Electrodynamic Levitator Research Articles

New experimental method for the simultaneous determination of concentration and size profiles of condensed combustion products around a burning aluminum droplet

When an aluminum particle burns in the diffusive regime, a cloud of nanometric particles of its primary oxide, alumina, is formed around the droplet. Characterizing this oxide smoke surrounding a burning aluminum droplet is essential. The original contribution of this paper is to introduce an experimental approach that allows for simultaneous size and concentration measurement of the particles composing the oxide smoke. This paper employs an electrodynamic levitator to facilitate the self-sustained combustion of an isolated aluminum particle, with a radius of 35μm, in atmospheric air. This levitation apparatus is paired with an optical setup that allows for a multi-spectral light extinction method. This technique is specifically designed to measure the size and concentration of alumina particles below the micrometric scale, which is scarcely documented in existing literature. Consequently, spatially resolved concentration and size profiles of the nanometric alumina particles are measured and compared to simulated data from the literature. The radii of alumina particles in the cloud are found to evolve closely around 80 nm. Volume fractions are evaluated from 2.8⋅10−4 near the particle surface to 3⋅10−5 further in the cloud. Therefore, significant concentrations of alumina particles are revealed during the short (15 to 20 ms) combustion process at distances of the order of magnitude of a few initial particle diameters. Such characterization could then lead to a better assessment of the interaction distance among aluminum particles as they burn collectively.Novelty and significance statementThis article introduces the first experimentally determined profiles of alumina concentration and size distribution surrounding a single burning aluminum droplet. The uniqueness of our experimental setup lies in its ability to study of the most fundamental case of the autonomous combustion of a single levitating particle, without any convective effects. A new non-intrusive method specifically designed provides unprecedented results in the field of metallic particle combustion. Data provided in the paper are crucial as they serve as the first reference case for future simulation and experiments. The results introduced in this paper highlight the inability of current simulation methods to accurately account for nucleation and condensation processes, and emphasize the need for a non-invasive characterization method.

Peculiarities of aluminum particle combustion in steam

This work experimentally addresses aluminum combustion in steam, pure or mixed with diluents, for aluminum particles in size range 40∼80 µm, using an electrodynamic levitator. High-speed videos unveil an unreported and complex mechanism in steam, not observed in other oxidizers. The detached flame is quite faint and very close to the surface. Alumina smoke around the droplet rapidly condenses and coalesces into a large, single orbiting alumina satellite. It eventually collides the main aluminum droplet while generating secondary alumina droplets. A unique feature is the presence of several distinct oxide lobes on the droplet, which merge only at the end of burning and encapsulate the remaining aluminum, possibly promoting an incomplete combustion. The measured burning times in pure water vapor are longer than expected and the efficiency of steam is found to be 30% that of oxygen, lower than the usually accepted value of 60%. A general correlation on burning time, including the major oxidizers, is proposed. Direct numerical simulations are conducted and are in line with experiments, in terms of burning rate or flame stand off ratio. Combustion in steam seems mostly supported by surface reactions, giving a faint flame with low gas temperatures and high hydrogen content. It is speculated that those two specific features could help explain the peculiarity of steam.

Scattering phase function measurements on single particles and on particle ensembles

We present measurements of the light-scattering phase function of selected carbon and ash particles in the geometric-optics regime in which the particle diameter is much larger than the wavelength of the light source. Measurements were performed on both single particles and particle ensembles. This was accomplished with two separate methodologies: an electrodynamic levitator for single-particle measurements and a particle feeder for the ensemble measurements. For each methodology, two irradiation sources were utilized: an argon-ion laser (lambda = 496 nm) and a xenon lamp. Results of the normalized phase functions are presented.

Shape oscillations and stability of charged microdroplets.

Classical Rayleigh theory predicts an instability of a surface charged liquid sphere, when the Coulomb energy E(C) exceeds twice the surface energy E(S). Previously, electrified liquid droplets have been found to disintegrate at a fissility X=E(C)/2E(S) well below one, however. We determine the stability of charged droplets in an electrodynamic levitator by observing the amplitude and phase of their quadrupolar shape oscillations as a function of the fissility. With this novel approach, which does not rely on an independent determination of the charge and surface tension of the droplets, we are able to confirm for the first time the Rayleigh limit of stability at X=1 for micrometer sized droplets of ethylene glycol.

Ignition and Combustion of Levitated Magnesium and Aluminum Particles in Carbon Dioxide

This article considers ignition and combustion of single particles of magnesium and aluminum in carbon dioxide at pressures 0.1-2 MPa. An experimental setup with an electrodynamic levitator inside a high-pressure chamber was employed. The CO2-laser was used for heating to ignition of the particles. The results show that ignition mechanisms of Mg and Al in CO2 are different. Experiments with Mg indicate the existence of the critical partial pressure of CO2, whereas the ignition probability of Al particles in CO2 is low but independent on pressure. Analysis of flame images and combustion parameters shows that the mechanism of Mg particle burning in CO2 corresponds to conventional models of vapor-phase diffusion-controlled combustion, whereas in the case of Al exothermic processes on the particle surface or close to it play a leading part in the burning process.

Fluctuation–dissipation theorem for supersaturated electrolyte solutions

Highly supersaturated electrolyte solutions are prepared and studied employing an Electrodynamic Levitator Trap (ELT) technique. The containerless suspension of pl-size solution microdroplets achieved by means of this technique eliminates dust, dirt, convection flows and container walls which normally cause heterogeneous nucleation. This allows very high supersaturations to be achieved and studied. The experimental results obtained for solvent activity versus solute concentration have been processed by applying the least-squares regression. This has allowed processing of the data with large error bars obtained for the very high supersaturations. Theoretical study is based on the development of the Cahn–Hilliard formalism for electrolyte solutions. In the approach suggested the metastable state for electrolyte solutions is described in terms of the local conserved order parameter ω( r,t) associated with fluctuations of the mean solute concentration n 0. Parameters of the corresponding Ginzburg–Landau free energy functional which defines the dynamics of metastable state relaxation are determined and expressed through the experimentally measured solvent activity. A correspondence of 96–99% between theory and experiment for the all solutions studied was achieved and reported earlier [Izmailov et al., Phys. Rev. 52E (1995) 3923]. The theoretical approach suggested in this study for thermodynamics of supersaturated electrolyte solutions is further developed in order to describe the transient and stationary limits of the metastable state relaxation. Knowledge of these limits has allowed derivation of the Fluctuation–Dissipation Theorem (FDT) for supersaturated electrolyte solutions by specifying a time-dependent product of macroscopic mobility (inverse viscosity) and microscopic diffusivity. It is understood from general relationships of non-equilibrium thermodynamics that this product has a maximum at saturation point and is equal to zero at spinodal point. Further analysis of the FDT has revealed that the product of macroscopic mobility and microscopic diffusivity for electrolyte solutions has another particularity: a local minimum in the deeply undersaturated region of solute concentrations. Numerical analysis of the FDT has been carried out for the supersaturated, binary, electrolyte symmetric solutions.

Convertible electrodynamic levitator trap to quasielectrostatic levitator for microparticle nucleation studies

This article describes an apparatus for obtaining nucleation data from a levitated solution microdroplet, automatically. A particularly novel feature is that it uses an electrodynamic levitator trap (ELT) which converts to a quasielectrostatic levitator (QEL), at any time during an experiment. The conversion is accomplished by using asymmetrically applied potentials on the ELT structure. With this modification one can trap a particle in the ELT mode and then convert to the QEL mode for automatic operation. By eliminating the need for the alternating gradient forces which are intrinsic to the ELT, the system in its QEL mode is shielded from unwanted noise and parametric instabilities associated with the ELT’s alternating potential. To test the system theoretically, we calculate the effect which molecular collisions have on the positional variance in a spherical void QEL. Following this, we describe the components of our servosystem, and demonstrate the robustness of our design by following the nucleation of a solution droplet as the ambient relative humidity is reduced by evacuation.

A statistical understanding of nucleation

In order to study a stochastic phenomenon such as nucleation it is necessary to collect a large enough set of nucleation data to obtain nucleation statistics. This is done by performing nucleation experiments with the same solution under exactly the same conditions many times N ( N∼150–300). Such an experiment, based on simultaneous levitation of N∼150–300 identical microdroplets (1–20 μm in diameter) of supersaturated solutions in a solvent atmosphere, is possible by employing the linear quadrupole electrodynamic levitator trap (LQELT). The LQELT is supplemented with a special optical system which is based on scattering of monochromatic polarized light. This will enable fast observation of nucleation and, thus, induction times in each of the levitated microdroplets. The N different induction times, counted from the moment t 0 at which supersaturation is established are recorded. This data provides nucleation statistics (induction time statistics). The numerical and analytical studies of nucleation statistics and parameters provide an insight into statistical properties of the underlying nucleation phenomenon.

Ignition and combustion of levitated magnesium particles in carbon dioxide

This paper considers ignition and combustion of small (50–100 μm) single particles of magnesium and 50-50 magnesium-aluminum alloy in the atmosphere of carbon dioxide or its mixtures with argon. This investigation is of interest for both basic combustion science and applications to rocket engines, including those using Martian CO2 as an oxidizer, An experimental setup with an electrodynamic levitator inside a high-pressure chamber was employed. A CO2 laser was used for heating to ignition of the particles. The laser was switched off after ignition. The experiments were conducted with the oxidizer at room temperature over the range of pressures from 0.1 to 2 MPa. Effects of the CO2 concentration and pressure on the critical ignition conditions, ignition delay times, and burning times have been determined for Mg particles. The results clearly indicate that ignition of Mg in CO2 is controlled by chemical kinetics and that its combustion is controlled by diffusion in gas phase. Quantitative disagreement of the observed critical ignition pressures with previous experimental data on ignition of Mg disks in CO2 is explained by the differences in heat-loss mechanisms. The measured values of the burning rate correlate well with previous experimental results on combustion of 2-mm particles and with a quasi-steady model of Mg particle burning in CO2. In contrast to pure Mg and Al, particles of Mg−Al alloy did not ignite in CO2 under the present experimental conditions.

Relationship between diffusivity and viscosity for supersaturated electrolyte solutions

Highly supersaturated electrolyte solutions are prepared and studied employing an electrodynamic levitator trap (ELT) technique. Very high supersaturations were achieved and studied. The theoretical study is based on the development of the Cahn-Hilliard formalism for electrolyte solutions. A correspondence of 96–99% between theory and experiment for all the solutions studied was achieved and reported earlier [Izmailov, Myerson and Na, Phys. Rev. E 52 (1995) 3923]. The theoretical approach suggested in this study for thermodynamics of supersaturated electrolyte solutions is further developed in order to describe the transient and stationary limits of the metastable state relaxation. Knowledge of these limits has allowed derivation of the fluctuation-dissipation theorem (FDT) for supersaturated electrolyte solutions by specifying a time-dependent product of macroscopic mobility (inverse viscosity), describing momentum dissipation, and microscopic diffusivity, describing local fluctuations of solute concentration. It is understood from general relationships of non-equilibrium thermodynamics that this product has a maximum at saturation point and is equal to zero at spinodal point. Further analysis of the FDT has revealed that the product has another particularity: a local minimum in the deeply undersaturated region of solute concentrations. Numerical analysis of the FDT has been carried out for the supersaturated, binary, electrolyte symmetric solutions such as NaCl, NaBr, KCl, KRr and NH 4Cl.

Thermodynamic studies of levitated microdroplets of highly supersaturated electrolyte solutions

Highly supersaturated electrolyte solutions are studied by employing an electrodynamic levitator trap (ELT) technique. The ELT technique involves containerless suspension of a microdroplet thus eliminating dust, dirt, and container walls which normally cause heterogeneous nucleation. This allows very high supersaturations to be achieved. A theoretical study of the experimental results obtained for the water activity in microdroplets of various electrolyte solutions is based on the development of the Cahn-Hilliard formalism for electrolyte solutions. A correspondence of 96–99% between the theory and experiment for the all solutions studied was achieved and allowed the determination of an analytical expression for the spinodal concentration nspin and its calculation for various electrolyte solutions at 298 K.

Studies on the Ignition and Burning of Levitated Aluminum Particles∗

Abstract An experimental set-up is developed to investigate the burning of levitated aluminum particles ignited by a C02laser in air under high pressure conditions. Residence times of aluminum particles burning in the electrodynamic levitator are long enough to observe the total burning process, under normal and high pressures. Experiments allow to measure the ignition delay and the burning times of aluminum particles. These measurements and the accompanying high speed images give useful information on the burning processes of aluminum particles under various regimes. A numerical model is also developed to predict the burning rates of aluminum particles and the sizes of the alumina residues.

Supersaturated electrolyte solutions: Theory and experiment.

Highly supersaturated electrolyte solutions can be prepared and studied employing an electrodynamic levitator trap (ELT) technique. The ELT technique involves containerless suspension of a microdroplet thus eliminating dust, dirt, and container walls which normally cause heterogeneous nucleation. This allows very high supersaturations to be achieved. A theoretical study of the experimental results obtained for the water activity in microdroplets of various electrolyte solutions is based on the development of the Cahn-Hilliard formalism for electrolyte solutions. In the approach suggested the metastable state for electrolyte solutions is described in terms of the conserved order parameter \ensuremath{\omega}(r,t) associated with fluctuations of the mean solute concentration ${\mathit{n}}_{0}$. Parameters of the corresponding Ginzburg-Landau free energy functional which defines the dynamics of metastable state relaxation are determined and expressed through the experimentally measured quantities. A correspondence of 96--99 % between theory and experiment for all solutions studied was achieved and allowed the determination of an analytical expression for the spinodal concentration ${\mathit{n}}_{\mathrm{spin}}$ and its calculation for various electrolyte solutions at 298 K. The assumption that subcritical solute clusters consist of the electrically neutral Bjerrum pairs has allowed both analytical and numerical investigation of the number-size ${\mathit{N}}_{\mathit{c}}$ of nucleation monomers (aggregates of the Bjerrum pairs) which are elementary units of the solute critical clusters. This has also allowed estimations for the surface tension \ensuremath{\alpha}, and equilibrium bulk energy \ensuremath{\beta} per solute molecule in the nucleation monomers. The dependence of these properties on the temperature T and on the solute concentration ${\mathit{n}}_{0}$ through the entire metastable zone (from saturation concentration ${\mathit{n}}_{\mathrm{sat}}$ to spinodal ${\mathit{n}}_{\mathrm{spin}}$) is examined. It has been demonstrated that there are the following asymptotics: ${\mathit{N}}_{\mathit{c}}$=1 at spinodal concentration and ${\mathit{N}}_{\mathit{c}}$=\ensuremath{\infty} at saturation.

Microparticle driven by parametric and random forces: Theory and experiment.

The confined motion of a charged microparticle within the Paul Trap (also known as the electrodynamic levitator trap) in an atmosphere near the standard temperature T and pressure ${\mathit{P}}_{\mathrm{atm}}$ is studied both theoretically and experimentally. The suggested theoretical model is based on the Mathieu differential equation with damping term and stochastic source. This equation describes the damped microparticle motion subjected to the combined periodic parametric and random external excitations. To solve the equation in an experimentally investigated regime of extremely strong damping and periodic excitations, the singular perturbation theory (WKB theory) is applied. In order to compare experimental data obtained in the long-time imaging limit with an analytical solution obtained for the autocorrelation function, the last is averaged by employing the Bogoliubov general averaging principle. This comparison is performed in terms of the standard deviation of the microparticle confined stochastic motion. It results almost in the perfect agreement between the analytical result and the data obtained experimentally in an entire region of the investigated experimental parameters. The only theoretical restrictions imposed on the model parameters are 1/\ensuremath{\alpha}\ensuremath{\ll}1 and 4\ensuremath{\beta}/${\mathrm{\ensuremath{\alpha}}}^{2}$\ensuremath{\ll}1 (where \ensuremath{\alpha} and \ensuremath{\beta} are the dimensionless drag and drive parameters). It is discovered both experimentally and theoretically that there is a minimum equal to [8kT/(m${\mathrm{\ensuremath{\omega}}}^{2}$)${]}^{1/2}$ in the standard deviation of the microparticle confined stochastic motion (m is the microparticle mass and \ensuremath{\omega} is the drive force frequency). The presence of this minimum, which takes place at \ensuremath{\beta}\ensuremath{\approxeq}1.518\ensuremath{\alpha}, reduces the thermal noise effects, providing unique opportunities for the spectroscopic studies. Comparison with the numerical simulation schemes developed in papers [Arnold, Folan, and Korn, J. Appl. Phys. 74, 4291 (1993); Blatt et al., Z. Phys. D 4, 121 (1986); Zerbe, Jung, and Hanggi, Phys. Rev. E 49, 3626 (1994)] is discussed.

Water activity in supersaturated aqueous solutions of organic solutes

Measurements of water activity in supersaturated aqueous organic solutions of glycine, alanine, succinic acid and itaconic acid were made far into the metastable zone by levitating micron-sized droplets electrodynamically in a spherical void electrodynamic levitator trap (SVELT) with a water vapor reservoir. The concentration dependent behavior of the activity was examined in relationship to the molecular interactions for solutions.

Parametrically driven microparticle in the presence of a stationary zero-mean stochastic source: Model for thermal equilibrium in the Paul trap.

An analytical approach is developed to consider confined motion of a charged microparticle within the Paul trap (an electrodynamic levitator trap) in an atmosphere near the standard temperature and pressure. The suggested approach is based on a second-order linear stochastic differential equation which describes dampled microparticle motion subjected to the combined periodic parametric and random external excitations. To solve this equation a new ansatz is developed. This ansatz is a generalization of the Bogoliubov-Krylov decomposition technique, which is usually used to reduce the order of a differential equation. The solution is obtained in the long time imaging limit by applying the Bogoliubov general averaging principle. In spite of the second-order form of the initial stochastic differential equation, the microparticle motion can be understood as a one-dimensional Markov process. Comparison in the long time imaging limit of the calculated data obtained from the analytically derived expression for the standard deviation of confined microparticle stochastic motion with the experimentally obtained data demonstrates asymptotic agreement for regions where the dimensionless parameter \ensuremath{\kappa} is much less than 1 (\ensuremath{\kappa}\ensuremath{\le}0.005). Simple extremum analysis of the expression obtained for the standard deviation reveals that for the particular case of a large drag parameter \ensuremath{\alpha} (\ensuremath{\alpha}\ensuremath{\gg}8 \ensuremath{\surd}12 ) there is a minimum in the standard deviation which is only \ensuremath{\alpha} dependent.

Cluster formation in highly supersaturated solution droplets

Cluster formation in supersaturated aqueous solutions was investigated. In this study, single microparticles of aqueous sodium chloride and glycine solutions were suspended electrodynamically without a container in a spherical void electrodynamic levitator trap (SVELT) in a water vapor reservoir environment. Measurements of water activities were made far into the metastable zone. The dependence of the critical cluster size on concentration in the metastable region was investigated employing nucleation theory. In addition, the number average cluster size was estimated from a relationship between the diffusion coefficient and cluster size.

Morphological resonances detected from a cluster of two microspheres

Morphological resonances are detected in elastic scattering and fluorescence excitation spectra from a cluster of two spheres, each ~5 μm in radius, isolated and oriented in an electrodynamic levitator trap. Light scattering at 90° and perpendicular to the line of centers of the particles, by broadside illumination with polarization in the scattering plane, shows that an independent particle model may be used in describing resonance positions and the shape of the spectrum, even though the joined particles differ in size by slightly less than 10 nm. In end-fire illumination the fluorescence excitation spectrum is missing a number of resonances that are seen with broadside illumination.

Determination of Water Activity in Ammonium Sulfate and Sulfuric Acid Mixtures Using Levitated Single Particles

Water activities of ammonium sulfate-sulfuric acid mixtures with ammonium to sulfate molar ratios between 0 and 2 were measured by a spherical void electrodynamic levitator at relative humidities (RH) of 0.18–0.90. Since the composition of the solid particles is subject to uncertainty, solution properties were determined relative to the known properties at RH about 0.75–0.90. The data were compared with other measurements and the estimates from the Zdanovskii-Stokes-Robinson (ZSR) method and SCAPE, a newly developed gas-particle equilibrium model, and generally were found to be in good agreement.

Photon-correlation spectroscopy for small spherical inclusions in a micrometer-sized electrodynamically levitated droplet

Micrometer-sized droplets of saturated sodium chloride solution were captured in an electrodynamic levitator and maintained at constant diameter for several days at a time. Both pure droplets and droplets containing 0.5-microm subparticles (guests) were studied by means of 90-deg scattering of polarized laser light. Analysis of the correlation function for the intensity of the scattered light gave characteristic decay times of &~9 ms, which has a good correspondence with the physical parameters of the solution and the size of the subparticles. Longer characteristic times are measures of morphological features of the electromagnetic field intensity in the host particles. A new phenomenon was observed when the exit polarizer was crossed to the input polarization. Scattered light corresponding to light from the guest particles could only be visualized from a shell near the exterior of the sphere and not from the interior of the host droplet.