Institute for Soft Condensed Matter Physics
General Overview
The Institute "Weiche Materie" was founded in jan. 2000. The main focus of the Institute is to understand macroscopic properties and microstructural order of colloidal systems, under equilibrium- and non-equilibrium conditions, on a microscopic level. To make progress in this area it is necessary to be able to synthesize model colloidal systems; where monodispersity of the desirable interaction potential is one of the main goals.. The properties of these model systems should be tuned in such a way that it exhibits the phenomena that one wishes to study.
The research in the Institute is done within "projects", which will be described below.
At this moment the further development of the Institute is severely hindered by lack of laboratory facilities. Most of the planned laboratories are not yet ready for use, for reasons that go beyond the jurisdiction of the FZJ. Some of the planned research can not be started, while other research themes can only be performed in part in improvised laboratories.
The following projects have been defined within the Institute Weiche Materie,
Non-equilibrium Phenomena
Project leader : | Prof. J.K.G. Dhont |
Personnel : | D. Triefenbach (Technical Engineer) |
Dr. S. Rathgeber (Habilitand) | |
Dr. P. Lettinga (Post Doe) | |
Dr. T. Lenstra (Post Doc) | |
Dr. Z. Dogic (Humboldt Fellow, started march 2001) | |
Dr. P. Carletto (EU Post Doc, started June 2001) | |
Dr. R. Tuinier (started october 2001) |
The subjects under investigation in this project are
Shear-banding and Rheology of "hairy Colloids" and Suspensions of rod-like
Particles (S. Rathgeber, P. Carletto, P. Lettinga, Z. Dogic)
Shear-banding is a hydrodynamic instability in systems under flow that occurs whenever the stress decreases with increasing shear-rate, or the stress is a multivalued function of the shear-rate. The stationary state is now a state where regions of different microstructure and sometimes different shear-rates "coexist". The kinetics of the shear-banding instability and the possible stationary states that can occur are studied for two kinds of systems : "hairy colloids" (colloidal spheres coated with long polymer chains) and rigid rod-like particles. A system of hairy colloids where shearbanding has been observed will be investigated in cooperation with Dr. J. Vermant (University of Leuven). Another, well-defined hairy colloid has been developed in the Institut for Neutron Scattering (see the next theme). So far, our investigations are focused on the shear-induced polymer brush defoprmation at lower concentrations. Simulations on hairy colloids in shear flow will be performed in the Insitute Theory II. We recently observed a very clear shear-banding transition in suspensions of fd-virus (a semi-flexible colloid). By adding free polymer, the rate of the transition can be tuned, so that kinetic studies will be feasible in these systems. At this moment we are determining, by rheology and light scattering, the full non-equilibirum phase diagram of these kind of systems (where the shear-rate is on of the control variables). The shear-banding transition lines will be located as well. There is an ongoing collaboration with prof W. Briels (University of Twente) on the theory of the shear-banding transition in systems of rigid rods.
Shear induced Polymer Brush Deformation (S. Rathgeber, P. Carletto)
As was mentioned above, colloidal particles with a small core in comparison to the length of the polymers that are attached to their surface have been developed in the Institute for Neutron Scattering by Dr J. Allgaier and Dr. L. Willner. These systems will be used to study the polymer brush deformation under shear flow by means of birefringence measurements and small angle neutron scattering on very dilute samples, in cooperation with Dr. L. Willner and Dr. W Pyckhout (Institute for Neutron Scattering) and Dr. P. Lindner (ILL, Grenoble). For this purpose we developed a quartz shear cell in cooperation with Dr. W. Pyckhout and Dr. P. Lindner. In a later stage we will also perform rheology experiments and scattering experiments at larger concentrations, in order to probe the effect of the brush deformation on the microstructure. It has been found that appreciable brush deformation in water occurs only at very high shear-rates, where the onset of a Taylor instability occurs. We therefore investigate at the moment the possibility to use more viscous solvents. These investigations are part of a European project (HUSC, Hard to Ultra Soft Colloids).
Critical Phenomena under Shear Flow (P. Lettinga, H. Wang)
In mixtures of stearyl silica spheres and small PDMS polymers (Poly Di Methyl Siloxane) in cyclohexane, depletion attractions can give rise to gas-liquid phase separation. Non-linear microstructural response to stationary and oscillatory shear flow near the gas-liquid critical point in these systems is studied by means of time resolved small angle light scattering. The results of the static experiments are being analysed at the moment, also by Dr. H. Wang. The analysis of the time dependent response is currently in progress.
Dynamics and Structure of "Polymer-colloids " (S. Rathgeber)
Suspensions of polymeric particles of a colloidal size are studied by means of small angle neutron scattering and neutron spin echo experiments. This concerns the shape, internal structur and dynamics of the colloidal particles. The systems that are studied are two different kinds of dendrimers, cellulose derivatives and bottlebrush macomolecules, in cooperation with T. Pakula (Max Planck Institute for Polymer Research in Mainz). Scattering experiments and rheology measurements at higher concentration are planned.
Nucleation and Crystal Growth under Shear Flow Conditions (R. Tuinier)
Small angle, time resolved static light scattering will be used to examine nucleation and crystal growth under the influence of shear flow. The shear induced shift of the liquid-solid binodal will be a point of interest also. Since the crystals will probably be anisometric, scattering experiments are planned where the incident light beam is either directed along the gradient direction or the vorticity direction. High angular resolution is achieved with a 2D CCD camera, with the photosensitive chip placed directly at the position of Bragg scattering angles.
Negative Thixotropy of Polymer Solutions (J. Buitenhuis)
This project has been started at the Technical University Berlin and was continued at the FZ-Jiilich. Negative thixotropy is the phenomenon of a time-of-flow dependent shear thickening, which is not well understood. A new explanation is proposed and compared to experimental results.
Structure and Dynamics of Colloids at Interfaces
Project leader : Dr. P. Lang
Personnel : M. Hoelzle (Technician, started june 2000)
The subjects under investigation in this project are
Structure and Dynamics of Interfaces between Coexisting Phases
The colloidal systems that will be investigated are rigid rods, like silica coated boehmite rods and semi-flexible fd-virus (in isotropic-nematic coexistence), soft systems like internally crosslinked cylindrical micelles (in isotropic-columnar coexistence), and spheres (in gas-liquid or liquid-crystal coexistence).
Structure and Dynamics close to a Wall
Like in the previous theme, rods and spheres will be investigated. We also plan to investigate depletion forces near a wall in mixtures of long polymers and small colloidal spheres. This will be done in cooperation with Dr. E. Eisenriegler (Institute Theory II).
To study the above mentioned phenomena, it is necessary to apply surface and interface sensitive measuring techniques. At this moment, Dr. P. Lang is constructing two instruments with options for static and dynamic light scattering of evanescent waves (EWLS), total internal reflection microscopy (TIRM), capillary wave spectroscopy (CWS), ellipsometry and Brewster angle microscopy (BAM). The TIRM option is being built in cooperation with Dr. C. Bechinger (University of Konstanz).
In addition, P. Lang is working on the synthesis of "soft colloidal rods" (see the project "Synthesis").
Within the entire project "Structure and Dynamics of Colloids at Interfaces", the yet non-existing laboratories are essential for it's further development.
Dynamics in Equilibrium Systems
Project leader : Dr. G. Meier
Personnel : M. Hoelzle (Technician, started june 2000)
The subjects under investigation in this project are
Dynamics of Polymer Mixtures and Colloids under high Pressure
Pressure is an external variable that can be used to continuously vary the compatibility of polymers. The polymers can either be free polymers or polymers bound to the surface of colloidal particles. Besides by pressure, monomer-monomer pair-interaction potentials can also be changed systematically by changing the H/D composition of the polymers. This offers the possibility to gain insight in the microscopic origin of the rich phase behaviour and dynamics of these systems. The phase behaviour and (critical) dynamics of polymer mixtures and mixtures of polymers and polymer-coated colloidal particles will be the topic of this project.
Phonon Dispersion in heterogeneous Systems
Colloidal systems represent heterogeneous systems, in which the dynamics of acoustic modes will be studied. There will almost certainly be a dependence of the properties of acoustic exitations on the structural properties of the colloidal suspensions. The degree of heterogeneity can be changed continously by changing the size and/or concentration of the colloidal particles.
Rotational Diffusion of Rods near Phasetransition Lines
Rotational diffusion of rod like colloids and polymers in the neighbourhood of a gas-liquid and isotropic-nematic phase boundary will be studied. Near a gas-liquid phase transition line, the attractive interactions between the rods will probably lead to a severe change of the rotational diffusion coefficient, since attractions favor parallel alignment of neigbouring rods over perpendicular alignment. On approach of the isotropic-nematic spinodals, the rotational diffusion coefficient tends to zero in a way that is not known, that is, the critical exponent for the rotational diffusion coefficient is as yet unknown.
Rotational Diffusion away from Phasetransition Lines (P. Lettinga),
Rotational diffusion coefficients of colloidal spheres and rods in bulk solution and within porous media are investigated by means of Time resolved Phosphorescence Anisotropy (TPA) and Recovery After Photobleaching (FRAP). These experiments are performed at the University of Utrecht in collaboration G. Koenderink and prof. A.P. Philipse. Also of
interest is rotational diffusion of rigid rods under stationary shear flow, where FRAP and heterodyne dynamic light scattering experiments are planned.
We plan to built a combination of set ups which allows one to investigate dynamics in the time range 10-13-102 s. Dynamic light scattering covers a dynamic range of 10-7-102 s. A Sandercock tandem interferometer will be used to cover the time range 10-9-10-11 s. The gap between these time ranges can be covered employing a confocal interferometer. The very fast dynamics will be studied by means of Raman scattering.
The project "Dynamics in Equilibrium Systems" can not start yet due lack of laboratory facilities.
Correlation between Interactions and ordered Structures
Project leader : Dr. J. Hauck
Dr. J. Hauck has been at the IFF for a longer time. His work in the Institute Weiche Materie is a continuation of part of the work that has been done by J. Hauck during the past, applied to ordered structures in various colloidal systems.
Synthesis
Project leaders : Dr. J. Buitenhuis (started in march 2001)
Dr. W. Sager (started in april 2001)
Personnel K. Sellinghoff
D. Triefenbach (Technical Engineer)
The aim of this project is the development of new colloidal systems, the investigation of synthesis mechanisms and the synthesis of (known) colloidal systems for cooperation projects.
Synthesis of known Colloidal Model Systems for Cooperation Projects
The first aim will be to build up additional know how concerning the synthesis of known colloidal model systems (like coated silica spheres, latex spheres, coated boehmite rods and gibbsite platelets, fd-virus and DNA fragments). D. Jablonski and D. Triefenbach have already synthesized silica-coated boehmite rods under the guidance of J. van Wijnhoven (Colloid Synthesis Facility in Utrecht, the facility was closed down recently). J. Buitenhuis synthesized aluminium chloro hydrate coated boehmite rods.
Synthesis of Soft Colloidal Rods (P. Lang, L. Willner)
The aim is to develop a model system for soft colloidal rods. These rods are synthesized by radically crosslinking of cylindrical micelles, which consist of amphiphilic blockcopolymers.. This will lead to flexible colloidal rods, of which the length is about 200 nn and the aspect ratio about 20. The main problem will be to fractionate the system in order to reduce the polydispersity in length.
Synthesis of Silica Coated fd-virusses (J. Buitenhuis)
Optimization and Mechanism of the Synthesis of Colloidal Boehmite Rods (J. Buitenhuis)
Colloidal boehmite rods can be synthesized with an average length ranging from 70 to 500 nm. Although several studies on dispersions of these rods were successful, the system still contains a small degree of aggregates or is to polydisperse
for certain other studies. To improve the quality of the rods is the aim of this project. To do so it will be important to obtain some knowledge about the mechanism of the particles formation. This subject is just starting.
In addition, J. Buitenhuis -has been working on the "Negative thixotropy of polymer solutions" (see the project "Nonequilibrium Phenomena").
The polymer synthesis know how that is available in the Institute for Neutron Scattering (Dr. L. Willner and Dr. J. Allgaier) will be of great advantage in developing certain classes of new colloidal systems.
Preparation of Nanostructured Materials in Self-organising Systems (W Sager)
This project is concerned with the characterisation of microemulsions as precipitation media for inorganic materials with at least one dimension in the nanometer range and the development of synthesis routes for nanostructured inorganic/organic composites employing the different structures encountered in microemulsion systems as templates. It is part of the PhD-project of Aurelie Autin and is performed together with Prof. R. Nolte at the University of Nijmegen.
Though microemulsions have increasingly been utilised over the last 2 decades, and in some cases even established themselves as precipitation medium for the preparation of small uniform inorganic particles of mostly spherical geometry, little systematic work has been performed on what actually controls the size, shape, crystallinity and stability of the particles formed. Until now there is no global picture at hand describing nucleation and growth processes for the different microemulsion morphologies investigated and a generally valid theory is still out of reach.
In this project we want to perform a systematic study on precipitation in well defined and characterised microemulsions with different morphology ranging from droplet-type microemulsions to cylindrical aggregate structures and bicontinuous microemulsions. The latter consist of an interwoven network of water and oil channels stabilised by the interfacial surfactant film. To establish a direct relation between microemulsion structure and particle morphology, the microemulsions and the precipitated inorganic materials will be characterised by phase diagram and conductivity studies, electron and atomic force microscopy, X-ray diffraction and scattering techniques. Underlying nucleation and growth processes will be studied with respect to microemulsion morphology, specific inorganic nuclei/surfactant interactions and properties of the interfacial surfactant film. Emphasis will be laid on solidifying bicontinuous structures which is of special interest for inorganic/organic nanocomposites. Employing nucroemulsions systems as templates for nanostructured composite materials has the advantage of offering the possibility of both simultaneous and consecutive formation of the organic and the inorganic solid phase by bringing in one precursor with the water and the other with the oil phase or the interfacial surfactant layer.
The project "Synthesis" can not be realized yet, due to the lack of laboratory facilities.
Theory
Project leader : Dr. G. Naegele (started jar. 2001)
Personnel : Dr. M. Kollmann (started in april 2001)
Dr. H. Wang (started in march 2001)
The aim of this project is two-fold : (i) developing theory on subjects that are, or may become, of experimental interest to the Institute, and (ii) to assist experimentalists with devising new experiments and with data interpretation.
Currently, the following subjects are being investigated
Electrokinetic Effects in Colloids
In particular, electrolyte friction is studied on the basis of mode coupling theory for a binary system : the large colloidal particles and the counter ions. A remarkable result is that hydrodynamic interactions play an essential role. A treatment of non-linear electrolyte friction response is within the validity of the present approach.
Diffusion in Two-dimensional Space
Inspired by experiments that are performed by Dr. K. Zahn and Prof G. Maret (University of Konstanz), diffusion coefficients have been calculated in case of a spatial dimension equal to 2. The surprising divergence of the ensemble averaged hydrodynamic interaction function (commonly denoted as H(q)) is confirmed by theory. In addition, unexpected scaling behaviour has been predicted theoretically.
Linear Viscoelastic Behaviour of spherical Colloids
A Green-Kubo formula for the linear shear viscosity of colloids, where hydrodynamic interactions are important, has been derived and evaluated numerically on the basis of a mode coupling approach.
Jan Dhont