Current Projects

Finished Projects

 

Current Projects

 

Finished Projects

Formation of multilamellar vesicles

Flow small angle light scattering
Flow small angle light scattering during MLV formation.

Multilamellar Vesicles (MLVs) have attracted much interest because of their encapusalted morphology. The conditions under which MLVs form are, however, not well understood.
The viscosity developement during MLV formation using creep flow is similar to the one observed for oscillatory shear. They can be compared by introducing an effective strain and an effective shear rate. This effective shear rate is related to an effective amplitude for a given frequency by the Delaware-Rutgers rule. Comparing the effective amplitude with the experimental one shows that a frequency independent minimum strain amplitude is needed for MLV formation. Only deformations greater than the minimum deformation contribute to MLV formation, smaller deformations act elastically. Therefore the ratio of the to contributions to deformation can be related to the loss angle.

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Electrosteric stabilisation

Overlapping electrosteric layers
Overlapping steric layers cuase steric stabilisation.

It is often important to stabilise colloidal dispersions against aggregation. One way to do so is steric or electrosteric stabilisation of the dispersions.
We studied steric stabilisation of paricles covered with PMAA. This hairy layer causes a repulsive interaction. Investigating all the components of the interaction potenial, we learned that an osmotic contibution is the most important one. It is caused by the increased volume fraction of polymers within a certain region, when the two layers overlap. The consequence of the high polymer consentration in the overlap region is an increase in osmotic pressure, which drives the particles apart.
Electrostatic interactions are also important, but their contribution is more indirect. The electrostatic repulsion between the particles is not very important, but the repulsion between the polymers within the layer has a string influence on the composition of the polymer layer and therfore on the osmotic pressure, as soon as the layers overlap.

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High frequency rheology

G' & G''
Loss and storage moduli of sterically stabilised dispersions

Measuring G' at high frequencies is a highly sensitive tool to probe interactions. Using torsional resonators (IdM, Ulm) it is possible to measure at frequencies up to several kHz.

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Higher stability of the non-linear least squares needed for GIFT

Hyper surface
Hyper-surface of the GIFT problem. The red point marks the global minimum

The generalised indirect Fourier transformation is based on the relationship

I(q)=n·P(q)·S(q)

where the intensity I(q) is related to the form factor P(q) and to the structure factor S(q). Solving this non-linear problem has to be done in an iterative way. Testing various algorithms for non-linear lest squares we found that Boltzmann simplex simulated annealing gave best results.

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Scattering of Interacting Charged Particles

MSA structure factor
Structure factor of CTAB micelles in 10 mM KCl solution. The surfactant concentration is increased from 1% to 20%

The new Generalised Indirect Fourier Transformation (GIFT) is an improved version of Otto Glatter's IFT. This software package calculates the pair distribution function and the structure factor for small angle scattering data.
The program separates inter- and intra particle contributions to scattering data. Model assumptions must be done for the inter-particle contributions. I added three models based on a Yukawa potential but with different closure relations to connect the potential with the pair correlation function, namely the rescaled mean spherical approximation (Hayter & Penfold, 1981; Hansen & Hayter 1982), the hypernetted chain approximation (van Leeuwen et al, 1981) and the Rogers Young closure (Rogers & Young, 1984).

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Application of Scattering Methods to Problems of Biological Membranes

Picture LPL activity
Change in size of triolein droplets due to LPL activity

This work was done in cooperation with the SFB Biomembranes of the Karl Franzens University Graz and the Technical Univeristy Graz. The following sub-projects were included:

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Density, Ultrasonic Speed and Viscosity Measurements

Picture of DMA 5000
DMA 5000

The DMA 5000 (by Anton Paar GesmbH, Graz) measures density by the oscillating tube method (Measurement principle): The sample is filled into an oscillating U-shaped glass tube. The volume is determined by the nodal points of the tube, mass is measured by the oscillation period. After calibrating the system with two well defined samples (typically water and air), density can be calculated with an accuracy of +/- 0.005 mg/ml. For viscous samples, another effect has to be taken into account: During the oscillation, the sample near the wall of the tube will perform an additional rotational movement to the normal translation up and down. This movement leads to a too high masses, leading to an error in density of up to 0.7 mg/ml.
To compensate this error the DMA 5000 measures the excitation force needed to keep the tube in oscillation. Using this value for compensation, the accuracy is 0.005 mg/ml.

We use a modified version of the DMA 5000 with an additional sound velocity cell with a accuracy of about 1 m/s.
I am interested in calculating viscosity out of these damping data. this can be done up to about 350 mPa s.
Additionally I write software to control the DMA from an external computer for time and temperature dependent measurements, and for evaluation of these measurements (Program WINDMA). These measurements can be used to set up phase diagrams quite easily, because they are performed automatically and give three independent values (Density, sound velocity and viscosity) or to follow reactions.

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