Abstract
We investigate a model system for the rotational dynamics of inertial many-particle clustering, in which submillimeter objects are acoustically levitated in air. Driven by scattered sound, levitated grains self-assemble into a monolayer of particles, forming mesoscopic granular rafts with both an acoustic binding energy and a bending rigidity. Detuning the acoustic trap can give rise to stochastic forces and torques that impart angular momentum to levitated objects. As the angular momentum of a quasi-two-dimensional granular raft is increased, the raft deforms from a disk to an ellipse, eventually pinching off into multiple separate rafts, in a mechanism that resembles the breakup of a liquid drop. We extract the raft effective surface tension and elastic modulus and show that nonpairwise acoustic forces give rise to effective elastic moduli that scale with the raft size. We also show that the raft size controls the microstructural basis of plastic deformation, resulting in a transition from fracture to ductile failure.
3 More- Received 17 June 2021
- Revised 14 February 2022
- Accepted 4 March 2022
DOI:https://doi.org/10.1103/PhysRevX.12.021017
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Focus
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Popular Summary
Many physical systems are composed of rapidly spinning objects, from rotating black holes to self-gravitating asteroids. The stability and shape of these rotating systems is determined by a balance between attractive forces, which bind the material together, and outward pressure from the rotation. Rotating liquid droplets are often used as experimental and mathematical models, with surface tension playing the role of the attractive force. Still, even in tiny liquid droplets, the number of molecules is so large that internal restructuring under rotation is difficult to assess. To probe the limit where individual constituents in a rapidly rotating system can be tracked with precision, we acoustically levitate a granular material.
When particles are levitated, sound scattering generates attractive forces that organize the granular material into a tightly packed single layer of submillimeter-sized particles. This levitated “raft” has an overall surface-tension-like cohesion. The acoustic trap also increases the rate of rotation of levitated objects, driving transitions in the shape of the rafts and their eventual breakup. Increasing the number of particles in a raft from 10 to 200, we find a crossover from soft solid to liquidlike behavior. In contrast to liquids, however, the effective surface tension and bulk modulus of acoustically levitated rafts increase with their size, similar to the attractive forces in spinning objects held together by gravitational forces.
These results provide insight into the emergence of liquidlike behavior in clusters of cohesive particles and might even open the door toward the systematic study of “tabletop asteroids.”