In order to generate the tailored acoustic field, the team used a square matrix array of about 1000 transducers, operating at ultrasound frequency (550 kilohertz), placed at the bottom of a water-filled chamber. Each one can be individually addressed, not only with on-off states, but also by programming up to eight discrete phase values. This device allows the researchers to imprint a spatial modulation on the emitted acoustic field. In this case, the generated patterns behave, in the region of interest, as a pair of plane waves whose wave vectors are symmetrically inclined towards the array centerline, where the target was suspended from a force balance. The two waves intersect at a certain height over the array plane, having a hollow core underneath; the height of the intersection point can be readjusted by changing the emission pattern from the array.
By varying the vertical position of the target, the researchers were able to obtain a detailed map of the acoustic force as a function of the distance from the object to the source plane, which was measured in terms of changes in weight of the target. Based on a theoretical analysis, two contributions of the force were clearly identified; a negative (pulling) force due to the reflection in the walls of the prisms, and a positive (pushing) one, owed to the absorbed radiation at the base. The qualitative behavior of the total force and its order of magnitude (millinewtons) agreed well with the experiments. The equilibrium position, where the two contributions of the force are exactly balanced, can be controlled by reconfiguring the acoustic field. All the measurements were done for different configurations of the acoustic pattern and for each of the two targets, whose volumes are of the order of tens of cubic centimeters.
Although the intensity gradients may play a role in the force towards the low-pressure regions, the careful design of the experiment for maximizing the forward scattering, the ability to control the equilibrium position, and the fair comparison with simulations guarantee that the main role is indeed played by the scattering force. Therefore, this is the first demonstration of a nonconservative pulling force in the acoustics realm, which complements previous demonstrations in optics.
In addition, the authors point out the potential impact that the control of acoustic forces and the generation of structured ultrasound fields may have in modern biomedical techniques. In therapeutic treatments involving focused high-intensity fields, for example, the precise control of energy deposition could be greatly improved with the use of complex beams.
Interaction between matter and waves always seems to surprise scientists in new ways. The more we advance in the study and generation of structured wave fields and novel materials, the higher the possibility of finding new effects and applications."
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