Acoustic tweezers manipulate multiple particles independently

Arthur Ashkin received half of the 2018 Nobel Prize in Physics for his development of optical tweezers, which allow researchers to manipulate nano- to micron-sized physical and biological specimens (see Physics Today, December 2018, page 14 ). But using light isn’t the only way to trap particles; researchers have manipulated them using acoustic radiation forces that originate from the scattering of their ultrasonic waves. Those acoustic tweezers are catching on as a useful tool because of their energy efficiency and their ability to control larger centimeter-sized particles. Now Asier Marzo at the Public University of Navarre and Bruce Drinkwater at the University of Bristol have improved that technology by developing an algorithm that can manipulate multiple particles simultaneously and independently.

Marzo and Drinkwater’s advance tunes the transducers (essentially high-frequency loudspeakers) of an acoustic array to create a field with multiple functional traps in different arrangements. The algorithm calculates the acoustic pressure that should be emitted from each transducer to trap particles within the minimum pressure regions. And because the algorithm runs in real time, those regions can be displaced by small increments and cause individual particles to move independently.

For the experimental setup, a grid of 256 transducers operating at 40 kHz was arranged about 15 cm from an acoustically reflective surface. With the improved algorithm, the acoustic tweezers, set up in a plane, managed to manipulate 12 millimeter-sized spheres that came as close as 1.3 cm to each other. When Marzo and Drinkwater replaced the reflective surface with another grid of transducers, they were able to manipulate another dozen particles simultaneously in three dimensions, as shown in the image. You can see the millimeter-sized polystyrene spheres move in the video.

Compared with the acoustic tweezers in 3D, optical tweezers can manipulate more particles, 27. But rather than compete, the two technologies may be complementary. Because acoustic radiation can act through tissue, acoustic tweezers could find use for in vivo biomedical applications. (A. Marzo, B. W. Drinkwater, Proc. Natl. Acad. Sci. USA, 2018, doi:10.1073/pnas.1813047115 .)