Capillary wave atomization

Ultrasonic atomization is sometimes also termed capillary-wave atomization. In its most common form, 142 a thin film of a molten metal is atomized by the vibrations of the surface on which it flows. Standing waves are induced in the thin film by an oscillator that vibrates vertically to the film surface at ultrasonic frequencies. The liquid metal film is broken up at the antinodes along the surface into fine droplets once the amplitude of the capillary wave exceeds a certain value. The most-frequent diameter of the droplets generated is approximately one fourth of the wavelength of the capillary wave,1 421 and thus decreases with increasing frequency. 

Analytical and empirical correlations for droplet sizes generated by ultrasonic atomization are listed in Table 4.14 for an overview. In these correlations, Dm is the median droplet diameter, X is the wavelength of capillary waves, co0 is the operating frequency, a is the amplitude, UL0 is the liquid velocity at the nozzle exit in USWA, /Jmax is the maximum sound pressure, and Us is the speed of sound in gas. Most of the analytical correlations are derived on the basis of the capillary wave theory. Experimental observations revealed that the mean droplet size generated from thin liquid films on... 

The second mechanism proposed for aerosol generation is based on the piezoelectric crystal operating at low frequency and imparting vibrations to the bulk liquid. This results in the formation of cavitation bubbles, which move to the air-liquid interface.The internal pressure within the bubbles equilibrates with that of the atmosphere, causing their implosion. When this occurs at the liquid surface, portions of the liquid break free from the turbulent bulk liquid, resulting in droplet formation. The dependence of atomization on cavitation phenomena has been demonstrated for frequencies between 0.5 and 2.0 MHz.Boguslavskii and Eknadiosyants combined these theories with-their proposal that droplet formation resulted from capillary waves initiated and driven by cavitation bubbles. 

For DOC, it can be seen that the results of Williams (1967), for example, show an extra 2.1 g m DOC in the microlayer. If the thickness of the water film obtained with the screen device is taken to be —200 pm, the surface excess of DOC can be calculated as 2.1 X 200 X 10 = 4.2 X 10" g m . A reasonable lower limit to take for the molecular weight of this extra organic material in the surface film is that of a relatively short-chain acid or alcohol with —14 carbon atoms, equivalent to —170 g mole carbon. Using this minimum value, the area per molecule in the ambient type of films sampled by Williams (1967) can be calculated as > — 170/4.2 X lO" X 6.02 X 10 3 = > 70 A. It can be seen from Fig. 1 that for all surface film types except gaseous films, such an area per molecule has no effect on the surface tension of seawater, as measured by the spreading drop method, or on the damping of capillary waves. Moreover, only relatively water-soluble surfactants remain in the gaseous state at film pressures of —10 N m" ... 

Ultrasonic nozzles are designed to specifically operate from a vibration energy source. In ultrasonic atomization, a liquid is subjected to a sufficiently high intensity of ultrasonic field that splits it into droplets, which are then ejected from the liquid-ultrasonic source interface into the surrounding air as a fine spray (Rajan and Pandit 2001). A number of basic ultrasonic atomizer types, like capillary wave, standing wave, bending wave, fountain, vibrating orifice, and whistle, etc., exist. 

Droplets from Free Surfaces It is also possible to dispense microdroplets from free surfaces, either formed on a sessile droplet or along the meniscus formed at a nozzle. In the latter case, the critical difference with the previous section is that the primary droplet formed from the nozzle is much smaller than the diameter of the nozzle, eliminating the direct influence of the nozzle s size oti the size of the droplet. The study of the formation of droplets from free surfaces has a long and controversial history, which has been suggested to be due to either capillary wave breakup, cavitation, cavitation-induced capDlaiy waves which subsequently break up to form droplets, or even cavitation-screened capillary wave breakup, where a cavitation layer beneath the surface acts to suppress atomization. It is likely that over the wide range of operating... 

The possibility of generating a cloud of droplets by means of ultrasonic waves was first reported by Wood and Lomis [29]. Two different mechanisms have been reported to explain the ultrasonic atomization capillary waves and cavitations. However, the interaction between these two approaches and hmits in which one could predominate over the other depending on the different atomizing situation are challenging for immediate understanding. 

DGE a AC AMS APCI API AP-MALDI APPI ASAP BIRD c CAD CE CF CF-FAB Cl CID cw CZE Da DAPCI DART DC DE DESI DIOS DTIMS EC ECD El ELDI EM ESI ETD eV f FAB FAIMS FD FI FT FTICR two-dimensional gel electrophoresis atto, 10 18 alternating current accelerator mass spectrometry atmospheric pressure chemical ionization atmospheric pressure ionization atmospheric pressure matrix-assisted laser desorption/ionization atmospheric pressure photoionization atmospheric-pressure solids analysis probe blackbody infrared radiative dissociation centi, 10-2 collision-activated dissociation capillary electrophoresis continuous flow continuous flow fast atom bombardment chemical ionization collision-induced dissociation continuous wave capillary zone electrophoresis dalton desorption atmospheric pressure chemical ionization direct analysis in real time direct current delayed extraction desorption electrospray ionization desorption/ionization on silicon drift tube ion mobility spectrometry electrochromatography electron capture dissociation electron ionization electrospray-assisted laser desorption/ionization electron multiplier electrospray ionization electron transfer dissociation electron volt femto, 1CT15 fast atom bombardment field asymmetric waveform ion mobility spectrometry field desorption field ionization Fourier transform Fourier transform ion cyclotron resonance... 

In an acoustic atomizer, high-frequency sound waves are used to create capillary ripples that ultimately break up into droplets. Ultrasonic atomization can produce a fairly narrow droplet size distribution. 

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