Several physical effects allow free floatation of solid and even liquid matter. Materials may be levitated by a jet of gas, by intense sound waves, or by beams of laser light. In addition, conductors levitate in strong radio-frequency fields, charged particles in alternating electric fields, and magnets above superconductors or vice versa. Although levitation by means of ferromagnets is unstable, supper-conductors may be suspended both above and below a magnet as a result of flux pinning. Levitation is used for containerless processing and investigation of materials, for frictionless bearings and high-speed ground transportation, for spectroscopy of single atoms and microparticles, and for demonstrating superconductivity in the new oxide superconductors.

https://science.sciencemag.org/content/sci/243/4889/349.full.pdf

Levitation in Physics

The study of metallic melts, their physical properties, and their solidification behaviour is of relevance to understanding of fundamental principles and to metallurgical applications. Many, if not...

Containerless processing in the study of metallic melts and their solidification

We demonstrate the trapping of elastic particles by the large gradient force of a single acoustical beam in three dimensions. Acoustical tweezers can push, pull and accurately control both the position and the forces exerted on a unique particle. Forces in excess of 1 micronewton were exerted on polystyrene beads in the submillimeter range. A beam intensity less than 50  W/cm^{2} was required, ensuring damage-free trapping conditions. The large spectrum of frequencies covered by coherent ultrasonic sources provides a wide variety of manipulation possibilities from macroscopic to microscopic length scales. Our observations could open the way to important applications, in particular, in biology and biophysics at the cellular scale and for the design of acoustical machines in microfluidic environments.

/pdf/observation-of-a-single-beam-gradient-force-acoustical-trap-50ijzk7f71.pdf

Observation of a Single-Beam Gradient Force Acoustical Trap for Elastic Particles: Acoustical Tweezers.

Ultrasound waves can levitate heavy balls of tungsten. This contact-free method of keeping items suspended in the air can be applied to the investigation and processing of new materials.

Acoustic physics. Suspended by sound.

Acoustic levitation has the potential to enable novel studies due to its ability to hold a wide variety of substances against gravity under container-less conditions. It has found application in spectroscopy, chemistry, and the study of organisms in microgravity. Current levitators are constructed using Langevin horns that need to be manufactured to high tolerance with carefully matched resonant frequencies. This resonance condition is hard to maintain as their temperature changes due to transduction heating. In addition, Langevin horns are required to operate at high voltages (>100 V) which may cause problems in challenging experimental environments. Here, we design, build, and evaluate a single-axis levitator based on multiple, low-voltage (ca. 20 V), well-matched, and commercially available ultrasonic transducers. The levitator operates at 40 kHz in air and can trap objects above 2.2 g/cm3 density and 4 mm in diameter whilst consuming 10 W of input power. Levitation of water, fused-silica spheres, small insects, and electronic components is demonstrated. The device is constructed from low-cost off-the-shelf components and is easily assembled using 3D printed sections. Complete instructions and a part list are provided on how to assemble the levitator.

/pdf/tinylev-a-multi-emitter-single-axis-acoustic-levitator-aplnx2uij3.pdf

TinyLev: A multi-emitter single-axis acoustic levitator

An ultrasonic levitation device operable in both ordinary ground-based as well as in potential space-borne laboratories is described together with its various applications in the fields of fluid dynamics, material science, and light scattering. Some of the phenomena which can be studied by this instrument include surface waves on freely suspended liquids, the variations of the surface tension with temperature and contamination, the deep undercooling of materials with the temperature variations of their density and viscosity, and finally some of the optical diffraction properties of transparent substances.

E. H. Trinh

Papers

Compact acoustic levitation device for studies in fluid dynamics and material science in the laboratory and microgravity