The dynamics and stability of thin layers, membranes, films, interfaces and surfaces of complex fluids is investigated. Of special interest is the relation between dynamical processes within a surface layer and the macroscopic motion of the layer itself. Examples are surface waves in polymers and amphiphilic sol-gel systems. Surface waves are exponentially damped going into the bulk and therefore these excitations probe surface material parameters.
Surface waves at the surface of visco-elastic media basically show a transition from solid behavior (Rayleigh elastic waves with a linear dispersion relation) to liquid behavior (capillary waves with dispersion ω ~ k3/2) depending on the time scale involved. For a fixed frequency this transition can also be obtained as a function of the magnitude of the transient elastic (plateau) modulus (H. Pleiner, J.L. Harden and P.A. Pincus, Europhys.Lett. 7, 383 (1988) and J.L. Harden, H. Pleiner and P.A. Pincus, Langmuir 5, 1436 (1989)).
Surface waves of visco-elastic films or interfaces constitute transverse undulations of the film or interface. The dispersion relation is a very complicated function involving a host of visco-elastic parameters of the film or interface, e.g. the dilational and transverse viscosities, transient elastic moduli, and relaxation times, as well as bending rigidity, static compressional modulus and surface tension.
Quite recently, surface waves at the free surface of ferrogels (magnetic gels) have been studied. Gels are a special case of the visco-elastic media considered above, but, in addition, the magnetic character allows for a distinct influence of an external magnetic field on the dispersion relation: for a given wave vector the frequency is reduced by the field. At a certain threshold field the frequency can become zero and the Rosensweig instability occurs (cf. also Unconventional Ferrofluids).
preprints of the theory group
Other research topics:
Banana and Tetrahedral Phases
Rheology of Complex Fluids
Non-magnetic Liquid Crystalline Polymers and Elastomers
Instabilities in Complex Fluids and Active soft Matter
Magnetic Fluids and Elastomers