3,171 research outputs found
Periodic forcing in viscous fingering of a nematic liquid crystal
We study viscous fingering of an air-nematic interface in a radial Hele-Shaw
cell when periodically switching on and off an electric field, which reorients
the nematic and thus changes its viscosity, as well as the surface tension and
its anisotropy (mainly enforced by a single groove in the cell). We observe
undulations at the sides of the fingers which correlate with the switching
frequency and with tip oscillations which give maximal velocity to smallest
curvatures. These lateral undulations appear to be decoupled from spontaneous
(noise-induced) side branching. We conclude that the lateral undulations are
generated by successive relaxations between two limiting finger widths. The
change between these two selected pattern scales is mainly due to the change in
the anisotropy. This scenario is confirmed by numerical simulations in the
channel geometry, using a phase-field model for anisotropic viscous fingering.Comment: completely rewritten version, more clear exposition of results (14
pages in Revtex + 7 eps figures
Level attraction in a microwave optomechanical circuit
Level repulsion - the opening of a gap between two degenerate modes due to
coupling - is ubiquitous anywhere from solid state theory to quantum chemistry.
In contrast, if one mode has negative energy, the mode frequencies attract
instead. They converge and develop imaginary components, leading to an
instability; an exceptional point marks the transition. This, however, only
occurs if the dissipation rates of the two modes are comparable. Here we expose
a theoretical framework for the general phenomenon and realize it
experimentally through engineered dissipation in a multimode superconducting
microwave optomechanical circuit. Level attraction is observed for a mechanical
oscillator and a superconducting microwave cavity, while an auxiliary cavity is
used for sideband cooling. Two exceptional points are demonstrated that could
be exploited for their topological properties.Comment: 5 pages, 4 figures; includes Supplementary informatio
Waterborne GPR survey for estimating bottom-sediment variability: A survey on the Po River, Turin, Italy
We conducted an integrated geophysical survey on a stretch of the river Po in order to check the GPR ability to discriminate the variability of riverbed sediments through an analysis of the bottom reflection amplitudes. We conducted continuous profiles with a 200-MHzGPR system and a handheld broadband EM sensor.Aconductivity meter and a TDR provided punctual measurements of water conductivity, permittivity, and temperature. The processing and interpretation of the GEM-2 and GPR data were enhanced by reciprocal results and by integration with the punctual measurements of the EM properties of the water. We used a processing flow that improved the radargram images and preserved the amplitude ratios among the different profiles and the frequency content at the bottom reflection signal.We derived the water attenuation coefficient both from the punctual measurements using the Maxwell formulas and from the interpretation of the GPR data, finding an optimal matching between the two values. The GPR measurements provided maps of the bathymetry and of the bottom reflection amplitude. The high reflectivity of the riverbed, derived from the GPR interpretation, agreed with the results of the direct sampling campaign that followed the geophysical survey. The variability of the bottom-reflection-amplitudes map, which was not confirmed by the direct sampling, could also have been caused by scattering phenomena due to the riverbed clasts which are dimensionally comparable to the wavelength of the radar pulse
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