Katsuo Negishi

Existence of Negative Group Velocities in Lamb Waves

It is known only fragmentarily that negative group velocities exist in some lower modes of Lamb waves whose phase velocities are very high. This paper reveals numerically that the existence of negative group velocities depends not only on the frequency-thickness product but also on Poissons ratio σ of the plate material. Negative group velocities exist in S1 mode for σ<0.45, and in A2 mode for σ<0.31. In addition, S3, S4, S6 and A5 modes exhibit negative group velocities in respective narrow ranges of σ.

Vibrational double relaxation in liquid pyridine

Ultrasonic and hypersonic property of liquid pyridine has been studied at 20° and 30°C with three techniques: pulse‐echo‐overlap, high‐resolution Bragg reflection, and Brillouin scattering. The sound velocity observed over the frequency range from 3 MHz to 7 GHz suggests two relaxation regions, one centered at 762 MHz and the other at 3.9 GHz at 20°C. The velocity‐dispersions in the UHF and in the hypersonic ranges are 94 m/s and 65 m/s, respectively. The same type of vibrational double relaxation as observed in liquid benzene is hypothesized. Quantitative analysis of the relaxation strength indicates that the vibrational specific heat associated with both the lowest and the second lowest modes relaxes in the hypersonic range and all the rest, in the UHF range.

Vibrational–translational relaxation in liquid chloroform

Ultrasonic measurements were made in liquid chloroform over the frequency range from 3 MHz to 5 GHz by means of three experimental techniques, pulse‐echo overlap, high‐resolution Bragg reflection, and Brillouin scattering. The observed velocity dispersion revealed two relaxation processes, one at 650 MHz and the other at 5.1 GHz at 20 °C. They are interpreted in terms of vibrational–translational relaxation. Quantitative analysis of specific heat shows the lowest (261 cm−1) and the second lowest (366 cm−1) fundamental vibrational modes should have a common relaxation time at 50 ps and the group of all above the third mode (667 cm−1) at 290 ps. The present results are combined with recent data obtained by Laubereau et al. with the picosecond spectroscopy technique; a diagram illustrating V–T and V–V energy transfer is presented. A brief comment is given also on V–T and V–V processes in dichloromethane.

Strobo-Photoelastic Visualization of Lamb Waves with Negative Group Velocity Propagating on a Glass Plate

An elastic wave with negative group velocity transports wave energy in the direction opposite to the phase velocity. This phenomenon has been predicted in certain modes of Lamb waves. This is still difficult to believe, despite the fact that some experimental evidence has been reported so far. In this paper, direct evidence of the negative group velocity is presented which was obtained by means of visualization of pulsed Lamb waves propagating on a glass plate in A2 mode.

Ultrasonic and Hypersonic Studies of Relaxations in Ethanol-Water Mixtures

The sound velocity and absorption in ethanol and water mixtures are measured with a new technique utilizing high-resolution Bragg reflection in the frequency range from 150 MHz to 500 MHz. The absorption in the range 20?150 MHz as well as the hypersonic velocity in the range 3?7 GHz are also measured. Relaxation spectra over the range 10 MHz?10 GHz at the temperatures of 15? and 20?C suggest two relaxations, one at about 100 MHz and the other above 1 GHz. The relaxation strength of the former has a maximum at the ethanol concentration of about 40 wt%(?20 mole %), whereas the latter at 50 wt%. The two-state model A+Bm\rightleftarrowsABm is applied to interpret the concentration dependence of the relaxation strength at 100 MHz.

Ultrasonic studies of relaxation in dichloromethane and dibromomethane with high‐resolution Bragg reflection method

A method of high‐resolution Bragg reflection was utilized to measure ultrasonic velocity and absorption in liquid dichloromethane and dibromomethane near the room temperature, over the frequency range from 60 to 700 MHz. In both liquids, considerable velocity dispersion and decrease in α/f2 were observed and described with single relaxation frequencies, 192 MHz for dichloromethane and 393 MHz for dibromoethane at 20° C. The observed relaxation strengths were roughly consistent with the theoretical values calculated from the hypothesis of the vibrational relaxation associated with all but the lowest mode. The hypersonic velocities were measured in dichloromethane with the technique of Brillouin scattering. No dispersion was observed between 700‐MHz and gigahertz region. The second relaxation involved with the lowest mode was expected to be in the range higher than 10 GHz, and the volume viscosity was estimated to be ηv?3ηs, the shear viscosity.

Measurement of UHF Ultrasonic Attenuation in Liquids by Optical Heterodyne Method

A new optical heterodyne system is established which is capable of measuring acoustic attenuation at frequencies too high for an ordinary pulse-echo technique and yet too low for Brillouin scattering. The direction of the light scattered by injected phonons suffers an angular spread introduced by the phonon decay. The technique employed is to measure the angular width which determines the attenuation constant. Experiments in water are described in the range 300–500 MHz. The value of α/f2 obtained is 25 (±0.5) ×10-17 sec2/cm. This technique is useful when the decay distance of the acoustic wave is shorter than 0.5 mm.

Effect of near‐resonant energy transfer on vibrational relaxation in liquid dichloromethane–benzene mixtures

The velocity and absorption of sound waves are measured in liquid dichloromethane–benzene mixtures in the frequency range from 3 MHz to 6 GHz using three ultrasonic techniques, high‐resolution Bragg reflection, Brillouin scattering, and pulse‐echo overlap. Vibrational double relaxation is observed in every mixture studied. The higher‐frequency relaxation is assigned to the lowest mode of benzene, and the lower‐frequency one to all but the lowest mode of both molecules. These modes of both molecules are shown to be coupled with each other by the near‐resonant intermolecular vibrational–vibrational energy transfers between the second lowest mode (704 cm−1) of dichloromethane and the modes (707, 675 cm−1) of benzene. The concentration dependence of the relaxation frequency is analyzed with the binary collision theory, and the rates of vibrational–translational energy transfer in collisions between different kinds of molecules are estimated.

Viscoelastic and Structural Relaxations in Liquid Cyclohexanol

The ultrasonic velocity and absorption were measured in liquid cyclohexanol over the frequency range from 80 MHz to 7 GHz at temperatures of 20, 25, 30, 40 and 50°C, using high-resolution Bragg reflection and Brillouin scattering techniques. A double relaxation process was observed, one at 450 MHz at 25°C being viscoelastic and the other at 5 GHz being structural. The structural relaxation was interpreted in terms of the formation and destruction of hydrogen-bonded dimers. The enthalpy difference between the monomer and dimer was estimated to be 5.7 kcal/mole of dimer.

A Model Experiment of Wave Propergation in Acoustic Microscope

Using stroboscopic photoelastic method together with schlieren technique, simultaneous visualization of ultrasonic waves in water and glass has been made as a model experiment of wave phenomenon occurring in a scanning acoustic microscope. Evidence of reradiation into water from leaky surface acoustic waves excited by the incident converging ultrasonic pulses of 5.5 and 1.2 MHz is shown.