Interfacial shock wave

Photochemical Generation of an Interfacial Shock Wave. Both the an-thocyanidine and the thioindigo monolayers showed a decrease in surface pressure at constant area during the photoisomerization reaction. A different behavior is observed with mixed monolayers of the surface active spiropyran SP and octadecanol (OD), molar ratio SP 0D = 1 5, on illumination with UV radiation. The isomerization of the spiropyran to the merocyanine MC causes an increase in surface pressure at constant area (5, 14). This is shown in Figure 4, where the sudden rise in surface pressure it upon repeated 0.5 s exposures (as indicated by the arrows) can be seen to occur in a wide surface pressure range (15). The kinetics of the relaxation process following the surface pressure increase depends on the surface pressure. 

The fast isomerization of the spiropyran to the merocyanine provides a possibility of generating an interfacial shock wave. The methods used so far in studying the transmission of waves in mono-layers and the adjacent bulk phases require mechanical (16) or electrocapillary (17) excitation of the interface which involves the displacement of the aqueous bulk phase. In addition, the range of frequencies accessible to the investigation of interfacial waves by the conventional techniques is very limited. The fast photochemical generation of an interfacial shock wave is strictly occurring in the monolayer and provides a larger spectrum of frequencies which can be fully explored only after the development of appropriate detection methods. 

Photochemical processes in monolayers at the air-water interface can be controlled externally by variation of the various parameters like matrix composition, subphase composition, temperature and surface pressure. When the product of the reactions has a different area per molecule, the surface pressure may change at constant monolayer area. An interfacial shock wave has been generated in this way. This technique permits the investigation of the kinetics of reorganization processes and the transmission of mechanical signals in monolayers. 

A study on the influence of the viscosity of the dispersed phase in the preparation of emulsions of vegetable oils (olive, soyabean and linseed) in water with US assistance revealed that replacing the oil with the highest viscosity and interfacial tension — olive oil — with soyabean oil, which has slightly lower viscosity and interfacial tension, caused virtually no reduction in droplet size. Linseed oil, with much lower viscosity and interfacial tension than olive oil, exhibited a much smaller Sauter diameter than the latter viz. 0.47 (xm versus 0.62 pm). Breaking low-viscosity droplets requires less vigorous cavitation shock waves than breaking more viscous ones [49]. 

The most pertinent effects of ultrasound in solid-liquid reactions are mechanical, which are attributed to symmetrical and/or asymmetrical cavitation. Symmetrical cavitation (the type encountered in homogeneous systems) leads to localized areas of high temperatures and pressures and also to shock waves that can create microscopic turbulence (Elder, 1959). As a result, mass transfer rates are considerably enhanced. For example, Hagenson and Doraiswamy (1998) observed a twofold increase in the intrinsic mass transfer coefficient in the reaction between benzyl chloride (liquid) and sodium sulfide (solid). In addition, a decrease in particle size and therefore an increase in the interfacial surface area appears to be a common feature of ultrasound-assisted solid-liquid reactions (Suslick et al., 1987 Ratoarinoro et al., 1992, 1995 Hagenson and Doraiswamy, 1998). 

The "interfacial process" is currently acceptable as far as capillary and adsorption-desorption surfaces. , shock waves, combustion and thin layers are concerned°... 

At high enough qualities and mass fluxes, however, it would be expected that the nucleate boiling would be suppressed and the heat transfer would be by forced convection, analogous to that for the evaporation for pure fluids. Shock [282] considered heat and mass transfer in annular flow evaporation of ethanol water mixtures in a vertical tube. He obtained numerical solutions of the turbulent transport equations and carried out calculations with mass transfer resistance calculated in both phases and with mass transfer resistance omitted in one or both phases. The results for interfacial concentration as a function of distance are illustrated in Fig. 15.112. These results show that the liquid phase mass transfer resistance is likely to be small and that the main resistance is in the vapor phase. A similar conclusion was reached in recent work by Zhang et al. [283] these latter authors show that mass transfer effects would not have a large effect on forced convective evaporation, particularly if account is taken of the enhancement of the gas mass transfer coefficient as a result of interfacial waves. 

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