Spray-droplet method

Figure 21.6 Time-lapse observation of synaptic acid crystal formation. The spray-droplet method forms very fine crystals inside and outside of the matrix drop (a-d), so that finer and more homogeneous crystals are generated (d) than those obtained by the droplet method (e-h). Observation with a scanning electron microscope of matrix crystals with the spray-droplet (i) and droplet methods (j). Reprinted with permission from Sugiura et al.7... 

Figure 21.7 Comparison of mass spectra obtained from rat brain. Optical observation of microspotted tissue sections employing spray-droplet (a), droplet (b), and spraycoating (c) methods. Scale bar, 1.0 mm. White squares (a-c) represent the cortex (A, d) and the medulla (B, e) of the cerebellum region, respectively. Accumulated mass spectra collected from each region are shown (d, e). In each spectrum, asterisks represent major unique signals for spectra using the spray-droplet method. The number of detected signals in the mass range of 2000 < m/z < 30,000 from each region is shown (f). Reprinted with permission from Sugiura et al.7...

The spray aeration method comprises a grid network of piping and nozzles over a pond or basin. Contaminated water is simply sprayed through the nozzles and into the air to form droplets. Mass transfer of the contaminant takes place across the air-water surface of the droplets. Mass transfer efficiency can be increased by multiple passing of the water through the nozzles. This method has three disadvantages ... 

Figure 21.8 Comparison of IMS indicating protein distribution in the rat brain section corresponding to Figure 21.7 (a-c) the spray-droplet (a), droplet (b), and spray-coating (c) methods are used. Reprinted with permission from Sugiura et al.7 See color insert.

Two numerical methods have been used for the solution of the spray equation. In the first method, i.e., the full spray equation method 543 544 the full distribution function / is found approximately by subdividing the domain of coordinates accessible to the droplets, including their physical positions, velocities, sizes, and temperatures, into computational cells and keeping a value of / in each cell. The computational cells are fixed in time as in an Eulerian fluid dynamics calculation, and derivatives off are approximated by taking finite differences of the cell values. This approach suffersfrom two principal drawbacks (a) large numerical diffusion and dispersion... 

Collection Techniques. The simplest mechanical method for normal liquid droplets is the slide collection slide sampling) or impression method. This method was extensively used three decades ago, and has been rarely employed since then. In this method, when the slide is exposed to a spray, droplets impinging on it make impressions. The impressions are then observed and measured usually using a Quantimet image analyzer, although a microscope fitted with a traversing scale may be used for the measurement. The measured data are subsequently converted to actual droplet sizes based on a correction factor proposed by May.[659]... 

With either method, a portion of the suddenly heated coarse spray droplet is vaporized. This serves as a propellent to disrupt the less volatile remainder of the droplets into smaller droplets, the diameters of which are determined by the temperature at which disruption occurs, by the rate of flow of the air, and by the pressure of injection, as well as by the particular characteristics of the solvent or formula used. 

I he experimental determination of drop-size distributions of fuel sprays is important for all studies involving the atomization of liquid fuels. Investigations of the mechanism of atomization, influences of the many factors that determine fineness of the spray, and methods of atomization and nozzle design all require some means for determining the extent to which the liquid is broken up into droplets in preparation for combustion. 

In a gas-continuous impinging stream device with liquid as the dispersed phase, the liquid is usually atomized into fine droplets with nozzles of an appropriate type, and ejected into gas flows to form droplets-in-gas suspensions before impingement. This can be called the Primary Atomization, and it defines the primary dispersity of liquids. The mechanism of primary atomization and the methods for predicting size distribution (SD) and mean diameter (MD) of the sprayed droplets have been widely reported and some sources of references may be found, e.g., in Ref. [69]. 

It is actually very difficult to obtain representative and reproducible data for the size distribution of sprayed droplets, no matter what kind of method is employed for measurement, because the dispersion of liquid by atomization, including re-atomization in impinging streams, is highly random. In the study the following factors are considered carefully in the arrangement of sampling for representative samples and thus statistically trustworthy results ... 

The gas-film mass transfer coefficient, kG, was determined based on the Sauter mean diameter of spray droplets. The results show essentially no influence of initial concentration of SOz on kG, suggesting that the process is controlled by diffusion through gas film and that the method proposed for the determination of kG is feasible ... 

Figure 2. Conceptual representation of spray-droplet charging methods. (Reproduced with permission from Ref. 10. Copyright 1978, American Society of Agricultural Engineers.)...

Spray drying methods can also be used. Here, one first dissolves the reactants in a medium, preferably H2O. Small droplets are transported in a gas stream and heated, resulting in a very fast evaporation of H2O. As a result, an intimate reaction mixture is obtained. 

The spray droplet size distribution has a major influence on the amount of oropharyngeal deposition and the amount of drug delivered to the lungs and the regions of lung deposition. Methods for spray droplet particle sizing from MDIs " have been reviewed. 

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