Researchers have successfully generated negative acoustic radiation forces, a phenomenon that counter-intuitively pushes objects in the opposite direction to sound wave propagation. This breakthrough, published in Nature, opens new avenues for precise manipulation of particles and cells in liquid environments, with significant implications for microfluidics and biomedicine. Traditionally, sound waves exert pressure that pushes objects in their direction of propagation; however, this study demonstrates the possibility of reversing this force through appropriate design of acoustic fields.

To achieve this, the team employed a specially designed acoustic transducer to create a complex sound field that induces these negative forces. The key lies in manipulating the phase and amplitude of the sound waves to generate pressure gradients that overcome conventional radiation force. This precise control of the acoustic field allows for the creation of regions where the net radiation pressure acts opposite to the acoustic energy flow, thus achieving the effect of "pulling" rather than "pushing" particles. Experiments were conducted with microspheres in aqueous suspension, validating the proposed theoretical model.

This discovery holds great potential for applications in contactless manipulation of microscopic objects. For instance, it could be used to trap and move individual cells, assemble microstructures, or separate particles based on their acoustic properties, all with unprecedented precision and control. The ability to exert acoustic attractive forces opens the door to new microfluidic techniques for diagnosis and biological research, where gentle sample manipulation is crucial. Next steps include optimizing the systems to increase the magnitude of these forces and exploring their application in complex biological systems.