IFF
Scientific Report 2000/2001

General Overview

Introduction: Soft Matter Research

The main research topic of the Institute is the theory of "complex fluids" and "soft matter" systems. Soft matter physics is an interdisciplinary research area encompasing statistical physics, material science, chemistry, and biology. The systems are characterized by

• Supramolecular structure and self-assembly
• Typical length scales ranging from nano- to micro-meters
• Typical energy scales comparable to the thermal energy k_BT.

• Polymer solutions, mixtures, and melts
• Mixtures of block copolymers and homopolymers
• Lyotropic liquid crystals
• Amphiphilic systems, i.e. mixtures of oil, water and amphiphiles
• Colloidal suspensions.

While these areas remain active fields of research, the focus has recently shifted to more complex systems which are obtained by combining two or more of the components listed above. A few examples are

• Colloidal particles in polymer solutions
• Mixtures of surfactants and amphiphilic block-copolymers
• Mixtures of several surfactants or lipids
• Colloids in liquid crystals

Since the structures in soft matter systems often contain a large numer of molecules, mesoscale modelling is typically required to bridge the length- and time-scale gap between the microscopic domain -- of atoms and their interactions -- and the emerging properties of supramolecular assemblies on meso- or macroscopic scales. Microscopic models are employed to study properties of complex systems on the molecular scale, and to provide a link of mesoscale models to molecular architecture.

A characteristic feature of soft-matter research is the fruitful interaction between theory and experiment. With a third of the IFF institutes [Neutron Scattering (Richter), Theory II and Soft Matter (Dhont)] now focusing on soft matter research, many of the essential aspects of these systems are investigated here.

Research projects and results:
(in alphabetic order)

1.

Polymer-mediated attraction between two small colloidal particles:
The second virial coefficient B₂ of a dilute solution of small colloidal particles shows an interesting non-monotonic dependence on the concentration n of free polymer chains in the embedding solvent. The quantitative form of this dependence, with a minimum at the overlap concentration $n \approx n^{\ast}$, is obtained by expressing B₂ in terms of the compressibility of the polymer solvent without particles. (E. Eisenriegler)

2.

Influence of chain self-avoidance on polymer depletion forces between colloidal particles:
Self-avoidance effects become more and more important the lower the dimension of the polymer-embedding space. For a dilute solution of long chains in two dimensions the density depletion profile around two touching disks is calculated exactly and the force between the disks is obtained. The force between two spheres in three dimensions can be estimated by interpolating between two and four dimensions. (E. Eisenriegler)

3.

Measuring bending rigidity in bicontinuous microemulsions:
We demonstrate a new approach to determine the bending rigidity of the amphiphile film in microemulsions and sponge phases from neutron scattering data. This method is precise enough to measure the logarithmic scale dependence of the bending rigidity and its universal prefactor for the first time. Furthermore, we show that in the mushroom regime the bending rigidity of a membrane decorated by amphiphilic block copolymers increases linearly with the polymer concentration on the membrane; the amplitude is found to be about a factor 1.5 larger than theoretical results for ideal chains. (G. Gompper, H. Endo, M. Mihailescu, J. Allgaier, M. Monkenbusch, D. Richter, B. Jakobs, T. Sottmann, R. Strey)

4.

Stability of inverse bicontinuous cubic phases in lipid-water mixtures:
We have investigated the stability of seven inverse bicontinuous cubic phases (G, D, P, C(P), S, I-WP, F-RD) in lipid-water mixtures based on a curvature model of membranes. Lipid monolayers are described by parallel surfaces to triply periodic minimal surfaces. The phase behavior is determined by the distribution of the Gaussian curvature on the minimal surface and the porosity of each structure. Only G, D and P are found to be stable, and to coexist along a triple line. The calculated phase diagram agrees very well with experimental results for 2:1 lauric acid/DLPC. (G. Gompper, U.S. Schwarz)

5.

Diffusion in glasses:
The diffusion of interstitial particles in disordered systems without lattice translational invariance is investigated by a novel Monte Carlo approach. Experimental and simulated structures of silicate and alkali-silicate glasses are used to calculate the positions and energies of the minima and saddle points for the interstitials. The resulting transition rates are then utilized in Monte Carlo simulations, which can be extended to sufficiently long times to extract asymptotic diffusion coefficients. These show approximate Arrhenian behavior as functions of inverse temperature. (K. Kehr, K. Mussawisade)

6.

Phase separation of binary fluid mixtures in shear flow:
The phase separation of binary fluid mixtures in uniform shear flow has been studied numerically in the framework of continuum convection-diffusion equations based on a Ginzburg-Landau free-energy functional. The main results show the existence of domains with two typical length scales, whose relative abundance changes with logarithmic-time periodic oscillations. (A. Lamura, F. Corberi, G. Gonnella)

7.

Stability of a protein pore in a lipid membrane:
A membrane protein pore embedded in a fully hydrated bilayer lipid membrane is investigated by molecular dynamics simulations. It is found that the melittin pore decays from an initial tetrameric configuration into a stable trimer and one monomer. The expansion-induced formation of an interface between the pore-lining acyl chains of the lipids and the pore water is transformed into an energetically more favorable toroidal pore structure, where some lipid heads are translocated from the rim to the central part of the interface. (J.-H. Lin, A. Baumgärtner)

8.

Effects of size ratio and inter-chain overlap in colloid-polymer mixtures:
The depletion of long flexible polymers near the surface of a colloidal particle is an entropic effect and depends apart from the distances of nearby particles in a crucial way on the ratio of the particle and chain sizes ond on the degree of overlap between the chains. We study both effects for the simple system of a single spherical particle embedded in a monodisperse solution of free nonadsorbing polymer chains. Both the density profile of the polymers and the solvation free energy of the particle are calculated. (R. Maassen, E. Eisenriegler, A. Bringer)

9.

Wetting behavior in amphiphilic systems:
The wetting behavior of ternary amphiphilic systems, containing water, surfactant and vapor, has been investigated. We first study interfacial wetting in ternary mixtures with the most general nearest-neighbor pair interactions, and find Cahn-type wetting transitions near the critical end points. In a second step, we investigate the dependence of the contact angles on the amphiphilic strength of the surfactant molecules. [Supported by DFG priority program ``Wetting and Structure Formation at Interfaces''.] (T. Schilling, G. Gompper)

10.

Dynamics of swollen lamellar phases:
Among the large variety of phases, which appear in amphiphilic systems, the lamellar phase plays a key role for the understanding of the physical properties of these systems, since its simple geometry allows for detailed theoretical and experimental investigations. We study the relaxation rates of lamellar phase in a ternary system of water, oil and amphiphile, which are governed by the hydrodynamics of the fluid layers. A direct comparison with light scattering and neutron-spin-echo experiments is possible. (T. Schilling, O. Theissen, G. Gompper)

11.

Hydrophobic interaction:
We investigate idealized discrete models of the hydrophobic interaction. Our findings suggest that a solubility-enhancing increase of symmetry of the solvent particles decreases the solvent-mediated part of the potential of mean force between solute particles. This weakening of the hydrophobic attraction is in agreement with the notion that the effect is entropic in origin. (G.M. Schütz, I. Ispolatov, G.T. Barkema and B. Widom)

12.

Shocks in driven diffusive systems:
Shocks in driven diffusive systems, i.e. abrupt changes in the local density, form collective excitations which are localized and stable over long periods of time. In a family of lattice models for driven diffusive systems we obtain detailed information about the microscopic structure of the shock as well as its large-scale properties by using special non-abelian symmetries for the exact analytical calculation of the time evolution of a shock measure. Numerical finite-size scaling analysis shows that the notion of a localized shock is meaningful also for very small systems, thus suggesting that coarse grained nonequilibrium theories involving shocks are applicable to small real systems. (G.M. Schütz, T. Antal, V. Belitsky, M. Dudzinski, C. Pigorsch)

13.

Reaction-diffusion systems:
The dynamics of a coupled two-component nonequilibrium reaction-diffusion system for dynamically activated hopping is examined by renormalization group analysis of a continuum field theory representing the corresponding master equation. For activators B subject to diffusion-limited reactions the activated particles A perform normal diffusion if the density of B particles attains a finite asymptotic value (active state), while strongly anomalous subdiffusive behavior occurs if the B density decays into an inactive state. For B pair annihilation the mean-square displacement of the A particles grows only logarithmically with time in $d \geq 2$ dimensions. For radioactive B decay, the A particles remain localized. (G.M. Schütz, S. Trimper, U.C. Täuber)

Some Remarks:

• Prof. Klaus Kehr, a member of the Institute Theory II from 1971 until 1999 and acting director during several periods (1976-1977, 1988-1990, and 1995-1999), unfortunately died last year shortly after his retirement. Many members of the IFF will remember Prof. Kehr as a helpful and friendly person, and as a very enthusiastic and excellent physicist.

• Prof. Erich Eisenriegler, who was a member of Institute Theory I from 1969 until 2000, has formally joined the Institute Theory II this year. Prof. Eisenriegler's main research interests have focused on soft matter systems already for quite a while. He brings his expertise on statistical field theory and the renormalization group, as well as on polymers near surfaces and colloids into the Soft Matter Theory group.

• Dr. G. Schütz spent the summer 2000 as visiting professor at the Cornell University in Ithaca, New York.

• Dr. Roland Winkler will join the Institute Theory II at the beginning of 2001.

Awards etc.:

• Dr. G. Schütz has been awarded the Gustav-Hertz Preis 2000 by the Deutsche Physikalische Gesellschaft, in recognition of an excellent recently completed research project from the group of young scientists.

• Prof. T. Burkhardt (Temple University, Philadelphia, USA) has received an award from the Alexander von Humboldt Stiftung, which he used to spend half a year (Sept. 1999 to Febr. 2000) as a visiting scientist at the IFF.

Personnel 2000/2001 and areas of activity

Scientific Staff

Dr. A. Baumgärtner	Statistical mechanics of proteins and membranes;	23.30.0
Prof. E. Eisenriegler	Polymers near surfaces, colloid-polymer mixtures	23.30.0
Prof. G. Gompper	Statistical mechanics of amphiphilic systems	23.30.0
Institute Director
Dr. G. Schütz	Driven diffusive systems, reptation models	23.30.0

Technical Staff

H. Paffen

Secretary

Postdocs

Dr. A. Lamura

Hydrodynamics of simple and complex fluids

23.30.0

Diploma and Graduate Students

T. Auth	Polymers at membranes	23.30.0
K. Mussawisade	Diffusion in disordered materials	23.15.0
J.-H. Lin	Membrane proteins	23.30.0
M. Paeßens	Finite-size effects in entangled polymers	23.30.0
T. Schilling	Wetting in amphiphilic systems	23.30.0
R. Willmann	Polymer dynamics in disordered media	23.30.0

Guests

Prof. T. Burkhardt	(Temple University, Philadelphia, USA) Statistical	23.30.0
	mechanics of polymers; stochastic processes
	(Sep. 1999 - Feb. 2000)
C. Pigorsch	(Universität Halle) Shocks in many-body systems	23.15.0
	(Mar. 2000)
S. Miller	(Universität Stuttgart) Polymerized Membranes	23.30.0
	(May - June 2000)
E. Fouladvand	(Sharif University, Tehran, Iran)	23.15.0
	Driven many-body systems (Aug. - Sept. 2000)
Dr. T. Ihle	(University of Minnesota, Minneapolis, USA)	23.30.0
	Mesoscale simulations of hydrodynamics
	(Sep. - Oct. 2000)
Dr. J. Santos	(TU München) Reptation dynamics (Oct. 2000)	23.30.0
Dr. W. Gozdz	(Polish Academy of Sciences, Warsaw, Poland)	23.30.0
	Membrane shapes (Oct. - Dec. 2000)