Pak-Kon Choi

A NEW LIGHT SCATTERING TECHNIQUE FOR A WIDE-BAND RIPPLON SPECTROSCOPY AT THE MHZ REGION

A new light scattering system was developed to investigate the propagation of liquid surface waves over a wide wave‐number range 4×104–3×106 m−1 (or 10 kHz–6 MHz in frequency). Thermal fluctuation of liquid surface displacement excites a surface tension wave called a ‘‘ripplon’’ whose behavior yields information on the dynamics of the liquid surface. The transmission diffraction method and some optical and electrical improvements are successfully applied to the light scattering system and the upper limit of the frequency measured was extended about two orders higher than the conventional one. The wave‐number dependence of the ripplon frequency and damping constant was measured for several kinds of liquids and the results agreed well with the classical hydrodynamic theory.

Ultrasonic spectroscopy in bovine serum albumin solutions

Ultrasonic absorption and velocity spectra in bovine serum albumin (BSA) aqueous solutions have been measured at 20 °C over the broad frequency range 0.1–1600 MHz in the pH range 1.5–13.2. Five different techniques were used: the plano–concave resonator, plano–plano resonator, pulse–echo overlap, Bragg reflection, and high‐resolution Bragg reflection methods. The absorption spectrum at neutral pH was well fitted to the relaxation curve assuming a distribution of relaxation frequency with a high‐frequency cutoff and long low‐frequency tail. The relaxation behavior was interpreted in terms of various degrees of hydration of BSA molecules. At acid pH’s, excess absorption over that at pH 7 was explained by double relaxation. The pH dependences of the relaxation frequency and maximum absorption per wavelength showed that the relaxation at about 200 kHz was related to the expansion of molecules and that at 2 MHz resulted from the proton transfer reaction of carboxyl group. At alkaline pH’s, the excess absorptio...

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.

Observation of cavitation bubbles and acoustic streaming in high intensity ultrasound fields

We observed the behavior of acoustic cavitation by sonochemical luminescence and ultrasound B-mode imaging with ultrasound diagnostic equipment in a standing-wave ultrasound field and focused ultrasound field. Furthermore, in order to investigate the influence of acoustic streaming on acoustic cavitation bubbles, we performed flow analysis of the sound field using particle image velocimetry. We found that acoustic cavitation bubbles are stirred by circulating acoustic streaming and local vortexes occurring in the water tank of the standing-wave ultrasound exposure system. We considered that the acoustic cavitation bubbles are carried away by acoustic streaming due to the high ultrasound pressure in the focused ultrasound field.

Anomalies of ultrasonic velocity in high-Tc ceramics YBa2Cu3Oy and BiSrCaCu2Oy

Abstract Ultrasonic longitudinal velocity and adsorption have been measured in ceramic superconductors YBa2Cu3Oy with various porosity and also in BiSrCaCu2Oy. A velocity drop of about 400 ppm was found at Tc only in the measurements on cooling. The magnitude of the velocity drop is anomalously large compared with the value expected from the thermodynamics. A hysteresis of velocity with respect to temperature was observed in all the samples studied. It is suggested that some structural change at pore size level is responsible for this phenomenon.

New ultrasonic resonator method using optical diffraction for liquids

A new resonator method has been developed for measuring ultrasonic absorption in liquids in the frequency range 0.3–10 MHz. This method utilizes Raman–Nath light diffraction to detect a resonance spectrum of standing waves in a cylindrical cavity. Comparison with the spectrum obtained by conventional resonator method demonstrates that distortion of plane‐wave peaks by higher modes can be avoided with the present method. The use of a concave reflector is found to reduce a cavity loss considerably, enabling us to measure absorption down to a few hundred kHz. Results for water obtained using three types of reflectors with curvature radius 100–400 mm are shown.

Surface-wave modes on soft gels

To understand behavior of surface waves on sols and gels, the dispersion relation of surface modes on isotropic materials with both surface tension and elasticity is studied. The dispersion relation consists of two separate branches: one corresponds to the truly localized Rayleigh-type modes which can exist only at low frequencies, while the other, which is connected continuously to the capillary modes, gives pseudo-surface waves that are damped, radiating elastic transverse waves into the bulk of the medium just like the ones known in surface waves on elastically anisotropic materials. The result is compared with recent experiments on sols and gels.

Sol-Gel Transition in Gelatin Observed with Surface Waves

Surface wave velocity and absorption were measured in the frequency range 250–750 Hz in gelatin solution at around the sol–gel transition temperature. We propose a dispersion relation for surface wave propagation on gel, which takes surface tension and viscoelasticity into account. The results obtained using the dispersion relation provide the shear elasticity and viscosity in the gelation process. The shear elasticity and viscosity were interpreted within the framework of the percolation theory. The critical exponents obtained experimentally t=1.7–2.2 and s=0.7–0.8 are in good agreement with the percolation theory.

A plano-concave resonator for ultrasonic absorption measurements

A new ultrasonic resonator method is described for measuring absorption in the frequency range 0.1-1.5 MHz in liquids. The method uses a plano-concave resonator with 56 mm diameter which shows high-Q characteristics enabling absorption measurements down to 0.1 MHz to be carried out. A resonance spectrum is detected using Raman-Nath light diffraction instead of the quartz transducer. The results in water and benzene demonstrate the resolution of the apparatus. Absorption spectra of methyl and ethyl formate are presented.

Rotational isomerism and ultrasonic relaxation in 1,2‐dibromoethane

Three different ultrasonic techniques, pulse echo, HRB (high‐resolution Bragg reflection), and Brillouin scattering were used to measure the sound velocity and absorption in 1,2‐dibromoethane at 11, 20, and 30 °C over the frequency range from 3 MHz and 6 GHz. The observed results showed a typical spectrum for a single relaxation process, from which the relaxation frequency and strength were determined. The temperature dependence of the relaxation frequency and strength suggested that the activation energy for the gauche molecule is 3.0±0.5 kcal/mol and that the energy difference is 1.3±0.2 kcal/mol, respectively. The magnitude of the relaxation strength was successfully described by taking the effect of the volume change into account, and ΔV/V was estimated to be 0.05.