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AKB: Biologische Physik

AKB 30: Poster Session I

AKB 30.9: Poster

Monday, March 27, 2006, 15:30–18:00, P1

Nonlinear Thermophoresis — •Stefan Duhr and Dieter Braun — Noether Group on Dissipative Microsystems, Applied Physics, Ludwig Maximilians Universität München, Amalienstr. 54, 80799 München, Germany

Thermophoresis is the drift of molecules in a temperature gradient. In the past, the effect was phenomenologically based on Onsager non-equilibrium thermodynamics: thermophoretic drift velocity rises linearly with the applied temperature gradient. We experimentally check this relation using fluorescence single particle tracking in microthermally heated microfluidics. For small particles and flat temperature gradients, the relation holds. Molecule concentration follows an exponential function of the applied temperature difference over two orders of magnitude, very similar to a Boltzmann-distribution. This confirms entropic, thermodynamic models of thermophoresis.

For large particles, we find a nonlinear drift relation for aS_T∇ T>1, violating the Onsager foundation of thermophoresis. In the limit relation, a is the molecule radius, S_T the Soret coefficient and ∇ T the temperature gradient. Thermophoresis is linear if thermophoretic directed drift is slower than diffusive random drift. Or in the thermodynamic description of thermophoresis, the interfacial enthalpy is symmetric within kT. Compared with the zeta-potential limit of electrophoresis, the limit of thermophoresis can be avoided by the experimenter by simply reducing the temperature gradient.