Chemical-acoustic coupling

Acoustic-chemical coupling, 151-54 Activation energy, 112-13 Activation energy, reaction of NH and oxygen, 109 Addition channel rate constant, 249-54... 

The chemical effects of ultrasound do not arise from a direct interaction with molecular species. Ultrasound spans the frequencies of roughly 15 kH2 to 1 GH2. With sound velocities in Hquids typically about 1500 m/s, acoustic wavelengths range from roughly 10 to lO " cm. These are not molecular dimensions. Consequently, no direct coupling of the acoustic field with chemical species on a molecular level can account for sonochemistry or sonoluminescence. 

Thus far we have discussed the direct mechanism of dissipation, when the reaction coordinate is coupled directly to the continuous spectrum of the bath degrees of freedom. For chemical reactions this situation is rather rare, since low-frequency acoustic phonon modes have much larger wavelengths than the size of the reaction complex, and so they cannot cause a considerable relative displacement of the reactants. The direct mechanism may play an essential role in long-distance electron transfer in dielectric media, when the reorganization energy is created by displacement of equilibrium positions of low-frequency polarization phonons. Another cause of friction may be anharmonicity of solids which leads to multiphonon processes. In particular, the Raman processes may provide small energy losses. 

Two models are available for interpreting attenuation spectra as a PSD in suspensions with chemically distinct, dispersed phases using the extended coupled phase theory.68 Both models assume that the attenuation spectrum of a mixture is composed of a superposition of component spectra. In the multiphase model, the PSD is represented as the sum of two log-normal distributions with the same standard deviation, that is, a bimodal distribution. The appearance of multiple solutions is avoided by setting a common standard deviation to the mean size of each distribution. This may be a poor assumption for the PSD (see section 11.3.2). The effective medium model assumes that only one target phase of a multidisperse system needs to be determined, while all other phases contribute to a homogeneous system, the so-called effective medium. Although not complicated by the possibility of multiple solutions, this model requires additional measurements to determine the density, viscosity, and acoustic attenuation of the effective medium. The attenuation spectrum of the effective medium is modeled via a polynomial fit, while the target phase is assumed to have a log-normal PSD.68 This model allows the PSD for mixtures of more than two phases to be determined. 

Figure 8.11. Portable Raman spectrometer coupled to an acoustic levitator. A — Dantec ultrasonic acoustic levitation device, B — Raman fIbre-optIc probe, C — control unit for the acoustic levitation device, D — quartz halogen light source and E — InPhotonIcs portable 785-nm Raman spectrometer. (Reproduced with permission of the American Chemical Society, Ref [120].)...

We summarize a number of simulations aimed at deciphering some of the basic effects which arise from the interaction of chemical kinetics and fluid dynamics in the ignition and propagation of detonations in gas phase materials. The studies presented have used one- and two-dimensional numerical models which couple a description of the fluid dynamics to descriptions of the detailed chemical kinetics and physical diffusion processes. We briefly describe, in order of complexity, a) chemical-acoustic coupling, b) hot spot formation, ignition and the shock-to-detonation transition, c) kinetic factors in detonation cell sizes, and d) flame acceleration and the transition to turbulence. 

Studies of chemical-acoustic coupling are concerned with the interactions between sound waves and the processes involved in... 

In the examples given above we have tried to describe some of the phenomena which arise as a result of chemical kinetic-fluid dynamic coupling. First, we described studies of the isolated effects of chemical-acoustic coupling, emphasizing the effects on the chemical kinetics. The major conclusion is that sound waves and entropy perturbations can alter chemical timescales, and that this effect can be quantified. We then described a system in which sound waves and entropy perturbations behind a shock wave caused early ignition at unpredictable locations and at reduced ignition times. A series of reaction centers formed and one of these close to the shock front eventually ignited. 

Oran, E.S. Gardner, J.H., A Review of Research in Chemical-Acoustic Coupling," NRL Memo. Report 5121, Naval Research Laboratory, 1983. 

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