Particle levitation

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. 

A number of schemes have been developed which circumvent this restriction. Some require the particle to be charged, while others will work with neutral objects. Arnold 121 has recently reviewed the history, design, and operating principles of charged-particle levitators, but a brief description is provided here for completeness. 

Fig. 12, Photophoretic force data of Lin and Campillo (1985) for crystalline ammonium sulfate particles levitated in an electrodynamic balance. Reprinted with permission from Lin, H.-B., and Campillo, A. J., Applied Optics 24, 244, Copyright 1985, The Optical Society of America.

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. 

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). 

Davis, E. J., A History of Single Aerosol Particle Levitation, Aerosol Sci. Technol., 26, 212-254 (1997). 

Reid JP (2009) Particle levitation and laboratory scattering. J Quant Spectrosc Radiat Transf 110(14-16) 1293-1306... 

Peng CG, Chow AHL, Chan CK. Study of the hygroscopic properties of selected pharmaceutical aerosols using single particle levitation. Pharm Res 2000, 17, 1104-1109. 

Negative dielectrophoresis causes particles and cells to be repelled from regions of high electric field strength. This effect can be used to levitate particles over, for example, a planar array of electrodes. A common geometry is a four-electrode field trap with electrodes forming the sides or comers of a square and with the particle levitated over the center of the square. The viscous forces from the medium will damp the motion of the particle. 

Fig. 10.29 Simulated heatup time in dependence of particle size for particles levitated in a vertical flow of warmer air...

Gas—solids fluidization is the levitation of a bed of solid particles by a gas. Intense soflds mixing and good gas—soflds contact create an isothermal system having good mass transfer (qv). The gas-fluidized bed is ideal for many chemical reactions, drying (qv), mixing, and heat-transfer appHcations. Soflds can also be fluidized by a Hquid or by gas and Hquid combined. Liquid and gas—Hquid fluidization appHcations are growing in number, but gas—soHds fluidization appHcations dominate the fluidization field. This article discusses gas—soHds fluidization. 

Mixed liberated particles can be separated from each other by flotation if there are sufficient differences in their wettability. The flotation process operates by preparing a water suspension of a mixture of relatively fine-sized particles (smaller than 150 micrometers) and by contacting the suspension with a swarm of air bubbles of air in a suitably designed process vessel. Particles that are readily wetted by water (hydrcmhiric) tend to remain in suspension, and those particles not wetted by water (hydrophobic) tend to be attached to air bubbles, levitate (float) to the top of the process vessel, and collect in a froth layer. Thus, differences in the surface chemical properties of the solids are the basis for separation by flotation. 

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. 

Levitation is a stable condition in which a particle responds to the oscillating fluid in such a way that the influence of finite buoyancy, or gravity, forces is completely neutralized so that the particle oscillates about a fixed position (Houghton, 1963, 1964, 1966, 1968 Krantz, Carley and Al-taweel, 1973 Tunstall and Houghton, 1968 Van Oeveren and Houghton, 1971). Figure 37 (Liu, 1983) shows the levitation of solid particles in air under oscillation caused by a sonic generator located at the bottom of the column. 

Figure 37. Levitation of solid particles in vertically oscillating air. (a) Dilute suspension of solids. (b) Solids concentration at node. (Liu, Keling, 1983.)...

The frequency of fluid oscillation at which levitation takes place is plotted in Fig. 40 against the corresponding amplitude A of oscillation, the asymmetry factor ka or ratio of the duration of the downstroke to that of the upstroke, and the resin particle diameter d. From these experimental data, the three parameters of the equation were correlated to the particle diameter ... 

Trajectories of a single resin particle computed from the equation are shown in Fig. 41, indicating that at/= 4.5 Hz the particle sinks, while at /= 7.5 Hz rising levitation takes place. 

In optical tweezer experiments, the optical scattering force is used to trap particles, but the force can also be used to control the shape of liquid droplets26. An infrared laser with 43-mW power focused onto a microdroplet on a superhydrophobic surface enabled up to 40% reversible tuning of the equatorial diameter of the droplet26. Such effects must naturally also be taken into account when exciting laser modes in droplets in experiments with levitated drops. 

On this subject notice that, possibly combined with various heating methods, several physical effects may be considered which allow free flotation of solid and even liquid matter. Materials may be levitated for instance by a jet of gas, by intense sound waves or by beams of laser light. Conductors levitate in strong radiofrequency fields, charged particles in alternating electric fields, magnets above superconductors or vice versa. A review on levitation in physics with the description of several techniques and their principles and applications was made by Brandt (1989). 

In order to stably levitate an object, the net force on it must be zero, and the forces on the body, if it is perturbed, must act to return it to its original position. The object must be at a local potential minimum that is, the second derivatives with respect to all spatial coordinates of the potential must be positive. This may seem, at first sight, to be trivial to arrange. However, any system whose potential is a solution to Laplace s equation is automatically unstable A statement in words of Laplace s equation is that the sum of the second partial derivatives of the potential is zero, and so not all can be simultaneously positive. This has long been known for electrostatic potentials, having been stated by Earnshaw(n) Millikan s scheme for suspending charged particles is thus only neutrally stable, since the fields within a Millikan capacitor provide no lateral constraint. 

A schematic diagram of the apparatus used in the energy transfer experiments is shown in Figure 8.22. The particles are produced and levitated in an electrodynamic levitator as described previously. Excitation is provided by the filtered output of either a Xe or Hg-Xe high-pressure arc. The intensity produced at the particle was found to be 10-50 mW/cm2. The fluorescence emitted from each of the levitated particles was monitored at 90° to the exciting beam using //3 optics, dispersed with a j-m monochromator, and detected with an optical multichannel analyzer. The levitator could be... 

The spectra are very smooth when compared to the emission spectra shown for polystyrene particles in Figure 8.1. The anticipated resonances are not observed in Figure 8.23. The evaporation of the particle at room temperature is slow, but is still rapid enough that over the integration time of the detection system the resonance structure is completely washed out. Figure 8.24 shows the effect of cooling the levitating chamber to 13°C. The upper curve shows the emission spectrum of a cooled particle. The next lower curve shows a room temperature spectrum of a similar particle. The lowest... 

Figure 8.24. The effect of cooling the levitator is to reveal the resonances in the upper curve. The curve just below it is a room temperature spectrum of a similar particle. The difference is displayed below.

S. Arnold, Spectroscopy of single levitated micron sized particles, in Optical Effects Associated with Small Particles (P. W. Barber and R. K.. Chang, eds.), World Scientific, New York (1988). 

The Wyatt and Phillips instrument has the features of primary importance for subsequent electrical levitators—specifically, feedback control for vertical positioning and scattered light detection. However, the restoring forces exerted on the particle were not adequate for a number of applications. Slight convective currents in the chamber would cause the particle to be lost, so it was not possible to provide flow through the device. The electrodynamic balance does not have this difficulty. 

The stability of the levitated particle is governed by the vector equation of... 

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