Acoustic levitation

Sound waves carry momentum and exert forces when they are absorbed or scattered. In an acoustic levitator, transducers and reflectors or phased arrays create a structured sound field. A standing wave has stationary pressure patterns, and the time-averaged acoustic radiation force can counteract an object's weight, holding it near an equilibrium position. In air, sufficiently small particles in a conventional standing-wave configuration are often trapped near pressure nodes, where pressure fluctuations are low and particle velocity is high.

This is a balance of forces, not an absence of gravity. The object remains suspended only while the acoustic field supplies suitable forces and the trap is stable against disturbances. Ultrasonic frequencies are commonly used, so the sound is above the range of human hearing, but the exact operating frequency and geometry vary among devices.

The relation between particle size and sound wavelength matters. Approximations developed for particles much smaller than the wavelength do not describe every levitation regime. Experiments and three-dimensional analyses have shown that larger particles can occupy different stable positions, including locations near pressure antinodes; the radiation force and trapping stiffness can change substantially with particle size. Material properties and the shape and structure of the sound field also affect the result.

A single-axis transducer-and-reflector arrangement can produce a standing wave, while phased arrays can electronically reshape acoustic fields. By changing the phases of array elements, researchers can move or rearrange traps, allowing more flexible manipulation than a fixed node pattern. These methods require careful control of transducer output, geometry and sample behavior.

Because levitated samples do not touch a container, acoustic levitation can support containerless analysis and processing. It is used in laboratory studies of droplets and materials, where contact with a vessel might contaminate a sample or influence how it dries or solidifies. Argonne National Laboratory has described using an acoustic levitator to suspend pharmaceutical-solution droplets while studying their evaporation and solidification with X-ray measurements. Such work is an analytical research application, not evidence that levitation by itself improves a medicine's clinical effectiveness.

Other research explores non-contact transport and manipulation, droplet interactions, and experiments in which sound fields are dynamically controlled. Acoustic levitation has also been investigated for high-temperature containerless processing, including NASA-supported work. The suitability of the technique depends on sample size and properties, required stability, acoustic heating or streaming, and the need to control the surrounding environment.

Acoustic levitation is not a universal way to suspend arbitrary objects. The available acoustic force must be sufficient relative to gravity and other disturbances, and stable trapping depends on particle size, density, compressibility, shape, wavelength and field geometry. Small forces, airflow, evaporation and acoustic streaming can alter a sample or its motion. Complex samples may require three-dimensional control or multiple emitters.

The method should also be distinguished from optical or magnetic levitation: it relies on acoustic waves and their radiation forces. In research settings, levitation can reduce contact with container walls, but the sound field itself can still influence a sample. Results therefore need to be interpreted in light of the particular levitator and experimental conditions.