Researchers have simultaneously and independently controlled the motion of several trapped particles in three dimensions (3D) using acoustic radiation forces, as reported in a recent issue of the Proceedings of the National Academy of Sciences (PNAS). The acoustic tweezers, the sound equivalent of the better-known optical tweezers, created by Asier Marzo from the Public University of Navarre (UNPA) and Bruce Drinkwater from the University of Bristol could be useful for positioning particles and assembling structures inside a range of materials—gases, liquids, plastics, or metals.

“With sound, we have the capability of positioning particles (spherical or elongated) at specific locations and orientations within a material,” says Marzo, who started this work at Bristol and is now at UNPA. “For instance, we could align glass fibers inside resin before curing it to get different orientations at specific parts. Or we could attach small rods together in mid-air,” he says.

Acoustic tweezers trap particles in the nodes (regions of low amplitude) of an acoustic pressure field. By controlling the motion of the nodes, an operator can manipulate the trapped particles. The technique works for particles ranging in size from micrometers to centimeters. Researchers have manipulated groups of particles in 3D with acoustic tweezers before, but have not been able to isolate the motion of individual particles within a group.

The development of acoustics tweezers has been largely motivated by advancements in optical tweezers, which use lasers to trap and move particles. In 1998, researchers pioneered a tool for moving multiple particles independently and in 3D using a dynamic hologram, known as holographic optical tweezers (HOT). With this new research, Marzo and Drinkwater created an analogous version of HOT but using sound, which they call holographic acoustic tweezers (HAT).

“In general, [acoustic tweezers] are 20 years lagged behind optical tweezers, but it is worth researching acoustic tweezers given their unique advantages,” Marzo says. Optical tweezers typically manipulate nano- or micrometer-scale particles, but acoustic tweezers can manipulate particles over a larger range of sizes. Acoustic tweezers also work in a wider range of media (air, water-based media, or living tissue) and are more efficient, demonstrating trapping forces five orders of magnitude higher per unit input power than optical tweezers.

To harness these advantages, Marzo and Drinkwater designed a 16 × 16 array of ultrasound (40 kHz) emitters. Each emitter had an adjustable phase shifter. The research team developed an iterative algorithm that calculated the necessary emission phase of each element to create nodes at the target positions. Another algorithm predicted the resulting acoustic field and forces in real time. By controlling the phases of the emitters, the researchers were able to prescribe and move the locations of the acoustic traps.

To demonstrate the two-dimensional capabilities of their HAT system, the researchers positioned the array 13 cm above a flat and sound-reflective surface—a distance at which the acoustic pressure was high. They placed several expanded polystyrene spheres with diameters 1-3 mm on the surface. Using the algorithm, the researchers generated a focal point at the location of each sphere. By controlling the emitted phases, they were able to trap and move 25 spheres independently in the x-y plane, just above the surface. The researchers also demonstrated independent control of particle orientation and the chirality of vortex traps that can be used to rotate particles trapped inside.

To demonstrate the 3D capabilities of their system, the researchers positioned two arrays facing each other, separated by a distance of 23 cm. By creating nodes at the desired trapping locations, Marzo and Drinkwater obtained independent and simultaneous control of multiple levitated spheres in 3D. They fully controlled up to 12 spheres at once, arranging them into an icosahedron that could rotate around different axes.

Using simulations and experiments, the researchers characterized the quality of the traps and observed an inversely proportional relationship between the number of trapped particles and the trapping force. Their analysis suggests that with smaller or more powerful emitters, they could fully control 27 particles—a number that rivals current HOT (i.e., optical) systems.

Marco Andrade, an expert in acoustic levitation at the University of São Paulo in Brazil, is enthusiastic about this work. “The novelty of this new study is that the authors can not only levitate many particles, but they can also control the position of each particle individually,” he says.

Andrade expects that HAT will open up many possibilities in the near future, such as the ability to levitate, transport, and mix liquid droplets in mid-air and to engineer biological tissue. It could also be combined with tools like x-ray diffraction to study how particles interact. “[HAT] could be applied to levitate and mix different substances while remote detection systems are employed to investigate what is going on. Perhaps new materials can be developed by employing this technique,” Andrade says.

Marzo is already working on an additive manufacturing technique using acoustic levitation to bring particles toward a central seed. If this works in air, he plans to adapt the technique for water-based media in order to assemble cells into patterns and attach them into scaffolds for building biological materials.  

The next step toward realizing such applications is optimizing the system to work with more particles. “With 100 particles we would be able to represent relatively complex objects in mid-air. These objects, formed by levitated particles, could be visualized by multiple people with coherent points of view,” Marzo says.

Read the article in Proceedings of the National Academy of Sciences.