Figure 1. Acoustic pressure, velocity and levitation force in an ultrasonic standing wave.
Funding
Basic Technology Research
Programme
Ultrasonic or acoustic levitation is a technique for 'containerless processing' of small samples of a solid or liquid. Samples with diameters of up to a few millimeters can be suspended in a gaseous carrier medium by means of an acoustic standing wave field. This is useful for many investigations on single liquid droplets or solid particles under well defined environmental conditions, such as temperature, gas pressure and composition and relative humidity.
The development of advanced hardware employing acoustic, electromagnetic and electrostatic levitation techniques was initiated more than 20 years ago by the microgravity research programmes of ESA and also in the United States through programmes undertaken by NASA. Numerous experiments on fluid physics and materials science, e.g. on glass and metal solidification demonstrated that contactless positioning of small samples against multiaxial residual accelerations of the order 1 cm/s ² =10-³ g0 were possible. Tests conducted on ground, in parabolic flight, with sounding rockets (TEXUS), and later during manned space flights, proved the suitability of ultrasonic levitation techniques for terrestrial experiments as well.
Figure 1 illustrates the levitation of a spherical sample in an ultrasonic planar standing wave maintained by a piezoelectric transducer (bottom) and a concentric reflector which may be flat or concave. Also shown are the axial velocity and pressure profiles of the standing wave containing characteristic nodes and anti nodes. From these, the profiles of the kinetic and potential energy densities are derived. The energy density profile creates a pressure field around the sample, producing an axial acoustic levitation force which counteracts the gravity field acting on the sample. The sample now appears to be suspended in weightlessness.
Figure 1. Acoustic pressure, velocity and levitation force in an
ultrasonic standing wave.
Under microgravity conditions, the sample assumes a stable position at a pressure node, while under terrestrial conditions its weight is compensated at a downwards displacement of the sample center below the pressure node. A symmetrical radial profile of the pressure and the velocity amplitude, resulting from a slight divergence of the standing ultrasonic wave, centers the levitated sample within the levitation axis by a force which is about one third to one fifth of the axial levitation force.
The standard ultrasonic levitator (Figure 2) uses a frequency of 58 kHz, and generates between four and five pressure nodes. Since the two outer node planes are influenced by destabilising near field effects from the transducer and the reflector, only the inner two or three nodes are useable for stable levitation (Figure 3).
Figure 2. Liquid samples can easily be deployed using a
microlitre syringe.
Figure 3. Two liquid samples levitated in the standing ultrasonic
wave field.
Integration of a small free-jet nozzle into the reflector turns an ultrasonic levitator into a hybrid aerodynamic acoustic levitator suitable for heat and mass transfer investigations involving large variations in the Reynolds and Nusselt numbers.
Although ultrasonic levitation was initially developed as a necessary tool for space experiments, this technique is also suitable for terrestrial use in various research fields. It has already been successfully used in analytical chemistry to perform micro trace analysis where it offers several benefits:
Crystallisation processes in small droplets have also been investigated under accoustic levitation; for example a droplet of an aqueous solution of sodium nitrate was continuously subjected to spectroscopic measurements. Raman spectroscopy provides access to inherent molecule torsions, i.e. phase changes can be acquired. Figure 4 shows the Raman spectrum as a function of recording time. The change of the peak characteristics at 1400 cm-¹ and 1670 cm-¹ clearly indicates the beginning of the crystallisation process. The rather broad peaks, caused by the ions of the solution, turn into very sharp peaks, caused by the crystals.
Figure 4. Raman spectrum of crystallising sodium nitrate solution
(Courtesy of Ruhr Universität Bochum, Lehrstuhl für
Laseranwendungstechnik).
A hybrid aerodynamic acoustic levitator has been used to investigate the flow around a droplet and to quantify the influence of the acoustic field on the flow. This investigation, performed to reveal heat and mass transfer properties of droplets, has provided an insight into basic phenomena of spraying systems, such as fuel injection and spray drying.
These examples are only a limited selection of a multitude of potential applications for which levitation techniques can provide useful tools for experiments on single droplets or particles, and where a contacting surface would strongly influence or inhibit the process under investigation. A miniaturised acoustic positioning system has been commercialised by DANTEC/invent Measurement Technology GmbH.
A technology initially developed within the TRP for space borne applications now proved to be appropriate for a variety of experiments performed under gravity. Other positioning techniques were derived from these studies, such as the hybrid levitator. The levitators simplify or enable investigations, more difficult or even impossible to perform with the presence of a contacting surface.
Support from M. Dieckmann and A. Lundström of the Instrument Technology Division, ESTEC and P. Behrmann of the Microgravity Facilities for Columbus Division, ESTEC is gratefully acknowledged.
Preparing for the Future Vol. 6 No. 3