Acoustic Levitation

Sound is usually thought of as something that moves through air, not something that can hold an object suspended in it, but focused ultrasonic waves can do exactly that. When two or more sound waves are aimed at each other and tuned to the right frequency, they combine to form a standing wave pattern with alternating zones of high and low pressure. At certain fixed points along that pattern, called pressure nodes, forces from different directions cancel out almost perfectly, and a small, lightweight object placed there can hang suspended in midair, held by sound alone. Researchers have used the technique to levitate water droplets, tiny insects, beads of foam, and even small electronic parts, moving them through the air by shifting frequency or the position of the sound source. The appeal for laboratories lies in what levitation avoids: touching an object at all. NASA and other research groups have explored acoustic levitation as a way to handle hazardous or easily contaminated materials without any container or gripping tool making contact, and to study how liquids and chemical reactions behave when suspended freely, closer to conditions in microgravity. The technique does have real limits -- larger or denser objects require dramatically more power to support, which for now keeps acoustic levitation confined mostly to small, light objects rather than anything human-scale.