Levitation studies

Gragerov and Levit studied the oxidations of some aromatic amines and nitroso compounds by peroxomonosulphate in H2 0, and found that the products were unlabelled. Therefore the formation of hydroxyl radicals (by attack of peroxomonosulphate on the solvent) and their participation in the oxidations is excluded. 

Gragerov and Levit studied the oxidation of diphenyl sulphide by peroxomonosulphate in ethanol-acetic acid solvent containing H2 0, and found that the products, diphenyl sulphoxide and diphenyl sulphone, were unlabelled. They concluded that the oxidation is heterolytic and does not involve free-radicals. 

When a sonic standing wave is set up in air the particles suspended in the air will migrate to the nodes of the sound wave and this phenomenon has been used in a variety of applications. One remarkable consequence of this is the ability to construct a levitation reactor. When the source of ultrasound is above a planar surface, small particles will migrate to the nodes from the surface, i.e. levitate. Studies in such a system are not dissimilar from studies in the zero-gravity conditions of a space shuttle. 

The importance of the particle levitation methods is that they allow the study of how a single particle responds to changes in environment. The infrared molecular spectroscopy of single particles is possible [253], as are photophysical studies using adsorbed or dissolved dyes. 

The DEP of numerous particle types has been studied, and many apphcations have been developed. Particles studied have included aerosols, glass, minerals, polymer molecules, hving cells, and cell organelles. Apphcations developed include filtration, orientation, sorting or separation, characterization, and levitation and materials handhng. Effects of DEP are easily exhibited, especially by large particles, and can be apphed in many useful and desirable ways. DEP effects can, however, be observed on particles ranging in size even down to the molecular level in special cases. Since thermal effects tend to disrupt DEP with molecular-sized particles, they can be controlled only under special conditions such as in molecular beams. 

In this section, we describe an attempt to study the role of nuclear magnetization in a levitating body by the magneto-Archimedes effect, and how the OPENCORE NMR spectrometer was utilized for this purpose. 

Tona, M., Study on Spherical Microlasers Levitated in an Ion Trap, Kochi University of Technology, Kochi, 2002... 

We note that since Q involves the scattering coefficients, the radiation pressure force has resonance or near-resonance behavior. This first was observed and analyzed by Ashkin and Dziedzic (1977) in their study of microparticle levitation by radiation pressure. They made additional measurements (Ashkin and Dziedzic, 1981) of the laser power required to levitate a microdroplet, and Fig. 19 presents their data for a silicone droplet. The morphological resonance spectrum for the 180° backscattered light shows well-defined peaks at wavelengths corresponding to frequencies close to natural frequencies of the sphere. The laser power shows the same resonance structures in reverse, that is, when the scattered intensity is high the laser power required to levitate the droplet is low. 

Aqueous salt solutions are particularly volatile in a dry gas, and they become supersaturated as evaporation proceeds, for in the absence of solid boundaries heterogeneous nucleation does not occur. Homogeneous nu-cleation of crystals ultimately occurs to complicate the scattering process. Highly supersaturated solutions can be examined using droplet levitation, and studies related to concentrated electrolyte solutions are surveyed later. 

Most of the studies of levitated droplets have involved low-vapor-pressure materials, but Tallin and his coworkers reported data for water droplets evaporating in dry nitrogen. The rapid evaporation of a water droplet requires that the experiment be automated, and this was accomplished by injecting the droplet by means of a 3,000 V dc electrical pulse applied to a flat-tipped hypodermic needle. The pulse triggered the data collection system so that phase functions and the resonance spectrum were obtained during the less than three-second duration of an experiment. From the phase function... 

This review of the chemistry and physics of microparticles and their characterization is by no means comprehensive, for the very large range of masses that can be studied with the electrodynamic balance makes it possible to explore the spectroscopy of atomic ions. This field is a large one, and Nobel laureates Hans Dehmelt and Wolfgang Paul have labored long in that fruitful scientific garden. The application of particle levitation to atmospheric aerosols, to studies of Knudsen aerosol phenomena, and to heat and mass transfer in the free-molecule regime would require as much space as this survey. 

James Davis is the inventor of the levitation machine, with which a single aerosol particle can be suspended in mid-air in order to study its equilibrium and rate processes without resorting to averaging among many particles. He contributes a very strong chapter on Microchemical Engineering that involves chemical reactions, transport processes, thermodynamics and physical processes. 

Kintner et al (K7) and Damon et al. (Dl) have discussed photographic techniques applicable to the study of bubbles and drops. Sometimes it is desirable to hold a bubble or drop stationary, to study internal or external flow patterns and transfer processes. To prevent the particle from migrating to the wall, it is desirable to establish a minimum in the velocity profile at the position where the particle is to reside, and various techniques have been devised (D4, FI, Gl, Pll, M15, R15, S20) to do this. Vertical wandering of such particles may occur (W7), and may be reduced by using a duct tapered so that the area decreases towards the top (D4). Acoustic levitation of liquid drops may also be used (A3). 

A related area is that of single-particle levitation, which has been used in a number of studies to isolate a single particle and study its properties (e.g., see papers by Tang and co-workers in Chapter 9). A review of this area is given by Davis (1997). 

It should be noted that the procedure described in this chapter is strictly limited to the interactions of droplets with surrounding gases, with much emphasis on the conditions that would yield powders of narrow size distributions. However, no attempt is made to describe the literature on particle formation from a single levitated droplet (7). The technique used in these studies also differs from those in which the... 

Savolainen et al. investigated the role of Raman spectroscopy for monitoring amorphous content and compared the performance with that of NIR spectroscopy [41], Partial least squares (PLS) models in combination with several data pre-processing methods were employed. The prediction error for an independent test set was in the range of 2-3% for both NIR and Raman spectroscopy for amorphous and crystalline a-lactose monohydrate. The authors concluded that both techniques are useful for quantifying amorphous content however, the performance depends on process unit operation. Rantanen et al. performed a similar study of anhydrate/hydrate powder mixtures of nitrofurantoin, theophyllin, caffeine and carbamazepine [42], They found that both NIR and Raman performed well and that multivariate evaluation not always improves the evaluation in the case of Raman data. Santesson et al. demonstrated in situ Raman monitoring of crystallisation in acoustically levitated nanolitre drops [43]. Indomethazine and benzamide were used as model... 

Sample levitation can be accomplished in different ways, one of which is by using ultrasonic energy. The phenomenon by which small samples of solids, liquids or suspensions can be levitated at the nodal points of a standing ultrasonic wave was first described by Bucks and Muller in 1933 [70]. The flexibility and potential of acoustic levitation in various fields are widely documented, mainly by studies in the analytical and bioanalytical fields [71-73]. Therefore, levitation can be considered a mature technique. Its development warrants inclusion of a dedicated section in this chapter to describe its fundamentals and compare the advantages and limitations of acoustic levitation with other levitation modes. The devices used for this purpose and the potential applications of each mode are also discussed. 

The importance of sample delivery is frequently overlooked in the studies on acoustic levitation, even though precision in the analyses relies heavily on an appropriate choice of the sample volume. Delivery systems are closely related to the nature of the sample (liquid, suspension, solid or gas). Any type of system e.g. a micropipette, capillary, or microsyringe) can be used to position a drop from a liquid or slurry sample in an ultrasonic Ievitator. 

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