Luminescence Sonoluminescence

Weninger KR, Camara CG, Putterman SJ (2001) Observation of bubble dynamics within luminescent cavitation clouds sonoluminescence at the nano-scale. Phys Rev E 63 016310... 

Objective To produce, visualise and quantify different forms of luminescence produced by ultrasound sonoluminescence and luminol sonochemical... 

Theory Collapse of gas/vapour cavities in an acoustic field produces extremely high pressures and temperatures capable of causing the emission of light from the core of the collapsing cavity (sonoluminescence) and also the formation of oxidising radical species that can react in the solution with molecules, such as luminol, to produce a secondary, chemical luminescence. 

Experiment 3 In general, sonoluminescence emission is not discemable with the naked eye. The luminosity of the secondary emission from luminol (oxidised by sonochemically produced OH radicals) however, is several orders of magnitude brighter and is easily seen in a dark room. Prepare a 0.1 mM aqueous luminol solution in 0.1 M NaOH. Sonicate this solution and observe the emission pattern. This will appear as bands of light and dark if a standing wave reactor is used or in more elaborate forms in different reactors. If a 20 kHz horn is used, a cone shaped zone of luminescence will be observed. Explain the emission pattern. 

In the presence of an alkali salt, strong metal atom emission can be seen both in the emission spectrum and visually. This form of emission is described in detail in Chapter 13. Long-time exposure photographs comparing sonoluminescence and luminol and Na sonochemical luminescence are shown in Fig. 15.5a-c. 

Triboluminescence Luminescence resulting from the mbbing together of the surface of certain solids. It can be produced, for example, when solids are cmshed. See sonoluminescence. 

An order-of-magnitude increase in the intensity of sonoluminescence is observed when either carbon disulfide or carbon tetrachloride is added to the water. The intensity of luminescence, /, for the reaction... 

Other types of luminescence are defined by the source of the energy that causes the light emission. These include chemiluminescence, bioluminescence, electroluminescence, sonoluminescence, triboluminescence, and thermoluminescence. 

Sonoluminescence mainly occurs inside the bubbles (or in their immediate surroundings) and thus cannot be representative of what happens in the liquid phase where most of the events used in ultrasound application occur. In particular the temperature and frequency dependence of sonoluminescence is quite different from that of other effects, thus luminescence decreases with temperature, while liquid-phase effects usually increase to a maximum, and then decrease [19]. 

Margulis [34] has shown that the cavitation threshold may be confidently determined by the method of liquid sonoluminescence. Although the mechanism of liquid luminescence with cavity closing is not conclusively established—we do not know which factor is the main one in the luminescence in the moment ol collapse thermal phenomena, electric charges, or the superimposition of these processes—the method is of practical use for early observation of cavitation origination. 

It is well known that when acoustic waves are generated in aqueous and other media luminescence can be observed [205], This type of luminescence is known as sonoluminescence and has been attributed to the process of cavitation during which bubbles are formed. The collapse of these cavitation bubbles during the compression portion of the sound wave represents a very high energy output which results... 

Depending on the excitation method used, luminescence techniques are divided into photoluminescence excited by photons, cathodoluminescence generated under the action of cathode rays, X-ray luminescence excited by X-rays, candoluminescence generated under the action of heat, and sonoluminescence excited by ultrasound. Emission generated under the action of a stream of ions from alkali metals in vaccum is called ionoluminescence radiation which atoms emit on optical excitation in plasma is known as atomic fluorescence chemiluminescence is the emission of radiation generated by the energy of chemical reactions, it does not require an external excitation source. The excitation source needed in each particular case is chosen on the basis of this classification. 

The terms Luminescence refers to all other forms of light emission and involves a radiative transition where by a molecule lowers its energy by emission of a photon. In order for the luminescence process to exist for any period of time, energy must be supplied to the system to maintain an excited state population. Different forms of luminescence have been classified based on the source of this energy. While several interesting forms of luminescence exist (eg. Electroluminescence, bioluminescence, radioluminescence, triboluminescence, and sonoluminescence) the type of luminescence, which is of analytical importance is photoluminescence. In photoluminescence the excited state is produced by the absorption of light. 

The observation that sonochemistry and luminescent activities are not necessarily coincident had never been reported and suggests that the conditions at the root of sonoluminescence and sonochemistry could be different in nature (or in intensity). Chemical species as tracers should serve to describe the liquid flows the knowledge of which is of prime importance in medical ultrasound (microstreaming, acoustic streaming and/or rectified acoustic streaming zy Even if synthetic exploitations are difficult to imagine, some fundamental aspects of sonochemistry and cavitation still to be explored are very probably involved, and in this respect deserve further development. 

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