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 also demonstrated, 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 )
Dynamics of permeable particles systems
A variety of particles dispersions of practical interest are solvent permeable to some extent. Examples in case 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.
Thermophoresis in Soft Matter systems
Thermophoresis, also known as the Ludwig?Soret effect or thermodiffusion, is the migration of particles or molecules induced by a temperature gradient. In the past the effect has been used to separate isotopes or to characterize polymers and colloids in solution. In the recent years it has been utilized to monitor biochemical reactions and to determine equilibrium constants, which are essential for the development of new pharmaceuticals. Thermophoresis is also discussed in the context of molecular evolution and it was possible to duplicate DNA strands with a device combining thermophoresis and convection. The microscopic mechanism of this effect is still not understood and especially in the case of aqueous systems hydrogen bonds play an important role.
( S. Wiegand )
Transversale Phononen in komplexen Systemen
In glassbildenden Flüssigkeiten, die aus optisch anisotropen Molekülen bestehen oder in Polymeren, die Segmente mit anisotropen Gruppen beinhalten, findet man propagierende transversale Phononen, die mittels Fabry-Perot Interferometrie im Bereich einiger Ghz gemessen werden können. Wir haben den Übergang dieser transversalen Mode von einer diffusiven quasielastischen zu einer propagierend inelastischen Mode als Resultat von Temperaturerniedrigung bzw. Druckerhöhung mittels depolarisierter Rayleigh-Brillouin Spektroskopie gemessen.
( H. Kriegs )
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 sphärischer Kolloide in Suspensionen von Stäbchen
Es gibt zwei Formen der Diffusion: Selbstdiffusion, die mit der Dynamik eines einzelnen Brown'schen Teilchens in einem homogenen System zusammenhängt und kollektive Diffusion, die mit der Relaxation von Inhomogenitäten der Konzentration verknüpft ist. Wir sind an der Langzeit-Selbstdiffusion von kugelförmigen Tracer Teilchen interessiert, die in einer konzentrierten Suspension stäbchenförmiger Teilchen gelöst sind. Hierbei ist neben der Konzentration der Stäbchen eine weitere wichtige Variable auch das Verhältnis des Kugeldurchmessers zur Länge des Stäbchens.
( J. Dhont )