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Near wall dynamics of colloidal suspensions

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If colloidal particles suspended in a quiescent solvent approach a flat solid interface, the so-called "viscous drag effect" causes the particles' mobility to decrease and to become ansiotropic. Analysing the time-dependence of the intensity scattered from an evanescent wave by an ensemble of particles, it is possible to measure their near wall diffusivity. At ICS-3 it is possible with a unique instrumental set up for evanescent wave dynamic light scattering (EWDLS) to distinguish between the particle mobility parallel and normal to the interface.

Although, theoretical predictions of the wall drag effect have been available for a long time, ICS-3 researchers were the first to prove experimentally that these models describe the translational and rotational Brownian motion of colloidal spheres correctly.

Using a flow-through cell or a shear cell the EWDLS setup can be used to measure velocity profiles and slip lengths with unsurpassed resolution. At present, the effect of particle volume fraction, long range interactions and shear fields on near wall colloidal dynamics is being investigated.

( P. Lang )

Interaction of colloidal particles with a wall

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To understand the structure formation and phase behaviour of colloidal systems close to solid interfaces at a microscopic level, it is indispensable to know the details of the particles’ interaction with the wall. By applying total internal reflection microscopy (TIRM), depletion interaction between colloidal spheres mediated by rod shaped and disc shaped depleting agents was investigated. It was further shown that the strength of depletion interactions caused by discs is weakened and eventually vanishes under shear.

Current research is focusing on non-specific protein interactions, to obtain a better understanding of the stability of protein solutions.

( P. Lang )

Filtration of Soft Particles: From Microgels to Proteins

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We model theoretically sustainable membrane filtration processes for the concen tration, purification, and separation of microgels. A suspension of microgel particles is pumped through a cylindrical fiber membrane, and the permeate flux and the formation of a concen- tration polarization layer and possibly of an irreversible cake layer at the membrane surface are analyzed.

This work is part of the B6 project of the SFB 985 on functional microgels and microgel systems.

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(G. Nägele and R. Roa )

Hydrodynamics of Particles at a Liquid-Gas Interface

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In quasi-two-dimensional (Q2D) dispersions, where the colloidal particles are confined to a planar or curved monolayer, the particle hydrodynamics is more complicated than in the bulk case since additional interface boundary conditions need to be satisfied. The resulting solvent-mediated hydrodynamic interactions (HIs) are thus distinctly different from the bulk ones.

We have analyzed the HIs between spherical particles in contact with a planar fluid-gas interface. The one-sphere resistance operator was calculated numerically, and a spherical multipole ex­pansion with symmetry-adjusted basis functions was used to obtain explicit results for the long-dis­tance terms of the two-sphere mobility. The accuracy of this far-field approxi­mation was assessed by comparing it with precise many-sphere calculations. Our Q2D mobil­ity tensors are an essential ingredient in any theory and computer simulation of the lateral particle dynamics.

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(G. Nägele)


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