Frieder Eggers

Broad-band ultrasonic measurement techniques for liquids

Various methods for broad-band ultrasonic absorption and velocity measurements in liquids, using CW and pulsed signals, are reviewed. Several different ultrasonic spectrometric techniques are required to cover the wide range of liquid wavelength and attenuation values that exist in the kilohertz to gigahertz frequency range. Advantages and problems of various ultrasonic and related methods are discussed. Sonic cells for liquids and electronic equipment for spectral measurements are presented. Together with applications in relaxation spectrometry, examples of broad-band ultrasonic absorption spectra, utilizing different CW resonator and pulse-modulated travelling wave techniques, are presented.

Ultrasonic Broadband Spectrometry of Liquids A Research Tool in Pure and Applied Chemistry and Chemical Physics

This review is a survey of the many scientific applications of ultrasonic broadbandspectrometry (absorption and velocity measurements with coherent sound waves)in liquids and liquid systems, covering, at present, a frequency range from nearly10 kHz to 10 GHz. Ultrasonic spectrometry has proved to be an almost universalresearch tool in many laboratories, one that is useful for investigation of variouschemical, biochemical, and physicochemical systems. Sound waves traversingliquids induce periodic perturbations in pressure and temperature, which can shiftequilibria, resulting in characteristic sound absorption and velocity dispersionspectra. An analysis of such spectra yields valuable information about thermodynamic and kinetic parameters of the particular system that is often difficult toobtain by other methods. Since such periodic perturbations imposed on the systemare incremental in nearly all cases, the system can be studied under equilibriumconditions. All nonlinear effects (heating, nonconstant fluid compressibility, andothers) are negligible, permitting, for instance, the application of linearized rateequations. In this review, various examples of measured broadband spectra arepresented. Related elementary processes are discussed. Among these are ionicand molecular reactions, including mechanisms of association and complexation,proton transfer, solvation, isomerization, interconversion, side-group rotation,hydrogen-bonding, as well as stacking processes and micelle formation. Specialattention will be given to the extensive research on chemical relaxation.Fundamental early and recent publications are cited and discussed. Many referencesare included with particular emphasis on less well known research and publicationsfrom countries of the former USSR. This review aims at a demonstration of thewidespread applications of modern ultrasonic techniques in many fields ofliquid-state research.

Method for measurement of shear-wave impedance in the MHz region for liquid samples of approximately 1 ml

A method for measuring both the real and imaginary parts of complex shear-wave impedance Zs* for low viscosity liquids using a planar AT-quartz resonator (thickness shear) in an RF bridge network between 2 and 34 MHz, is described. This technique employs a PLL circuit and permits continuous quantitative recording of the sample impedance (probe volume below 1 ml). The mechanical vibration amplitude at the transducer-liquid interface is in the Angstrom range, preventing any heating and structural damage effects. Incremental Zs-resolution is better than 50 kg m-2 s-1 ( approximately 1% of water impedance at 6 MHz), as is demonstrated by test measurements on different newtonian liquids. These measurements show that Zs* values determine quantitatively the shear quartz resonator parameters. This method permits measurements of interface and surface layer properties.

New plano-concave ultrasonic resonator cells for absorption and velocity measurements in liquids below 1 MHz

Ultrasonic resonator cells for liquid attenuation and velocity measurements below 1 MHz with one planar and one concave piezoelectric transducer are described. Such cells with a limited sample volume are needed for broad band ultrasonic relaxation spectrometry and for studies in chemical reaction kinetics, particularly for frequencies below 200 kHz. Measurement and evaluation procedures are described and an example of a broad band spectrum is shown.

Ultrasonic spectrometry of liquids below 1 MHz. Biconcave resonator cell with adjustable radius of curvature

A new ultrasonic resonator technique for liquid attenuation and sound velocity measurements below 1 MHz is described. To reduce undesired diffraction losses at low frequencies the cell utilizes the focussing effect of the concave transducer disc shape that results at low hydrostatic overpressure of the sample liquid. The mode spectrum of the biconcave cell is discussed in analogy to optical resonators. It is shown that this spectrum significantly differs from that of biplanar ultrasonic resonators discussed so far. Measurement and evaluation procedures that consider higher-order modes appropriately, and examples of measurements, are presented.

Oscillating plate viscometer in the Hz to kHz range

A low and audio frequency viscometer with a frequency range 2 Hz to 1 kHz and possibly up to 10 kHz is described, which shears a liquid sample (shear volume < 0.3 ml) between an oscillating glass plate and the wall of a cuvette. Alternating velocity and force exerted on the oscillating plate are measured by a mechanical impedance head, designed for the frequency range 1 Hz to 10 kHz. This device permits the determination of frequency dependent viscous liquid properties. Real and imaginary part of the complex viscosity can be obtained from the complex mechanical impedance. Inertial forces from the oscillating plate are compensated in an adjustable electronic circuit.Significant shear parameters, as the frequency, the exciting wave form and the amplitude, can be set independently. In this way viscous spectral data and also nonlinear properties can be studied, e.g., in aqueous DNA solutions. In addition, processes with time constants τ>0.1 s in (relaxing) viscoelastic systems, which do not obey the Boltzmann superposition principle, can be monitored by means of the (nonlinear) modulation of an LF sampling signal. Some examples of measurements will be shown. Model calculations for water and for glycerol give typical values for the complex impedances to be obtained with this oscillatory rheometric device.

Ultrasonic resonator energy loss determination (chemical kinetics study).

Ultrasonic resonators are used to study chemical kinetics in liquids (relaxation spectrometry). Resonator loss, proportional to Q-1, is frequently determined from half-power bandwidth (amplitude 3 dB down from peak value); but other methods, in particular those utilising the phase characteristic at resonance, are often advantageous. The author describes three different techniques, employing an AM driving signal for the resonator network, affecting the modulation envelope; a resonance peak compensation (RF amplitude and phase of the output signal); the phase slope (group delay) of the output signal. Relations for the one-dimensional ultrasonic resonator, circuit diagrams and measurement examples are given.

Ultrasonic absorption measurements with a millilitre short-path pulse cell

A simple ultrasonic pulse cell with reduced variable pathlength for millilitre liquid samples (frequency range about 15-155 MHz) is described. The liquid cavity made from stainless steel has PTFE seals and gaskets and is resistant against many liquids. Attenuation and velocity measurements have been performed for H2O, CH3OH, C2H5OH and C2H5OC2H5 at 25 degrees C. The measurements, using a quartz effective diameter of 9 mm and pathlengths from 7-21 mm, show that diffraction correction terms are apparently not necessary.

Piston attenuator as RF signal vernier in the 10-300 MHz range: determination and correction of spurious mode errors

Systematic errors in H11-mode piston attenuators from unwanted mode excitation have been measured for CW, and pulse modulated RF signals as well, up to 300 MHz with a new calibration circuit based on a linear RF-IF mixer. The attenuation deviation by spurious mode influence can be calculated from the determined amplitude ratio of superposed modes and can be corrected. In this way piston attenuators, originally designed for IF application, can also be used as RF verniers with 0.01 dB resolution up to 300 MHz and possibly higher with an accuracy depending on mixer linearity in the calibration circuit.

Frequency Range Extension Below 100 kHz and Above 50 MHz for CW Ultrasonic Absorption and Velocity Measurements in Liquids with Resonator Cells

Piezoelectric–liquid resonators with concave reflectors for measurements below 500 kHz with liquid volumes ≤ 100 ml are discussed and half-power (-3 dB) bandwidth spectra for several resonator devices are shown. A nonresonant CW method, achieving crosstalk elimination by algebraic vector subtraction, permits measurements up to 500 MHz (and higher) at reduced bandwidth, as compared to pulse methods, with mW driving power.