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Physikalisches Institut

Experimentalphysik IV – Professor Dr. Werner Köhler

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Our research topics (in loose order):

Thermodiffusion (Soret effect)
A temperature gradient across a multicomponent fluid leads to a net mass flux and the build-up of a concentration gradient. This effect is known as thermal diffusion, thermodiffusion, or Ludwig-Soret-effect and is described within the framework of linear nonequilibrium thermodynamics, where Onsager's phenomenological equations constitute a linear relationship between the thermodynamic forces and the fluxes of heat and matter.

Critical polymer blends and forced spinodal decomposition
At the critical point the correlation length of fluctuations diverges and a critical slowing down of diffusion is observed. At the same time the Soret coefficient increases by orders of magnitude and small temperature gradients are sufficient to induce very strong composition changes. Equilibrium phase diagrams loose their conventional meaning.

After crossing the spinodal a polymer blend spontaneously demixes and random spinodal patterns develop that are characterized by a certain length scale. Laser heating allows for a, otherwise hardly possible, control of the demixing morphology by forcing the demixing sample locally into a certain composition state.

Light-induced interaction of ferrofluids
Some ferrofluids can be 'glued' together by irradiation with visible light to form huge micrometer-sized aggregates which contain more than 10^7 colloidal particles. Once the light is switched off, the aggregates dissolve again. The mechanism of this fully reversible effect is not understood.

Optically heated colloids
Metal colloids absorb laser radiation very efficiently and can be utilized as nanometer-sized highly localized heat sources. If dispersed in binary solvents, their temperature field couples to the order parameter of the mixture.

Ternary mixtures
The complexity of diffusion and thermodiffusion processes in multicomponent liquid mixtures grows rapidly with the number of components. While non-isothermal binary mixtures are readily described by two independent coefficients, ternary mixtures
already require six, reflecting the cross-coupling among all diffusion processes.

Non-equilibrium fluctuations
In the presence of a macroscopic gradient of concentration and/or temperature, the thermodynamic variables exhibit also non-equilibrium fluctuations that are drastically different from equilibrium ones. Their intensity is strongly enhanced by several orders of magnitude and grows in particular for small wave numbers. In laboratory experiments their amplitude is quenched by gravity, but under microgravity the limit is only set by the size of the container.

Marangoni convection in microfluidic devices
Marangoni convection along a liquid-gas interface within a microfluidic structure leads to a redistribution of sub-micrometer sized tracer particles. Because of collisions with the meniscus, the particles are trapped within the vortex, whereas other parts of the volume are particle-free. A potential application is sorting of particles according to their size.

We are members of two several microgravity research projects aboard the International Space Station ISS. The DCMIX project aims at the measurement of diffusion and thermodiffusion in ternary mixtures. The NeufDix (Giant Fluctuations) project together with the joint European and Chinese NESTEX project investigate giant fluctuations and non-equilibrium phenomena in soft matter and complex fluids.

Verantwortlich für die Redaktion: Univ.Prof.Dr. Werner Köhler

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