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What is 'Soft Matter'?

A rather large group of mostly organic materials is called soft condensed matter, for instance polymers, supramolecular structures of small organic molecules, liquid crystals, colloids, emulsions, biopolymers and biomembranes. Also included are structured and "interface dominated" materials such as microemulsions, block copolymers, foams, etc. All these materials are easily influenced by small external forces, they are "soft".

Well Known: Hard Condensed Matter

Large interatomar/intermolecular interaction energies (200-800 kBT/Molecule)
    due to electrostatic, metallic or covalent bonds
    stable if subject to high pressures, high temperatures, high electric fields, ...

Soft Condensed Matter

Weak Intermolecular Energies (< 25 kBT/Molecule)
    van der Waals and electrostatic force
    hydration and hydrophobic Force
    molecular size and shape
    entropic effects
    cooperative effects
When Does which Force Win? ... Very Complex Systems!
Intriguingly, a small change of one external parameter can induce a phase transition. For instance, a very prominent application is the liquid crystal display (LCD) used in watches or computer monitors. Still, many of the astonishing mechanical, electrical or optical properties found in nature are not yet realized in technical systems. Because they are not yet understood.

Research Topics

Biomimetic Systems

Even though Nature uses only very few chemical elements to build living matter (mainly H, C, N, O and P, with a little bit of Na, K, Cs, Cl, F, J, Sr, Ca, Mg, Mn, Fe, Zn) biomaterials are very complex. The cell of a bacterium consists of about 6000 different molecules. However, most biomolecules are just variations of a few basic molecular groups, mainly amino acids, nuclear bases, fatty acids, polyenes, phosphates and sugas. Biomaterials are self-organizing materials. The shape of the molecular groups, and the arrangement of hydrophobic, hydrophilic and charged groupes determine the supramolecular structures.

Structure of Self-Organized Interfaces

Monolayers at the Air Water Surface serve as model systems to probe the lateral interactions. In a Langmuir Trough many external parameters can be adjusted: temperature, area per molecule, content of salt, polymers or proteins in the water, organic molecules in the air side, etc. Additionally, the fluid interface is a large self-annealing surface, which is easily accessed by many techniques. We did and do research to address the following questions:
The structure within the many phases of monolayers of fatty acids or lipids. These small molecules consist of hydrophilic and hydrophobic molecular groups (cf. sketch on the left). They serve as model systems for cell membranes (phospholipid bilayers). 1996 1991
Specific protein binding to lipid monolayers with incorporated ligand molecules to mimic processes at cell membranes 1991
Scaling behaviour, counter ion incorporation & swelling of grafted polyelectrolytes. Polyelectrolytes, ie charged polymers, are some of the high-tech materials of everyday life. This industry was subject to tremendous changes in the last years, due to scientific progress. Applications include waste water treatment, dyes, cosmetics, diapers, food industry and paper making. (Our group focusses somewhat on those polymers relevant for pulp and paper.) 1998 1997
Unspecific adsorption of polymers and artificial proteins (radius of surface >> radius of the swollen polymer coil). What is the conformation, & how can we influence it? Is it equilibrium, or kinetically trapped? 2001 1999
Nanocolloids have about the same radius as a swollen polymer coil. Like polyelectrolytes they are multiions. One may imagine two extrme cases, the polymer is either wrapped around the nanocolloid, or the nanocolloids are aligned on the polymer like beads on a string (bridging flocculation).
Gold nanocolloid/polymer interaction is probed by measuring the gold-plasmon resonance which is an indicator for the nanocolloid surroundings (Mie-theory). Thus, it is even possible to measure polymer adsorption kinetics in solution.
2001 1999

Physics in Confined Geometry

Ultrathin layers:   structure, dynamics, swelling, optical properties
Polyion multilayers are technically interesting because the film composition parallel to the surface normal can be controlled on the nm-level. They are prepared by alternating adsorption of polycations and polyanions.
1999 1998 1997
Crystal nucleation and growth in two dimensions are (theoretically) simpler than in three dimensions, because there are less free parameters. The most intriguing and least understood problem concerns nucleation: the system is subject to large-scale fluctuations between saturated and supersaturated domains before a stable nucleus is formed.
Phase transitions, surface induced ordering or demixing and multilayer formation of macromolecules is a fun problem. Especially if the forces from the two interfaces and the inter- and intraparticle forces are of the same order of magnitude, a rich phase behavior can be observed. 1999 1997
Wetting and nanopatterning induced reversibly by adjusting the molecular density (on fluid substrates) or the temperature (on solid substrates) is another example for the self-organization of macromolecules.
2001 2000

© Mar 12 2001, Heiko Ahrens , Christiane A. Helm

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