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Phasenverhalten und Mikrostruktur

The Isotropic-nematic phase transition of rod-like viruses


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We investigate the kinetics of phase separation for a mixture of rod-like viruses (fd) and polymer (dextran), which effectively constitutes a system of attractive rods. Quenches are performed in two ways: concentration quenches are performed using hydrostatic pressure; orientation quenches are performed from a flow-induced fully nematic state. We show experimental evidence that the kinetic pathway of the phase separation depends on the overall concentration. Depending on the initial phase and the depth of the quench the system is rendered meta-stable and or unstable after the quench and thus either nucleation-and-growth or spinodal decomposition is observed. Thus we are able to locate the isotropic-nematic and nematic-isotropic spinodal point.

( P. Lettinga )

Phase Behaviour of Proteins and Colloid-Polymer Mixtures


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The exploration of protein crystal structures by x-ray spectroscopy has been proven to be a notoriously difficult task, since crystallisation often competes with unfavourable protein aggregation. Our aim is to understand the protein phase behaviour by examining the dominating (effective) interaction forces. In this project, we have devised and analyzed a simple patchy colloid model describing the experimentally observed equilibrium phase diagram of lysozyme. As a novelty, our model incorporates both the anisotropic patchy attraction and the electrostatic repulsion.

In a related joint experimental-theoretical work, we study the effect of additives such as NaCl, glycerol and dimethyl sulfoxide (DMSO) on the phase diagram of aqueous lysozyme solutions.

In another study, we investigate experimentally and theoretically the effect of non-adsorbing polymers on the non-equilibrium phase behaviour of colloid-polymer mixtures, where gelation interferes with gas-liquid-like phase separation.

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(G. Nägele, C. Gögelein, J. Buitenhuis, J. Dhont )

Adhäsive kolloidale Dispersion unter hohem Druck


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Uns interessiert die Struktur und das Phasenverhalten von sterisch stabilisierten kolloidalen Systemen, die aus oberflächenmodifizierten Silika-Teilchen bestehen, dispergiert in marginalen Lösungsmitteln. Solche Systeme zeigen eine Phasenseparation und eine Perkolation in Abhängigkeit von Temperatur, Druck und Volumenfraktion. Experimentelle Methoden zur Bestimmung von Phasengrenzen und Perkolation sind dynamische Lichtstreuung, "diffusive wave" Spektroskopie und Neutronenkleinwinkelstreuung. Die experimentellen Ergebnisse werden mit Simulationen verglichen und zeigen gute Übereinstimmung, wenn realistische Modelle für die Kolloid-Kolloid Wechselwirkung verwendet werden.

J. Chem. Phys. 130, 154903 (2009)

( G. Meier, J. Buitenhuis, P. Lettinga )


Lichtstreuung in selbst organisierenden Systemen

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Statische und dynamische Lichtstreuuntersuchungen sind ausgezeichnet dazu geeignet, Formen und Größen in einem fluiden System zu bestimmen, die in dem Bereich von wenigen Nanometern bis hin zu einigen Mikrometern liegen. Lichtstreuung ist eine berührungsfreie Meßmethode, die die Untersuchung der Strukturbildung in transparenten Lösungen erlaubt. Dies ist ein wesentlicher Vorteil gegenüber anderen Techniken, wie der Elektronenmikroskopie und der Kraftmikroskopie. Wir haben mit der Lichtstreutechnik zwei neuartige molekulare Systeme untersucht, die spontan eindimensionale Strukturen ausbilden: Knäuel-Makrozyklus-Knäuel und Dendrimer- Stab-Knäuel Moleküle.

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( S. Wiegand )

A glass in suspensions of colloidal rods: particle- and texture-dynamics

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A glass transition is observed in dispersions of very long and thin, highly charged rod-like colloids (fd-virus particles), at low ionic strengths where thick (27 nm) electric double layers exist.  Structural arrest as a result of particle-caging due to strong overlap of these extended double layers is observed by means of dynamic light scattering. The glass-transition concentration is found to be far above the isotropic-nematic coexistence region (1.5-3.4 mg/ml), at an fd-concentration of 11.7 mg/ml. The morphology of the system therefore consists of (chiral-) nematic domains with different orientations.

As the cuvette is filled with suspension, shear alignment occurs; leading to large nematic domains. Below the glass-transition concentration the initial morphology with large shear-aligned domains breaks up into smaller domains, and equilibrates after typically 80- 100 hours. With a technique that we termed “image-time correlation” (where transmitted-intensity correlation functions are constructed  from a time series of  depolarized  images),  the dynamics of texture is quantified by the inital slope of these correlation functions.  

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( K. Kang)


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