Our research focuses on subjects in the area of soft matter science. Meanwhile, colloid and polymer science are of course classical areas of research. However, due to the strong interest in nano- and mesoscale materials soft matter research is a strongly growing field at the moment and still has enormous potential with respect to applications. Moreover, the new custom designed particles, which can now be made also can serve as interesting model system to tackle more fundamental physical questions.
Within this framework our main research topics are microgels, microemulsions, block copolymers, and nanoparticle/polymer hybrid materials. The experimental techniques we use are advanced scattering experiments, but also imaging techniques are of growing importance.
Microgels are colloidal particles comprising a gel structure internally. When the polymer component of these particles is based on acrylamides (e.g. N-Isopropylacrylamide (NIPAM), N-n-Propylacrylamide (NNPAM) or N-Isopropylmethacrylamide (NIPMAM)) they exhibit a reveesible so-called volume phase transition. This reveresible change in volume grants them the name "smart microgels".
They have attracted considerable interest as model systems and for their potential applications in drug delivery, as chemical separation media, as sensors or as carrier particles of catalysts. Most of these systems are based on poly(NIPAM) and its homologous. In the case of poly(NIPAM) the volume phase transition is triggered by a change in temperature and the transition point is nearly at 32°C.
Using a second monomer during the sythesis of the micogels or the incorporation of nanoparticles (NPs) opens the possibility to create a wide variety of different architectures like core-shell paricles (mono- or double responsive), interpenetrating networks or particle containers with randomly distributed NPs or only one catalytical core.
We focus on the developement of tailor-made microgels systems using a great varity of copolymers and NPs. A detailed characterisation of the obtained systems is done by different scattering methods (light-, x-ray- and neutron scattering) in combination with imaging techniques (AFM, SEM, (cryo-)TEM), ellipsometry, ...).
From a physical point of view the phase transition of these colloidal gels is still of great interest. In this context we are interested in the network fluctuations and the behavior of the respective correlation length.
Karg et al. "Poly-NIPAM Microgels with different cross-linder densities" Intelligent Hydrogels, Progress in Colloid and Polymer Science, 140, 63-76
Microemulsions are thermodynamically stable mixtures of the two in principle immiscible liquids water and oil, which are stabilized by surfactant. A rich phase behaviour is found in this kind of systems. These systems are very interesting as reaction media or for oil recovery.
One of our recent research topics focuses on the diffusion of proteins in bicontinuous microemulsions. In the figure below (left picture), a schematic drawing of a bicontinuous microemulsion containing water, oil, a sugar surfactant and a co-surfactant is shown. If proteins, like GFG+ (green fluorescent protein), are present in the water phase of a bicontinuous microemulsion, the biopolymer is confined and its diffusion is expected to be affected by the microemulsion. The obtained experimental fluorescence correlation functions (obtaind by FCS measurements) are presumed to reveal a deviation from simple Fickian diffusion. By varying the oil/water ratio in the microemulsion, the correlation length of the water domains can be tuned in a controlled way. This allows to subsequently increase the confinement for the protein in the water domains. The performed FCS measurements (right picture below) confirm that the GFP+ mobility depends on the correlation lengths of the water domains. The confinement increases for higher oil/water ratios, corresponding to smaller water domains. As a consequence the anomalous diffusion exponent α reveals a sub-diffusive behavior.
Neubauer et al. "Protein diffusion in a bicontinuous microemulsion: Inducing subdiffusion by tuning the water domain size" , Soft Matter X XX-XX
The optical properties of microgels are rather poor, which is due to the low refractive index of the solvent swollen polymer network. On the other hand nanoparticles have outstanding optical properties but are not able to change size. Hence, it seems to be straightforward to combine both systems to generate new hybrids combining the advantages of both particle types.
Membrane BiophysicsUsually, soft matter materials as polymers, vesicles, microemulsions, microgels, liquid cystals etc. exhibit several relaxation processes on the local and global time scale.
For example, vesicles are model systems to study the dynamics of biological cells because they are topologically and structurally equivalent to biological membranes. In these systems the dynamics range form the macroscopic scale (shape fluctuations observable under an opical microscope), over translation diffusion (observable with photon correlation spectroscopy) and down to the local dynamics of the membrane measurable with neutron spin-echo (NSE) .
At the moment we use small angle scattering (SANS and SAXS) and NSE to study the influence of saponins on the lipid bilayer. Saponins are a huge class of natural soaps which are produced by plants as protection against e.g. bacteria. From a pharmacological point of view these compounds are very interesting and have divers applications. However, from a fundamental point of view only very little is known about their action. Recently, we have intensely studied the interaction of the horse chestnut saponin aescin with DMPC model membrane . Aescin drastically changes the membrane properties and at higher volume fractions a vesicle is completely decomposed into smaller entities.
 Mell et al. ‘Bending stiffness of biological membranes: What can be measured by neutron spin echo?’; Euro. Phys. J. E, 36, 75 (1-13)
 Sreij, et al. ‘DMPC vesicle structure and dynamics in the presence of low amounts of the saponin aescin’ Physical Chemistry Chemical Physics, 2018, Accepted
Scattering MethodsSince several years our group is working on the design and implementation of dedicated soft matter sample environments for small angel neutron scattering instruments. These projects are funded by the German Federal Ministry of Education and Research (BMBF). At the moment we have an ongoing instrumentation project for the ESS (hier link zur ESS) in collaboration with the von Klitzing group at TU Darmstadt and the Müller-Buschbaum group at TU Munich. In this project we develop an in-situ DLS/DWS setup for SKADI instrument.
This project is done in close collaboration with JCNS and ESS.