Phase behaviour and microstructure
The Isotropic-nematic phase transition of rod-like viruses
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
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.
Adhesive colloidal dispersions under high pressure
We are interested in investigating the structure and the phase diagram of sterically stabilized colloidal system consisting of grafted silica particles dispersed in marginal solvents. These system are known to exhibit gas-liquid phase separation and percolation, depending on temperature T, pressure P, and concentration ?. Phase boundaries and percolation threshold are determined by applying various techniques like dynamic light scattering, diffusive wave spectroscopy and small angle neutron scattering. Comparing this with recent simulation results shows good agreement especially concerning the predictions for the percolation threshold if realistic models for the colloidal interactions are used.
Light scattering applied to self-assembling systems
Static and dynamic light scattering provide a unique tool to study the shape and size of systems on a length scale ranging from 1 nm to several µm. Light scattering techniques are non-evasive and can be used to learn about the in situ structure of a system as it forms in solvent, contrary to for example electron microscopy or atomic force microscopy. We applied light scattering to two newly developed molecular systems that were designed to spontaneously self-organize into a one-dimensional structure: Ring-coil triblock copolymers and dendron rodcoil molecules.
( S. Wiegand )
A glass in suspensions of colloidal rods: particle- and texture-dynamics
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.
( K. Kang)