Forget Lasers. The Hot New Tool for Physicists Is Sound
Sound waves can also steer objects inside organisms. Daniel Ahmed, an engineer at ETH Zurich in Switzerland, recently used ultrasound to move hollow plastic beads inside a live zebrafish embryo. By doing these experiments, Ahmed aims to demonstrate the potential of using sound to guide drugs to a target site within an animal, such as a tumor. Similar to the acoustic tweezer, the ultrasound creates a repeating pattern of low and high pressure areas within the embryo, allowing Ahmed to use the pressure pockets to push the beads around. Other researchers are investigating the steering capability of sound to treat kidney stones. A 2020 study, for example, used ultrasound to move the stones around in the bladders of living pigs.
Other researchers are developing a technology known as acoustic holography to shape sound waves, in order to more precisely design the location and shape of the pressure zones in a medium. Scientists project sound waves through a patterned plate known as an acoustic hologram, which is often 3D-printed and computer-designed. It shapes the sound waves in an intricate, predefined way, just like an optical hologram does for light. In particular, researchers are investigating how they can use acoustic holograms for brain research, focusing ultrasound waves to target a precise place in the head, which could be useful for imaging and therapeutic purposes.
Andrea Alù also explores new ways of shaping sound waves, but not necessarily tailored to specific applications. In one recent demonstration, his team controlled sound with Legos.
In order to control sound propagation in new ways, his team stacked the plastic blocks on a platter in a grid pattern, making them stick up like trees in a forest. By shaking the platter, they produced sound waves on its surface. But sound traveled bizarrely over the platter. Normally, a sound wave should disperse symmetrically in concentric circles, like the ripple from a pebble falling into a pond. Alù could make the sound only travel in particular patterns.
Alù’s project draws inspiration not from light, but from the electron—which, according to quantum mechanics, is both a wave and a particle. In particular, the Legos were designed to mimic the crystal pattern of a type of material known as twisted bilayer graphene, which restricts the motion of its electrons in a distinctive way. Under certain conditions, electrons only flow on the edges of this material. Under others, the material becomes superconducting, and the electrons form pairs and move through it without electrical resistance.
Because electrons move so strangely in this material, Alù’s team predicted that the crystal geometry, scaled up to Lego size, would also restrict the movement of sound. In an experiment, the team found that they could make the sound emanate in an elongated egg shape, or in ripples that curve outward like the tips of a slingshot.
These unusual acoustic trajectories illustrated surprising parallels between sound and electrons, and hint at more versatile ways of controlling sound propagation, which could prove useful for ultrasound imaging or the acoustic technology that cell phones rely on for communicating with cell towers, says Alù. For example, Alù has created a device with similar principles that allows sound to only propagate in one direction. Thus, the device can distinguish a transmission signal from a return signal, which means it can enable technology to transmit and receive signals of the same frequency simultaneously. That’s unlike sonar, which sends out an acoustic wave and has to wait for the echo to return before pinging the environment again.
But applications aside, these experiments have changed how scientists think about sound. It’s not just something you can blast from the rooftops, whisper in someone’s ear, or even use to map an undersea environment. It’s becoming a precision tool that scientists can mold, direct, and manipulate for their needs.
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