manual levitator ultrasonic

Ultrasonic Levitator

Manual

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Ultrasonic Levitator Manual 2

Contents 1 Introduction...................................................................................................3 2 Basics About Ultrasonic Levitation ............................................................4

2.1 General Working Principle ................................ ................................ ................................ ................4 2.2 Typical Effects Occurring with Ultrasonic Levitation ................................ ................................ .....8

3 Description of the Ultrasonic Levitator....................................................10 3.1 The Power Supply Unit................................ ................................ ................................ .....................10 3.2 The Levitator with Process Chamber................................ ................................ .............................. 11 3.3 Options ................................ ................................ ................................ ................................ ...............12 3.4 Maintenance and Service................................ ................................ ................................ ..................13 3.5 Safety Instructions ................................ ................................ ................................ ............................14 3.6 Technical Specifications................................ ................................ ................................ ....................15

4 Working with the Ultrasonic Levitator ....................................................16 4.1 Start, Tuning, and Calibration of the Levitator ................................ ................................ .............16 4.2 Deployment and Extraction of Liquid Samples ................................ ................................ ..............17 4.3 Experimenting with the Levitator and its Unique Features ................................ ..........................21 4.4 Some helpful tricks for experimenting with the levitator ................................ .............................. 23 4.5 Potential Applications................................ ................................ ................................ .......................24

5 Literature.....................................................................................................25


1 Introduction The ultrasonic levitator is a powerful tool which facilitates the performance of a variety of investigations on single particles or droplets. As it suspends the lev i-tated object contactlessly in a fixed position, the process under investigation is not disturbed by the influence of a contacting surface. The ultrasonic levitator with its various optional features is designed to be applicable to manifold scientific disc i-plines.

Note Before installation of the ultrasonic levitator, the present manual and in par-ticular the safety and maintenance instructions should be read carefully. Chapter 2 of the present manual gives a brief description of the basic working principles of ultrasonic levitation. A more detailed description of the different parts of the levitator and of the available optional accessories is given in Chapter 3. Instructions for using the ultrasonic levitator are provided in Chapter 4. To gain a better and more detailed understanding of this levitation technique and its applications, a selection of relevant literature is cited in Chapter 5. We hope, you enjoy working with the ultrasonic levitator, and we wish you suc-cess in your specific application. As we are continuously trying to improve the general performance of the ultrasonic levitator and to increase its field of applica-tions, any ideas or suggestions from our customers are highly appreciated.


2 Basics About Ultrasonic Levitation 2.1 General Working Principle Standing wave As a result of multiple reflections between an ultrasonic radiator and a solid, flat

or concave reflector - which is adjusted concentrically at a distance of some mul-tiple half wavelengths - a standing wave with equally spaced nodes and antinodes of the sound pressure and velocity amplitude will be generated (see Figure 2-1). Solid or liquid samples with effective diameters of less than half a wavelength will be levitated without contact below the pressure nodes as a result of axial r a-diation pressure and radial Bernoulli stress. Minimal acoustic power is required for this levitation, when the sound wavelength is about three times the sample d i-ameter (see Table 2-1).

Reflector

Bernoulli-stress

Sound-radiation-pressure

Piezoelectrictransducer

Droplet

λ0

n ∗ λ0/2

Figure 2-1: Schematic set-up and pressure distribution in the resonator.

Ultrasonic frequency f [kHz] 20 30 45 58 100

Optimal drop diameter ds, opt [mm] 6.9 4.6 3.1 2.4 1.4

Optimal drop volume Vs, opt [µl] 120 36 11 5 1

Opt. sound pressure level Lmin [dB] 161 159 157 156 154

Table 2-1: Specifications for the levitation of water drops in ambient air.


58 kHz standard The tec5 AG ultrasonic levitator works at a standard frequency of 58 kHz frequency (optional: 100 kHz) in ambient air with a standard wavelength of about 5.9 mm.

The largest drop diameter that can be suspended with the standard frequency, without significant drop deformation, is about 2.5 mm, while the smallest diame-ter is around 15 µm. The environmental temperature for operating the ultrasonic levitator should be between 0°C and +70°C. The levitator is designed for applications with liquid and solid samples with de n-sities in the range 0.5 - 2 g/cm³. The large dynamic range of the levitation force of 1:10 allows, however, short time levitation experiments with heavy samples hav-ing a significantly higher density. Figure 2-2 illustrates schematically the levitation of a spherical sample in a plane standing ultrasonic wave between a piezoelectrically driven transducer and a con-centrically spaced flat or curved reflector.

-

+

-

+

z z

Pressure Velocity Force

z

stableunstable

under microgravity"on earth"

Figure 2-2: Acoustic pressure, velocity and levitation force in an ultrasonic standing wave.


The axial profiles of the acoustic pressure and the sound particle velocity ampl i-tudes (time average) of the standing wave

$ $ cos

$ $ sin

max

max

p p kz

v v kz

= ⋅

= ⋅

with the well known nodes and antinodes result in respective profiles of the ki-netic and potential energy densities:

E pc

kzpot =$ cosmax

2

0 02

2

2 ρ

E v kzkin =ρ0

22

2$ sinmax

In the above equations, k represents the wave number : k = 2π / λ .

Levitation force These energy densities provide a pressure distribution around the sample which, after integration over the sample surface, results in an axial acoustic levitation force:

F c v d d gac w ac s s s= ⋅ ⋅ ⋅ ⋅ = ⋅ ⋅ ⋅, max$ρ π π

ρ0 2 2 32 4 6

This levitation force compensates the sample weight ρsVs g in the gravity field. Under microgravity conditions (g ≈ 0 m/s²), the sample will be stably positioned exactly at a pressure node, while under terrestrial conditions its weight is compe n-sated at a downwards displacement of the sample center below the pressure node. Due to the fact that the standing ultrasonic wave is not a perfect plane wave, but shows a slight divergence, a radial profile of the sound particle velocity exists. This results in a symmetrical radial force which is centering the levitated sample on the levitator axis. The radial force is about one order of magnitude smaller than the axial levitation force.

Pressure nodes In the standard ultrasonic levitator, 4 to 5 pressure nodes exist which can be visu-alized by atomizing a drop of water at the transducer frontface. The atomized aerosol will agglomerate in the pressure nodes. Only the inner 2 to 3 pressure nodes can be used for stable levitation. The two outer nodes are influenced by de-stabilizing effects from the transducer and the reflector.

Droplets with diameters ds ≥ 2/3 λ can not be levitated. The optimal droplet di-ameter, for which minimal ultrasonic power is required, is ds,opt = λ/3 . Table 2-2 gives an overview of different liquids, their surface tension and density, and the corresponding maximal droplet diameters and volumes for ultrasonic levitation. These values correspond to the critical Bond-number Bocrit. = 1.5 and are irrespec-tive of the ultrasonic frequency, which means that these figures are absolute va l-ues. Droplets with diameters larger than indicated in Table 2-2 are not acousti-cally levitatable at all, for stability reasons.


Medium σs [dyn/cm] ρs [g/cm³] dmax [mm] Vmax [ml]

Ethanol 22.3 0.7894 4.16 37.6

Acetone 23.3 0.7910 4.24 40.0

Benzene 28.9 0.8790 4.48 47.0

Glycerin 65.7 1.2610 5.64 94.1

Methanol 22.6 0.7915 4.17 38.1

Mercury 465 13.546 4.58 50.3

CS2 32.2 1.2630 3.95 32.2

CCl4 26.8 1.5940 3.20 17.2

Toluol 28.5 0.8669 4.48 47.0

Water 72.75 0.9982 6.67 155

Xylol 30.1 0.8802 4.57 50.0

Cyclohexane 25.0 0.7784 4.43 45.6

Table 2-2: Overview of different liquids and their max. droplet diameter and vol-ume (for Bocrit. = 1.5) for ultrasonic levitation.


2.2 Typical Effects Occurring with Ultrasonic Levitation 2.2.1 Drop Deformation

The acoustic energy density profile around a levitated drop results in a drop d e-formation by balancing the gravitation and capillary forces of the liquid sample. With increasing sound intensity the drop will be spheroidally deformed with a continuous growth of the horizontal diameter. Beyond a certain critical value , the shape of the drop may change into a "donut". The center of the donut flattens more and more with further increased acoustic power. Larger samples or drops with small surface tension show a typical selfinflation or buckling until they f i-nally explode. Small drops of a liquid with high surface tension remain almost spherical. Figure 2-3 shows the characteristic behavior of the acoustic power intensity and the corresponding drop deformation. The left diagram shows the dynamic power range between the minimum power for safe levitation and the maximum power for a deformed droplet just before destruction. The right diagram represents the ratio between the horizontal diameter d* of the deformed drop and the diameter d0 of the non-deformed drop. It can be seen from both graphics that drops with Bond-numbers larger than 1.5 can not be levitated.

Pac

Bo1.50

min.

max.

Bo

d*/d0

1.501

2

Figure 2-3: Acoustic power intensity and drop deformation as a function of Bond-number.

2.2.2 Thermal Effects Due to sound absorption, a radial and axial temperature profile exists, similar to the energy density profile in the standing wave field. As a result thereof, a lev i-tated drop will be heated up to the equilibrium temperature existing at the levit a-tion position.

These temperature effects are usually small and negligible, but they should be mentioned and possibly have to be taken into account for certain applications. The sound absorption increases with lower ultrasonic frequencies. With the standard 58 kHz levitator, a temperature increase at the pressure nodes of less than 0.5°C was measured.


2.2.3 Acoustic Streaming The energy density distribution in the standing wave field induces an acoustic convection flow. This streaming effect slightly enhances heat and mass transfer phenomena, characterized by an increased Sherwood and Nusselt number. Sher-wood number and Nusselt number are describing the resulting convective en-hancement of heat and mass transfer between a drop and its environment, com-pared to the non-convective transfer based on conduction and diffusion, respec-tively.

The effective acoustic convection flow velocity can be used to define a corr e-sponding Reynolds number. The resulting Reynolds number is rather low, like e.g. Re = 1.83 for a 1 mm water droplet. Slight asymmetries in the acoustically induced convective flow field might result in mostly undesired and uncontrolled drop rotations.

2.2.4 Sample Oscillations A levitated sample represents, together with the radial and axial levitation force profile, a system which is able to perform damped oscillations. Such oscillations might be self-induced or externally activated by low frequency amplitude modula-tion of the ultrasonic carrier wave and are in general characterized by certain resonance frequencies.

These resonance frequencies can be monitored by connecting the piezoelectric sensor output to an oscilloscope. When the mass of the levitated sample changes, the frequency of the resonant oscillation will also change and this can be deduced by analyzing the monitor signal. This provides a means to determine e.g. diffusion constants by measuring the mass variation of a droplet as a function of time. Externally applied amplitude modulation of the ultrasonic wave can also be per-formed in a way, that self-oscillations of a drop (oscillating drop deformations) are induced. This enables investigation of droplet stability performance which is important in connection with liquid atomization processes.


3 Description of the Ultrasonic Levitator The standard ultrasonic levitator system comprises two subsystems: • 13L10 Ultrasonic Levitator Power Supply Unit (ultrasonic generator) and one of the following levitators: • 13D10 Ultrasonic Levitator with single-walled process chamber

• 13D11 Ultrasonic Levitator with double-walled process chamber.

The system is further equipped with all required cable connections.

3.1 The Power Supply Unit The ultrasonic generator has a standard operation frequency of 58 kHz (optional: 100 kHz) and radiates 0.65 - 5 Watt RF power. Other frequencies according to Table 2-1 are available upon request. Standard AC conditions are 220 V, 50 Hz and 110 V, 50-60 Hz respectively . The generator housing measures 165 mm x 177 mm x 261 mm at a total weight of approx. 2 kg. The front panel contains an LED control (red for AC power and green for HF power), a potentiometer for the continuous variation of the transducer amplitude and a liquid crystal display of the tranducer voltage (displayed in arbitrary units). The red LED is on when the AC power is properly connected and the power sup-ply unit is switched on. The green LED is on when the phase-locked-loop (PLL) is locked in and the transducer oscillates on its resonance frequency. A blinking of the green LED indicates that the PLL is not locked in due to a too high damping or an improper cable connection. The power supply unit enables the resonance tuning control of the reflector distance at constant transducer amplitude setting. The back side of the generator contains the AC power entrance safety plug, the on/off switch and a 3-pole round socket for the connection of the ultrasonic trans-ducer via a 2 m long shielded cable. A second socket allows the connection of a commercial low frequency oscillator for modulation of the ultrasonic carrier wave.


3.2 The Levitator with Process Chamber The levitator is shown schematically in Figure 3-1 (without process chamber). It consists of the reflector flange with integrated concave reflector and a micrometer screw for the variation of the reflector distance, and the transducer flange with in-tegrated ultrasonic transducer, shielding tube and round socket. Both flanges are coaxially interconnected with 4 anodized spacing pins. The stan-dard distance between transducer frontface and reflector is about 2.5 times the wavelength, which means 14.8 mm at the standard frequency of 58 kHz in air. The micrometer screw allows a distance variation by ± 6 mm. The standard version of the levitator is equipped with a single-walled processing chamber. This chamber is a glass tube with 70 mm inner diameter and 2 mm wall thickness. Three windows with Spindler & Hoyer Microbench adapters in the sample plane provide access for manipulation, illumination and observation of the sample. A sound absorber, around the transducer shaft eliminates undesired acoustic reflections and resonances inside the process chamber. The levitator can also be operated in an open version, without the process chamber, thus providing better access to the levitated sample but also being more sensitive to undesired disturbances induced from external air flows.

HF-connector

Absorber

Ultrasound transducer

Reflector

Piezoelectricsensor connector

Micrometeradjustment screw

Piezoelectric sensor

Figure 3-1: The ultrasonic levitator (shown without process chamber).


3.3 Options 3.3.1 Double-walled Process Chamber

An optional double-walled glass tube allows the connection of a commercial wa-ter thermostat and a simple subsystem for a controlled external humidification or dehumidification of the processing chamber gas. This turns the processing cham-ber into a "miniature climatic chamber" and enables measurements under exactly defined environmental conditions.

3.3.2 Piezoelectric Sensor An optional piezoelectric wide band sensor can be mounted behind the reflector, which can be connected to an oscilloscope via a socket at the top flange. Its output voltage is proportional to the acoustic pressure amplitude of the standing wave and can be used for the resonance tuning control of the standing wave and for the recording of low frequency sample resonances.

3.3.3 Free Jet Nozzle As another option, a free jet gas nozzle can be integrated into the reflector, which allows air or any other gas to be blown around the acoustically levitated sample. This resembles an acoustic/aerodynamic "Hybrid" levitator with the unique fe a-ture of extreme variation of the Reynolds-, Nusselt-, and Sherwood number. In this configuration, the levitator is equipped with four additional support pins and has to be operated in upside down position. In the standard unit, without the optional free jet nozzle, the axial bore hole of the ultrasound transducer can be used to apply a flow around the levitated sample. This, however, is limited to gas temperatures below 70°C in order not to damage the piezoelectric transducer element. The free jet nozzle integrated into the refle c-tor allows hot gas, with temperatures up to a few hundred degrees Celsius, to be blown around the levitated sample thus facilitating the investigation of fundame n-tal processes of e.g. spray drying.


3.4 Maintenance and Service The ultrasonic levitator is robust and easy to be maintained under proper operat-ing conditions. The processing chamber can easily be disassembled and its components, espe-cially the glass tube, should be cleaned frequently with ordinary dishwasher fluid. The sound absorber (foam rubber) can be cleaned with water or replaced if neces-sary. In case of algae deposits inside the water carrying section of the double wall proc-essing chamber tube, we recommend cleaning with some low concentrated acid. In case of problems with the ultrasonic generator please send the power supply unit together with the levitator back to tec5 AG for servicing.


3.5 Safety Instructions

• Switch off power before you change cables.

• Do not touch the vibrating ultrasonic transducer.

• Maintain operation temperature of the levitator below +70°C.

• Check compatibility of sample liquid and its vapor with critical levitator co m-ponents


3.6 Technical Specifications 13L10 Power Supply Unit

Operation frequency (standard): 58 kHz or 100 kHz

Particle diameter range: ≈ 15 µm to ≈ 2.5 mm

Wavelength of standing acoustic wave: 5.71 mm

HF power: continuously variable 0.65 - 5 Watt

AC power: 220 V / 50 Hz or 110 V / 60 Hz

Fuse: 160 mA (220 V) 320 mA (110 V)

Modulation frequency 10 Hz - 2 kHz

Modulation amplitude 0 - 2 Vpp

Modulation input impedance 20 kΩ

Operation temperature range 0 – 70°C

Rel. humidity (non-condensing) 10 – 90 %

13D10 Ultrasonic Levitator with Single-walled Process Chamber Material of process chamber: Duran-Glass

Connectors: 3 Linos Microbench adapters 1 GL 14 connector with septum

Diameter of process chamber: 75 mm ± 1 mm

Length of process chamber: 140 mm

13D11 Ultrasonic Levitator with Double-walled Process Chamber Material of process chamber: Duran-Glass

Connectors: 3 conical openings (cone NS 24) 1 GL 14 connector with septum 2 GL 14 connectors for thermostat

Diameter of process chamber: 100 mm ± 1 mm

Length of process chamber: 140 mm

13K10 Reflector with Piezoelectric Sensor

Sensor output voltage: typ. 200 mVpp

13K11 Reflector with Free Jet Nozzle Inner diameter of nozzle: 1 mm


4 Working with the Ultrasonic Levitator 4.1 Start, Tuning, and Calibration of the Levitator

• Make sure that all cables are properly connected.

• Start ultrasonic generator with switch on back side and adjust amplitude with the potentiometer on the front side until LED-display on front side reads about 3.8 - 4.0.

• Turn micrometer screw to the setting given in the data sheet until LED-display or voltage of reflector sensor (via oscilloscope) shows the expected resonance maximum.

• Increase reflector distance with micrometer screw by about 0.2 mm above the optimum, in order to avoid possible self-excitation of sample oscilla-tions.

• The levitator is now ready for "boarding".


4.2 Deployment and Extraction of Liquid Samples The standing wave can be made visible before sample deployment by atomizing a small amount of water from the transducer front face (contact with wet Q-tip). The atomized fine aerosol will agglomerate at the 4 or 5 (standard) pressure nodes. Only the inner 2 to 3 pressure nodes are suited for save levitation (see Fig-ure 4-1). The other two nodes are distorted by "nearfield structures".

Figure 4-1: Levitated droplets in the 2nd and 3rd pressure node above the ultra-sonic transducer.

Sample Liquid samples can be introduced from a µl-syringe (see Figure 4-2), deployment either free handed or by using a commercial support sleigh. Detachment of a drop-

let from the needle of a syringe can be facilitated by changing slightly the refle c-tor distance. After detachment from the syringe needle, the levitated droplet may perform oscillations which can be damped by changing the resonator distance and /or the ultrasonic power level.


Figure 4-2: Sample deployment using a µl-syringe.

Sample Stabilization of the levitated sample is achieved by adjusting the stabilization distance between transducer and reflector and by proper setting of the ultrasonic

power. Excessive power may result in disintegration of a large drop into smaller droplets or at least yields a significant drop deformation (see Figure 4-3). With optimal setting of the HF-power, a spherical shape of the levitated droplet (see Figure 4-4) can be achieved. With insufficient power the drop will fall down. The deployment of tiny droplets requires some experience. It is recommended to start with atomized aerosols (see above) and to remove unwanted droplets from some pressure nodes with a Q-tip. One can also start with a larger drop, which - after controlled evaporation - shrinks down to the desired size.


Figure 4-3: Drop deformation due to excessive HF-power.

Figure 4-4: Spherical droplet with optimal HF-power.


Power The required transducer amplitude increases with sample size and is adjustment proportional to the sample density. A good control of the optimal power setting is

a resonance excitation of the sample by low frequency amplitude modulation, where the resonance frequency should be near 20 Hz. Without this resonance measurement the drop deformation provides a sufficient control of the proper power setting. With insufficient power, too large drops might fall down; too flat drops indicate excessive power. Operating with "aggressive" liquids requires a compatibility test with the proces s-ing chamber components which might come in contact with the fluid or its vapor. The problem is only critical however, to the sound absorber of the lower process-ing chamber flange, which can easily be replaced. Possible self-excitation of sample resonances can be suppressed by increasing the reflector distance and readjusting the amplitude setting. This variable deviation from resonance provides a very large dynamic range of the levitation power.

Sample Sample extraction can be accomplished either with the syringe (see extraction sample deployment) or by switching the levitator off and letting the drop fall

either onto the transducer frontface or onto an acoustically transparent, flexible but stiff enough wire mesh, introduced slightly below the sample. Solid samples can be introduced and extracted with the same, acoustically trans-parent wire mesh or by using tweezers.


4.3 Experimenting with the Levitator and its Unique Features The "open levitator" as well as the "closed tube levitator" allow ideal access for optical equipment through side windows in the sample plane. Coaxial access for a laser beam for illumination or spot heating is possible through the axial bore hole of the transducer. The levitator has been designed mainly for experiments with liquid samples with densities in the range 0.5 - 2 g/cm³, excluding mercury. The large power dynamic range of 1:10 allows however a short time levitation of heavy solid samples (den-sity below 8 g/cm³), if the fast transducer heat up is considered and limited to T ≤ 80°C.

4.3.1 Low-frequency Modulation Many of the experiments with the ultrasonic levitator require a low frequency resonance excitation of the levitated drop. This can be accomplished by ampl i-tude modulation of the ultrasonic carrier wave with a commercial oscillator, co n-nected to the generator via the BNC-socket at the back plate of the power supply unit. The best modulation depth depends on the resonance frequency and physical properties of the sample and must be found by trial and error. To perform low frequency amplitude modulation of the ultrasonic wave, perform the following steps: • Connect a frequency generator (sine wave) to the BNC socket (modulation in-

put) at the back plate of the power supply unit.

• Choose the proper frequency range (approx. 10 Hz to 2 kHz) of the sine wave generator.

• Set amplitude of the frequency generator to zero.

• For monitoring of low frequency resonance oscillations, connect the BNC socket of the piezoelectric sensor (if available) at the top of the reflector flange to an oscilloscope.

• Deploy and stabilize a droplet in the levitator.

• Switch on the sine wave generator.

• Change the modulation frequency slowly from 10 Hz to 2 kHz at a low ampli-tude (50 mV - 100 mV).

• If axial or radial droplet oscillations occur, the modulation amplitude must be reduced.

These vibration experiments may take advantage of the reflector sensor, which a l-lows a simple recording and monitoring of resonance curves with a commercial oscilloscope and might save costs for expensive optical recording equipment.


4.3.2 Double-walled Process Chamber Several experiments will require exactly defined environmental conditions inside the processing chamber, including temperature, humidity and chemical composi-tion of the carrier gas. This is possible by using the optional, double wall process chamber, hooked up to a commercial thermostat and a simple external gas humidi-fier/dryer (no tec5 AG product), which allows a gentle gas-stream of exactly de-fined humidity and temperature to be blown into the processing chamber. The gas inflow is released through the axial bore hole of the ultrasonic transducer. Some experiments will require processing temperatures which might considerably differ from room conditions. The recommended operating condition range of the processing chamber is between –100°C and +80°C, with the obligation to main-tain the resonance tuning of the levitator in consideration of the temperature d e-pending sound velocity.

4.3.3 Additional Aerodynamic Levitation The bore hole of the ultrasonic transducer can also serve as a jet nozzle, when a gas stream is to be blown axially against the levitated sample. A separate and ex-changeable gas nozzle, integrated into the reflector, provides a better defined free jet flow and thus allows a combination of ultrasonic and aerodynamic levitation, even at considerably increased temperatures. This unique feature is essential for heat- and mass transfer experiments at extremely different Reynolds-, Nusselt- and Sherwood numbers.


4.4 Some helpful tricks for experimenting with the levitator

• Small styrofoam spheres are ideal samples for getting familiar with the levita-

tor.

• Silver iodine is an ideal tracer material to make streams inside a levitated drop visible.

• Air flows around a levitated drop can be made visible with tracing fume and light-sheet illumination using a cylinder lens mounted in one window opening of the processing chamber.

• The self-excitation of sample oscillations can be prevented by tilting the levit a-tor axis by approximately 10 - 15 °. This is of particular importance for very small samples (see comment below).

• Sulphur powder is an excellent tracer material for fluid currents at the sample surface.

• Water drops with dissolved Sodium chloride or Lithium chloride can be used to demonstrate the shrinking and growth of a levitated drop by condensation and diffusion from the environment. Depending on the actual vapour pressure of the solution and the environment, the evaporation-shrinking or condensa-tion-growth can be stopped (stable equilibrium of the drop diameter).

• For stable positioning of very small droplets or particles, with diameters smaller than ca. 100 µm, the following conditions are of importance:

− closed process chamber to avoid disturbing influence of external air flows

− slightly tilted (10° - 15°) position of the levitator stabilizes the levitation conditions

− the levitator should be mounted on a base plate which is isolated against vibrations and oscillations


4.5 Potential Applications The following list is a selection of experiments with the acoustic levitator: • Experiments using low frequency modulation technique in order to find char-

acteristic sample resonances for :

− Calibration of the acoustic field or measurement of physical sample properties.

• Experiments using the exactly defined environmental conditions inside of the processing chamber for the study of:

− Controlled evaporation and condensation growth of drops (incl. multi component liquids).

− Growth of single crystals from evaporating solution drops (oversatura-tion of solutions).

− Dissolution of solids and liquids in a liquid matrix.

− Diffusion of gases into a liquid matrix.

− Heat and mass transfer between drops and their environment at different convection flow (incl. the influence of drop resonance vibrations).

− Melting and solidification processes.

− Overheating and supercooling of liquids and melts.

− "Critical point" phenomena in binary systems under controlled tempera-ture variation.

− Spontaneous nucleation in supersaturated or supercooled drops by laser spot heating.

− Marangoni currents, excited by polar laser spot heating of levitated drops.

− Reaction kinetics of liquid/liquid and liquid/gas systems.

− Measurement of thermo-physical properties with extremely small sam-ples.

− Investigation of biomedical cell cultures under ideal isolation conditions.

• Other experiments:

− Investigation of drop deformation and disintegration (atomization).

− Investigation of liquid shells.

− Combustion of single fuel drops.

− Micro- and trace analysis with small samples.

− Light scattering at droplets and non-spherical particles.

− Calibration os particle sizing instruments.


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Annamalai, O., Trinh, E.H., Wang, T.G. Experimental Study of the Oscillations of a Rotating Drop, J. Fluid Mech., 158 (1985) 31.

Barmatz, M., Allen, J.L., Gaspar, M. Experimental Investigation of the Scattering Effects of a Sphere in a Cylindrical Resonant Chamber, J. Acoust. Soc. Am., 73 (1983) 725.

Barmatz, M., Collas, P. Acoustic Radiation Force on a Sphere in Plane, Cylindrical and Spherical Standing Wave Fields, Proc. of the 11th Int. Cong. on Acoustics, Paris, France, Vol. 1 (1983) 245.

Barmatz, M. Acoustic Containerless Processing - Overview, Proc. of Novel Methods for Material Synthesis, DOE Workshop (1984).

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Barmatz, M., Collas, P. Acoustic Radiation Potential on a Sphere in Plane, Cylindrical and Spherical Standing Wave Fields, J. Acoust. Soc. Am., 77 (1985) 928-945.

Busse, F.G., Wang, T.G. Torque Generated by Orthogonal Acoustic Waves - Theory, J. Acoust. Soc. Am., 69 (1981).

Collas, P., Barmatz, M. Acoustic Radiation Force on a Particle in a Temperature Gradient, J. Acoust. Soc. Am., 81 (1981) 1327-1330.

Collas, P., Barmatz, M., Shipley, C. Acoustic Levitation in Presence of Gravity, J. Acoust. Soc. Am., 86 (1989) 777-787.

Croonquist, A.P., Elleman, D.D., Jacobi, N., Wang, T.G. Acoustic Positioning and the Response of Large Liquid Drops in Low Gravity, Materials Research Soc. Symp., Vol. 9 (1982).

Crum, L.A. Acoustic Force on a Liquid Droplet in an Acoustic Stationary Wave, J. Acoust. Soc. Am., 50 (1971) 157.

Crum, L.A. Bjerknes Forces on Bubbles in a Stationary Sound Field, J. Acoust. Soc. Am., 57 (1975) 1363-1370.

Daidzic, N., Stadler, R., Melling, A. Droplet Evaporation and Deformations in an Amplitude Mo-dulated Ultrasonic Field, Report LSTM 373/TE/93, Lehrstuhl für Strömungsmechanik, Universität Erlangen-Nürnberg, Erlangen (1993).

Daidzic, N., Stadler, R., Domnick, J. Experimental Techniques for Measurements of Droplet Evaporation, Proc. Int. Conf. on Liquid Atomization and Spray Syst., ICLASS-94, Rouen, France (1994) 875-882.

Daidzic, N., Stadler, R., Melling, A., Tropea, C. Nonlinear Droplet Stability in Atomization Proc-esses, Proc. Int. Conf. on Liquid Atomization and Spray Syst., ICLASS-94, Rouen, France (1994) 142-149.

Elleman, D.D., Croonquist, A.P., Wang, T.G. Low Gravity Acoustic Positioning Chamber for Liq-uid Shell Studies, Proc. of the Conf. on Internal Confinement Fusion, Cleos/ICF´80 (1980).

Gammell, P.M., Croonquist, A.P., Wang, T.G. Application of Microgravity and Containerless En-vironments to the Investigation of Stabilized Acoustic Levitation of Dense Materials Using a High Powered Siren, NASA JPL Publ. 82-92 (1982), also J. Acoust. Soc. Am., 83 (1988) 496-513.


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