Joseph L. Hunter

Ultrasonic and Hypersonic Studies of Vibrational Relaxation in Benzene

Ultrasonic absorption measurements have been made in benzene over the frequency range from 7 to 570 Mc/sec at 6° and 25° C. Velocity measurements at both ultrasonic and hypersonic frequencies also were made in the temperature range 6°–60°C. The observed relaxation effects may be interpreted in terms of double relaxation of the vibrational specific heat. Analysis of the absorption data indicates that the vibrational specific heat associated with all but the lowest vibrational mode relaxes in the ultrasonic range, with the measured relaxation frequency varying from 510 Mc/sec at 6°C to 560 Mc/sec at 25°C. Correlation of the ultrasonic and hypersonic data also indicates a second relaxation frequency (associated with the lowest vibrational mode) of the order of 10 Gc/sec.

Ultrahigh‐Frequency Ultrasonic Absorption Cell

An ultrasonic absorption cell has been constructed that maintains parallelism sufficient for absorption measurements well above 500 Mc/sec. Constructional details are described and results presented showing absorption in benzene, dioxane, and polysiloxane at 510 Mc/sec. The system as a whole will permit an attenuation of about 35 dB before the signal is lost in the noise. The absorption in benzene divided by the square of the frequency is substantially lower than values obtained at low frequencies, and indicates a relaxation frequency of approximately 600 Mc/sec.

Relaxation experiments in binary mixtures of Kneser liquids. II

In a paper published in this Journal in 1971, relaxation curves for many binary mixtures of CS2, CH2, Cl2, CH2Br2, C6H6, and CCl4 were presented, but no attempt was made to interpret these results quantitatively. It has always been our intention to interpret these results in a following paper. In the present paper, it is found that reaction rates for vibrational relaxation energy exchange varied considerably with concentration in many binary mixtures. For those mixtures where variations of reaction rates are strongly dependent on concentration, an empirical formula has been found. Although this variation in reaction rates was empirically determined at low frequency, theory shows that reaction rates, though concentration dependent, should be independent of driving frequency. We have therefore used the empirically determined reaction rates in dichloromethane‐carbon tetrachloride to predict relaxation curves of this mixture over frequencies and concentrations where the strength of relaxation is very sizable.

Relaxation Phenomena in Electrolytic Solutions

Sound velocity and absorption measurements were made in a number of aqueous electrolytic solutions, principally the 2–2 valent sulfates. Ultrasonic pulse measurements were used in the frequency region 30–900 MHz, and the technique of Brillouin light scattering was employed in a number of measurements between 6000 and 7000 MHz. In several of the solutions studied, the ultrasonic data indicate the existence of two relaxation processes with relaxation times lying in the range 5 × 10−9–2 × 10−10 sec. These two relaxations, considered together with the previously observed long relaxation time process, can be interpreted in terms of the Diebler–Eigen model of ionic association which predicts three relaxation processes. The shortest relaxation time process was identified with the diffusion controlled approach of the hydrated ions. The calculation of the appropriate reaction rate constants using data by other workers below 50 MHz and the present data above 50 MHz was carried out. The results are compared with ear...

Vibrational Relaxation in Liquid Dichloromethane

Measurements of ultrasonic absorption have been conducted in liquid dichloromethane over a frequency range of 30 to 510 Mc/sec, and over a temperature range of −60° to 25°C. The relaxation is exceedingly strong. The experimental values are predictable by the assumption that the vibrational specific heat of the molecule is the chief cause of the absorption, if it is assumed that the first vibrational mode is inactive, as hypothesized by Andreae. The frequencies of relaxation, arrived at through a curve of best fit, are 171 Mc/sec at 25°C, 147 Mc/sec at 0°C, and 117 Mc/sec at −60°C. The measured velocity dispersion is in agreement with the magnitude of the relaxing specific heat of vibration.

Classical Ultrasonic Absorption in Liquid Gallium

Because of the high values of wave velocity and density and the low value of viscosity in gallium, the Stokes‐Kirchoff value of α/f2 is only 1.6×10−17 sec2 cm−1, of which approximately 75% is due to the thermal term. Measurements at 270 Mc/sec over a temperature range of 30°–60°C have checked this value to an accuracy of 2%. Velocity measurements were also carried out over this range. The accuracy of the measurement and the certainty of the physical constants entering into the classical value are discussed. The measurement was made over an inch of path length, and the attenuation in this distance was approximately 29 dB. As far as is known, this is the lowest value of ultrasonic absorption occurring in liquids. [Work supported in part by the U. S. Office of Naval Research.]

Propagation of Sound in Critical Binary Liquid Mixtures near the Critical Point

It is well known that very large attenuation of sound is produced in critical binary liquid mixtures as the critical temperature is approached, but there is disagreement whether this large attenuation is due to scattering or to true absorption. We have scanned the radiation emanating from a scattering cell containing a binary liquid mixture at several closely spaced temperatures near the critical temperature. Whereas there is a very large drop in the strength of signal at zero bearing angle, there is no consequent rise at any other angle, as there is in the case of true scattering (e.g., as has been experimentally found in scattering from air bubbles.). We conclude that the attenuation measured by previous observers should be assigned primarily to true absorption, and that the effect due to scattering is negligible. [Work supported by the Office of Naval Research, Acoustics Branch.]

Measurements of Ultrasonic Absorption in Several Liquids in the 1‐GHz Region

Measurements of ultrasonic absorption by direct pulse technique have been made in some 15 liquids in the frequency region running from 0.8 to 1 GHz. Some of the liquids used have also been investigated by Plass at 0.4 and 1.5 GHz, and in these liquids, agreement between the two measurements is very good. In general, the measurements confirm the expected values in those liquids that have given previous evidence of relaxation and agree with the low‐frequency value in those liquids that have not. [Work sponsored by the Office of Naval Research.]

Acoustic Absorption and Dynamic Viscosity in Polymers

We have obtained results that show that acoustic absorption in linear polysiloxanes varies with viscosity and frequency in a unique manner. At any constant temperature, a significant relaxation is found, with a broad spectrum of relaxation frequencies. On the other hand, the absorption shows a monotonic rise with increasing viscosity up to very high viscosities. For this reason, there is some doubt as to how to treat the data by means of reduced variables. Shear impedance has recently been measured in the same liquids over a wide range of frequency and viscosity, and dynamic viscosities have been computed. We have also attempted to show that there exists in general a simple relaxation between the acoustic absorption and the dynamic viscosity. It is not clear, however, that the two are uniquely related. [Work sponsored by the Acoustics Branch, U. S. Office of Naval Research.]

Ultrahigh‐Frequency Pulse Measurements in Relaxing Liquids

Ultrasonic‐absorption measurements have been carried out to 550μ Mc/sec in several relaxing as well as nonrelaxing liquids, including carbon disulfide and halogenated methanes. Problems of measurement where the absorption is extremely high (of the order of 25 000 dB/in. are discussed and absorption data are presented. Marked contrasts are found between the behaviors of different relaxing liquids over wide frequency and temperature ranges. The data are interpreted in terms of the vibrational specific heats of the liquids, with particular attention given to the problem of deciding between single and multiple relaxation times. The apparatus was a two‐crystal absorption cell with special refinements to preserve parallelism. [This work was supported by the Acoustics Branch of the U. S. Office of Naval Research.]