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DY: Dynamik und Statistische Physik

DY 23: Ferrofluids / Liquid crystals

DY 23.4: Hauptvortrag

Dienstag, 25. März 2003, 12:30–13:00, G\"OR/229

black & mag(net)ic — •Reinhard Richter, Bert Reimann, Alexander Rothert, and Peter Rupp — Experimentalphysik V, Universität Bayreuth

Magnetic fluids – or ferrofluids – are colloidal suspensions of magnetic particles in the nanometres range and thus react susceptible to static and alternating magnetic fields. E.g. a horizontally extended layer of magnetic liquid, subjected to a transversally oriented magnetic field forms liquid spikes when a threshold of the field amplitude is surpassed. We investigate this Rosensweig instability and compare our measurements with predictions of linear stability theory. Because of the subcriticality of the underlying bifurcation the suitable range of amplitudes is only accessible in a transient way by pulsing the magnetic field. The wave number of maximal growth as well as the critical oscillation frequency for subcritical pulses turn out to be in good agreement with theory. In the nonlinear regime we present recent results on the scaling behaviour of the static pattern amplitude versus the magnetic field control parameter, obtained by means of x-ray absorption and magnetic array sensors.

When superimposing an alternating magnetic field on to the static field a quasi one dimensional realization of the instability can be driven to spatio-temporal intermittency. The measured scaling exponents of the order parameter follow in part the ones given by directed percolation.