analyzed the interdiffusion process in mixtures of spherically shaped particles. Exact analytic expressions have been derived for the interdiffusion coefficient of a semi-dilute mixture of colloidal hard spheres, with and without hydrodynamic interactions. These expressions are useful for analyzing dynamic scattering data from mixtures. The near-field part of the HIs is shown to be strongly influential on interdiffusion. Furthermore, we have analyzed whether the interdiffusion coefficient is expressible in terms of the self-diffusion coefficients of the individual components.

( G. Nägele )

## Electrohydrodynamic Effects in Charged Colloids and Biomolecules

Charge-stabilized suspensions consist of strongly charged colloidal particles or biomolecules dispersed in a solution of small electrolyte ions. The dynamics in these systems is subtly influenced by electrostatic, excluded-volume, and hydrodynamic interactions (HIs). Particularly intriguing are electrokinetic effects on the colloidal long-time dynamics, arising from the non-instantaneous relaxation of the electrolyte ion cloud formed around each charged colloid.

We have derived a semi-analytical mode-coupling theory (MCT) scheme in which colloids and electrolyte ions are treated equally as dynamic entities and, most importantly, the HIs between all ionic species are included. Using this scheme, several electro-hydrodynamic effects have been discussed and explained such as electrolyte friction, long-time self-diffusion in non-dilute suspensions, and the non-monotonic concentration dependence of the self-diffusion coefficient of DNA fragment solutions.

In addition, in a joint theoretical-experimental study, crowding and salt effects on the viscosity and diffusion in bovine serum albumin (BSA) protein solutions have been explored.

(G. Nägele )

## Diffusion of spherical colloids in suspensions of rods.

There are two different types of diffusion; self-diffusion that is related to the dynamics of a single Brownian particle in a homogeneous system and collective diffusion that is related to relaxation of inhomogeneities in concentration. We are interested in long-time self-diffusion of spherical "tracer" particles embedded in concentrated suspensions of rod-like colloids. Besides the concentration of rods, an important variable is the ratio of the size of the tracer sphere and the length of the rods.

( J. Dhont )