Aberystwyth, Wales 7–9 July, 1965 molecular relaxation processes

of each phase. Theoretically, ',his relaxation effect should always be present. But clearly it is not very apparent at the low frequencies and medium intensities used in most liquidprocessing work. An important contribution to the literature on acoustic cavitation which has appeared since my paper was prepared is the excellent review by H. G. Flynn published in Physical Acous'tics. u This contains a good short section on cavitation activity measures which should certainly be closely studied by anyone interested in this subject. The Ultrasonics Committee of the American Manufacturers Association has now standardized on a chemical method of assessing ultrasonic cleaning

activity. We have already considered this method, and decided that while it has some points in its favour, it cannot be ideal for this purpose. The experimental justification for using this method will be published soon. We welcome Bradfield's contribution to the discussion of these problems of measurement in liquids 13 and agree that the correspondence columns of this journal could well be used for discussion of these problems, which are certainly attracting much interest just now. E. A. NEPPIRAS Physics Department Imperial College, London, S.W.5. REFERENCES

1. NEPPIRAS, E. A., UltrasotHcs, 3, 9 (1965). 2. POHLMANN, R., Acustica, 10, 217, 229 (19601. 3. BROWN, B., and GOODMAN, J. E., "High

intensity ultrasonics Industrial applica-

tions," lliffe (1965). 4. NEpPIRAS, E. A., Soriet Ph)'sics Acoustics, 8, 4 (1962). 5. NFPPIRAS, E. A., Report to Scientific Instrument Manufacturers" Association, January, 1902. 6. NI-pPIRAS, E. A., and PARROTt, J., Fifth International Congress on Acoustics, Liege (September, t965). Paper No. D51. 7. HUNT, F. V., Private Communication (April, 1965). g. BOU('HER, R. M. G., and POLANSKY, B., "The measurement of ultrasonic cavitation." Report of Macrosonics Corp., Carterct, New Jersey, U.S.A. (1964). 9. DEGROIS, M., and BADILAN, B., Co#~lple.v Remlus de /'Academic des' Sciences, 254, 231. 837, 1213, 1943 (1962) and 257, 2409 (1963). 10. DE¢;ROIS, M., and BADILAN, B., Fourth International Congress on Acoustics, Copenhagen (1962). Paper No. K44. 1. FLYNN, H. a . , in MASON, W. P. (Ed.), "Physical acoustics," IB, Academic Press, New York (1964}. Chapter 9. 2. NIPPIRAS, E. A., Ultrasonics, 3, 157 (1965). 3. BRAt)Villi), G., UItraso#tics, 3, 100 (1965L

CONFERENCE REPORTS Aberystwyth, Wales 7-9 July, 1965 MOLECULAR RELAXATION PROCESSES The molecular relaxation processes discussed fell into two main categories. Firstly structural or bulk relaxation in liquid and glassy-state systems, in which the main lines of investigation are dielectric relaxation, nuclear spin resonance and ultrasonic dispersion and absorption. In the second category are vibrational and rotational relaxations in liquids and gases, and here ultrasonics has been one of the most powerful experimental methods for research. The emphasis at the symposium was more on experimental results and their interpretation in terms of the chemical nature of the systems than on experimental procedures, and most of the work used well-tried techniques. Litovitz (Catholic University, Washington) stressed the connection which must exist between the various macroscopic relaxation times obtained from dielectric relaxation, nuclear magnetic resonance and ultrasonic measurements in liquids. Although ultrasonically more than one relaxation process is present, the structural relaxation time must be connected, like the dielectric and n.m.r. relaxation times, with the microscopic lattice relaxation time or "jump time." A major difficulty was that some experimental techniques appeared to give a distribution of relaxation times, and others a single relaxation time for processes which should be directly connected with the "jump time." Professor Litovitz showed that a combination of the Arrhenius and "free-volume'" expressions for viscosity could qualitatively explain the results, and convincingly demonstrated that further

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analysis and comparison of relaxation data from various methods will throw light on the problems of liquid state theory. Other papers by Schwartz, Atkinson, Barlow et al and Jones showed that ultrasonic techniques with both longitudinal and shear waves are being used to investigate shear, structural and rotational isomeric relaxation in a wide range of liquid and glassy-state systems. Perhaps the most important work reported dealt with vibrational and rotational relaxation. It appears that the considerable uncertainty and disagreement between experimental results and between experiment and theory in this field has been largely resolved. As Millikan (General Electric Research Laboratory, Schenectady, N.Y.) pointed out in his introductory review paper, the fault has mainly lain with the experimentalist. The reasons chiefly arise from the inability of the older techniques, namely shock tube and ultrasonic methods, to overlap in temperature range and the underestimation of the importance of certain impurities, e.g. water vapour, in catalytically shortening the relaxation times. However the development of further techniques such as flash photolysis, fluorescence and the spectrophone and the refinement of experimental procedure with respect to purity has radically changed the situation over the last five years. The temperature variation of relaxation time given by the Landau-Teller theory can now be shown to hold for most gases over an impressive range of temperature down to room temperature. Further-

U L T r A S O N i c s ~ O c t o b e r - D e c e m b e r 1965

more, the theories often successfully account for the effect of the reduced mass of the colliding pair upon the relaxation time. However, some systems, notably those containing molecules with hydrogen atoms, still show anomalous behaviour. One such example was given by Cottrell, who presented some ultrasonic dispersion measurements on carbondioxide noble gas-mixtures. Inert gas molecules appear to be much less efficient at de-exciting carbon dioxide in collisions than is predicted by the standard theories. He suggested that the noble gas interaction potential is unusually soft. Finally, it is of interest to note one unusual experimental technique described by Campargue, A supersonic molecular beam is generated by directing the core of a nozzle expansion through defining orifices into a high vacuum region maintained by continuous pumping. Extreme non-equilibrium between the various degrees of freedom is observed in such a beam owing to the expansion, and this allows the study of vibrational and rotational relaxation effects. The molecular beam gives a higher intensity and better defined system than normal nozzle expansions. The papers were of a good standard and essentially reflected the chemists" outlook to relaxation phenomena in fluids. They will be published by the Chemical Society. The discussions indicated that ultrasonics has still an important role to play in these investigations. B. J. LAVERCOMBF