arXiv subjects

Ingo Rehberg

Publications and source records attributed to Ingo Rehberg.

42 records · Page 3Linked to original sources

arXiv2002

Swirling Granular Matter: From Rotation to Reptation

A novel effect in a granular material under swirling motion of the container is presented. At low packing densities the material rotates in the same direction as the swirling motion of the container (rotation). At higher densities the cluster of granular material rotates in opposite direction (reptation). The change of the direction of the motion of the cluster takes place at a critical packing density while the diffusion coefficient changes significantly. The measured critical density of the packing is in good agreement with results obtained by molecular dynamics simulation.

cond-mat.stat-mech↗

arXiv2000

Deviations from linear theory for fluctuations below the supercritical primary bifurcation to electroconvection

We report measurements of thermally-induced mean-square director-angle fluctuations below primary supercritical bifurcations to electroconvection of the nematic liquid crystals I52 and "Merck Phase V". For epsilon_mf = V^2/V_c,mf^2 - 1 < -0.1 (V is the amplitude of the applied alternating voltage) we find proportional to epsilon_mf^(-gamma) with gamma given by linear theory (LT). Closer to the bifurcation there are deviations from LT with a smaller gamma and with V_c^2 > V_c,mf^2. For I52 measurements as a function of the conductivity sigma of above V_c^2 suggest a tricritical bifurcation at sigma_t = 4.0 x 10^(-9) Omega^(-1)m^(-1), and (V_c^2 - V_c,mf^2)/V_c^2 increases strongly as sigma_t is approached from above.

cond-mat↗

arXiv1998

Parametrically excited surface waves in magnetic fluids: observation of domain structures

Observations of parametrically excited surface waves in a magnetic fluid are presented. Under the influence of a magnetic field these waves have a non--monotonic dispersion relation, which leads to a richer behavior than in ordinary liquids. We report observation of three novel effects, namely: i) domain structures, ii) oscillating defects and iii) relaxational phase oscillations.

patt-sol↗

arXiv1998

Magnetic Faraday-Instability

In a magnetic fluid parametrically driven surface waves can be excited by an external oscillating magnetic field. A static magnetic field changes the restoring forces and damping coefficients of the various surface waves. This property enables the excitation of both subharmonic and harmonic responses of the standing waves.

patt-sol↗

arXiv1997

Nonlinear Dynamics of a Single Ferrofluid-Peak in an Oscillating Magnetic Field

If a magnetic field normal to the surface of a magnetic fluid is increased beyond a critical value a spontaneous deformation of the surface arises (normal field instability). The instability is subcritical and leads to peaks of a characteristic shape. We investigate the neighborhood of this instability experimentally under the influence of a temporal modulation of the magnetic field. We use a small vessel, where only one peak arises. The modulation can either be stabilizing or destabilizing, depending on the frequency and amplitude. We observe a cascade of odd-numbered response-periods up to period 11, and also a domain of even-numbered periods. We propose a minimal model involving a cutoff-condition which captures the essence of the experimental observations. PACS: 47.20.-k, 47.20.Ky, 75.50.Mm Keywords: magnetic fluid; nonlinear oscillator; subharmonic response; surface instability;

patt-sol↗

arXiv1996

Non-Monotonic Dispersion of Surface Waves in Magnetic Fluids

The dispersion relation of surface waves of a magnetic fluid in a magnetic field is studied experimentally. We verify the theoretically predicted existence of a non-monotonic dispersion relation. In particular, we demonstrate the existence of two different wave numbers occuring at the same frequency in an annular geometry.

patt-sol↗