Apparatus for real-time acoustic imaging of Rayleigh-Benard convection

We have designed and built an apparatus for real-time acoustic imaging of convective flow patterns in optically opaque fluids. This apparatus takes advantage of recent advances in two-dimensional ultrasound transducer array technology; it employs a modified version of a commercially available ultrasound camera, similar to those employed in non-destructive testing of solids. Images of convection patterns are generated by observing the lateral variation of the temperature dependent speed of sound via refraction of acoustic plane waves passing vertically through the fluid layer. The apparatus has been validated by observing convection rolls in both silicone oil and ferrofluid.Comment: 20 pages, 11 figures, submitted to the Review of Scientific Instrument

Globule to Helix Transition in Sodiated Polyalanines

International audienceThe structures of sodiated poly-alanine peptides containing 8-12 residues are investigated using infrared multiple photon dissociation (IRMPD) spectroscopy and classical and quantum modeling. Calculations indicate that the a-helical structure is the most stable conformation for the peptides whatever their size. The IRMPD spectra provide evidence for the coexistence of helical and globular shapes for Ala(8)Na(+), and possibly for Ala(9)Na(+) The turning point from globule to helix is thus found at Ala(8-9)Na(+). The N-H and O-H stretching region allows identifying a new spectroscopic pattern typical for alpha-helical structures of polyalanines