Implementation of the Laser Diffraction Technique for Acoustic Cavitation Bubble Investigations

Knowledge of the acoustic cavitation cloud would be useful for improving ultrasound reactor design. Among the characterisation techniques, few are adapted to bubble investigations in an intense ultrasound field. Some problems raised by these measurements result from interactions between the acoustic pressure wave and the measuring light wave. This paper reports the implementation of the laser diffraction technique to determine the size and volume concentration of bubbles generated by a dipping horn operating at 20 kHz. Measurements were performed with a Malvern 2600 instrument. The size distribution, deduced from the diffraction pattern scattered by the bubble cloud crossed by a laser beam, is disturbed by the acoustic pressure wave involving deviation of a light beam at low diffusion angles (acousto‐optic effect). A bubble size correction procedure based on the subtraction of the light energy due to the ultrasound wave is described. The size measurements, and thus the correction procedure, were validated by a second laser technique based on a different measuring principle: phase Doppler interferometry. The measurement reliability was further confirmed by an original application of laser diffraction based on measurements performed just after sonication. These three methods lead to a mean bubble size (Sauter mean diameter) of about 10 μm at a high ultrasound power input. Concerning the void fraction, only measurements achieved after sonication and by laser diffraction predict a correct estimation of this parameter.     

MR detection of mechanical vibrations using a radiofrequency field gradient

A new method for NMR characterization of mechanical waves, based upon radiofrequency field gradient for motion encoding, is proposed. A binomial B1 gradient excitation scheme was used to visualize the mobile spins undergoing a periodic transverse mechanical excitation. A simple model was designed to simulate the NMR signal as a function of the wave frequency excitation and the periodicity of the NMR pulse sequence. The preliminary results were obtained on a gel phantom at low vibration frequencies (0-200 Hz) by using a ladder-shaped coil generating a nearly constant RF field gradient along a specific known direction. For very small displacements and/or B1 gradients, the NMR signal measured on a gel phantom was proportional to the vibration amplitude and the pulse sequence was shown to be selective with respect to the vibration frequency. A good estimation of the direction of vibrations was obtained by varying the angle between the motion direction and the B1 gradient. The method and its use in parallel to more conventional MR elastography techniques are discussed. The presented approach might be of interest for noninvasive investigation of elastic properties of soft tissues and other materials.     

Probing Proteins in Solution by Xe NMR Spectroscopy

The interaction of xenon with different proteins in aqueous solution is investigated by ¹²⁹Xe NMR spectroscopy. Chemical shifts are measured in horse metmyoglobin, hen egg white lysozyme, and horse cytochrome c solutions as a function of xenon concentration. In these systems, xenon is in fast exchange between all possible environments. The results suggest that nonspecific interactions exist between xenon and the protein exteriors and the data are analyzed in term of parameters which characterize the protein surfaces. The experimental data for horse metmyoglobin are interpreted using a model in which xenon forms a 1:1 complex with the protein and the chemical shift of the complexed xenon is reported (Locci et al., Keystone Symposia “Frontiers of NMR in Molecular Biology VI”, Jan. 9–15, 1999, Breckenridge, CO, Abstract E216, p. 53; Locci et al., XeMAT 2000 “Optical Polarization and Xenon NMR of Materials”, June 28–30, 2000, Sestri Levante, Italy, p. 46).     

Setting up an experimental apparatus for the study of multimodal acoustic propagation with turbulent mean flow

An experiment has been set up to study multimodal acoustic propagation inside a cylindrical duct in presence of a turbulent mean flow. This paper describes the preliminary work which has been found necessary for assembling the experiment together with first measurement results. In order to set up this experimental facility, a high level acoustic source was developed to generate higher propagating modes in the presence of mean flow. A microphonic antenna was designed for detecting the propagating modes. LDV measurements were performed and synchronous detection was used to extract both the mean flow and the acoustic components of the particle velocity. Results of aeraulic measurements are presented. Then, results of acoustic velocity measurements are compared to results obtained from the microphonic antenna.     

Acetylene on Cu(111): imaging a molecular surface arrangement with a constantly rearranging tip

Acetylene on Cu(111) is investigated by scanning tunnelling microscopy (STM); a surface pattern previously derived from diffraction measurements can be validated, if the variation of the STM image transfer function through absorption of an acetylene molecule onto the tip apex is taken into account. Density functional theory simulations point to a balance between short-range repulsive interactions of acetylene/Cu(111) associated with surface stress and longer range attractive interactions as the origin of the ordering.     

Master Equation and Fokker–Planck Equation: Comparison of Entropy and of Rate Constants

A usual approximation of the master equation is provided by the Fokker–Planck equation. For chemical systems with one species, we prove generally that the prediction of the rate constant of the metastable state given by the Master equation and the Fokker–Planck approximation differ exponentially with respect to the size of the system. We show that this is related to the fact that the entropy of the metastable state is not described correctly by the Fokker–Planck equation. We prove that the rate given by the Fokker–Planck equation overestimates that rate given by the Master equation.     

A Proof of Haag-Swieca's Compactness Property for Elastic Scattering States

 It is proved that in any massive relativistic quantum field theory satisfying two-particle asymptotic completeness, all the bounded energy components in the elastic two-particle range of all subsets of states which are excitations of the vacuum state by uniformly bounded observables localized in a given finite region of spacetime are compact in the Hilbert space of states. This result, which is in agreement with Haag-Swieca's conjecture, is also given a more precise form in terms of the rate of decrease of the ``N–dimensional thickness' (or approximation number) of such sets of states when N tends to infinity. A similar computation, valid at arbitrarily high energies, is also given for the massive free-field case. Received: 7 February 2003 / Accepted: 5 April 2003 Published online: 13 May 2003 Communicated by H. Araki, D. Buchholz and K. Fredenhagen