Design and operation of dynamic meta-bricks, showing the concept of in situ reconfigurability within a set of assembled metamaterials. a) Metasurface photograph, showing acoustic levitation of a pink polystyrene bead in the inset. Scale bars, 1 cm. b) (i) Schematic of a multilayer metasurface, showing the section within a dotted red box in (a). (ii) Photograph of the unactuated meta-brick (A0) and actuated meta-brick (A1) states, showing the meta-brick highlighted in the dotted blue box in (i). Scale bars, 2 mm. c) Diagrams of the cross section of three meta-bricks. (i) The first meta-brick has no flap. The black arrows represent the direction of propagation, with a 40 kHz input from the bottom. (ii) The second meta-brick shows how the static flaps (boxed in yellow) form maze-like obstacles inside the channel to increase the overall path length of the moving sound waves. (iii) For the third meta-brick in the dotted orange box, the lower flap is replaced by a black dynamic flap. In the inset, it shows that the dynamic shutter can move downward in the direction of the arrow within the 90 degree range delineated by the dotted lines. Credit: Communication mediado I:
Spatially coiled acoustic metamaterials are static and require manual reconfiguration for sound field modulation. In a new report published in Communication mediaChristabel Choi and a team of computer science and engineering scientists in the UK and Italy, developed an active reconfiguration approach with autonomous dynamics for space coil unit cells known as dynamic meta-bricks.
The meta-bricks housed an actuable, magnetorheological, elastomeric shutter to function as a switch and directly regulate the transmitted ultrasound. Scientists have shown the synergy between active and passive reconfigurability to develop multifunctional metamaterials with additional degrees of freedom, for design and implementation.
Smart materials
The current era of smart materials has seen the rise of metamaterials to innovate sound manipulation technologies. Reengineering projects have recently explored acoustic metamaterials to enhance complex wave shaping applications, including acoustic levitation, masking, and holographic imaging.
Researchers can strategically regulate the fitness and composition of a structure on demand to enable greater functional flexibility and deployment. To achieve real-time functionality, scientists modulated the sound field upon actuation using a transmissive acoustic metamaterial as a platform to explore the synergy between active and passive reconfigurability of a metasurface to achieve a amended.
Next-generation metamaterials engineering
In this work, Choi and colleagues showed that a metasurface does not require a fully dynamic nature to generate dynamic output. Conventionally, an active metasurface can be formed from a complete set of actively reconfigurable unit cells exhibiting a high degree of electronic and computational complexity.
The scientists combined static and dynamic meta-bricks to create hybrid meta-brick stacks within the metasurface. The researchers placed the dynamic meta-bricks on the edges of the metasurfaces and magnetically regulated them to enable precise sound modulation through simulations and experiments.
Sound engineers have so far only achieved acoustic levitation with static metamaterials. The ability to modulate ultrasound in real time has implications in various fields, including energy harvesting. Commercial audio applications can, for example, use metamaterials to allow a narrow beam of sound to be dynamically directed to specific locations on demand. This work demonstrates a method for designing next-generation versatile, tunable, and multifunctional metamaterials.
Design the dynamic meta-brick
The presence of internal protrusions on the side walls of a meta-brick can create a labyrinthine path through which sound waves can travel. While meta-bricks can be sized to operate at lower frequencies, flaps can be designed to operate at an airborne ultrasonic frequency of 40 kHz; suitable for contactless handling and haptic feedback.
Using a magnetorheological elastomer, the team avoided conventional hinge-type mechanisms due to the high amounts of associated friction, in order to achieve a maximum deflection angle for the meta-brick. The active binary beat facilitated the path inside the meta-brick to form an editable maze to transfer acoustic waves in real time.
Make a dynamic meta-brick
Choi and colleagues developed a dynamic meta-brick in which the external components referred to the shell of the meta-brick and the internal components referred to static and dynamic flaps of different lengths. The team developed the meta-brick shell as well as the static and dynamic flaps via three-dimensional printing and molding methods.
For casting, materials scientists used planar glass plates, developed from synthetic magnetic nanoparticles mixed with Ecoflex and cast into 3D printed molds.
An illustration of the meta-brick stacks and respective layers in the metasurface. a) Photograph of stacked metasurface layers and inset showing (i) “static” and (ii) “dynamic hybrid” meta-brick stacks. Scale bar, 1 cm. b) 3D maps for the upper and lower metasurface layers. The “top layer” map shows the dynamic meta-bricks (purple) located along the edge of the metasurface. The fully static map of the “bottom layer” shows the meta-brick identifications (IDs) (based on a set of 16 static meta-bricks) and the corresponding locations of the assembled static meta-bricks. Credit: Communication mediado I:
They placed the molds on a magnet during the curing process and used a combination of washing and soaking at high temperatures to remove polymerization inhibitors. The team molded each flap with a consistent thickness and coefficient of variation.
After assembling the dynamic meta-brick, they operated it with a permanent magnet. When actuated, the shutter quickly moved towards the wall. In the presence of the magnetic field, the shutter was held and stable, while when not operated by a magnet, the shutter remained in its original state.
Binary ultrasonic modulation, meta-bricks and metasurfaces
The team conducted simulations and experimental plots to show how combined actuation states affected transmission in a small dynamic network; the results were in good agreement. While each meta-brick allowed a specific phase shift, the physically combined meta-bricks in a metasurface formed a combined phase shift as a collective acoustic output.
The researchers achieved the desired output sound field by predefining the phase values to determine the type of meta-brick required to evaluate their placement relative to each other.
By including a small number of locally actuated dynamic meta-bricks, they made an otherwise static global metasurface dynamically function. First, they regulated the magnetic shutter within the dynamic meta-brick, then evaluated the meta-bricks within a metasurface by stacking. While static stacks were formed by placing a static meta-brick on top of another similar structure, dynamic stacks combined the two to create a vertical supercell.
a) Each transducer t contributes to the complex pressure incident on the underside of the AMM. b) The dynamic metasurface stack concentrates at two different points when the dynamic meta-bricks are actuated and unactuated, respectively. c) We create a stack of static and dynamic metasurfaces to generate the two focal points. Note that focal points are not drawn to scale in the diagrams. d) We average the central bricks because they must remain static. e) For edge bricks, we review brick by brick to determine whether a fully static pier or a hybrid static-dynamic pier is more appropriate for that particular location. f) We add π to half of the metasurface to transform our attention into a twin compartment. g) Output of the algorithm. Credit: Communication mediado I:
Dynamic acoustic levitation
Cho and colleagues performed pressure measurements by turning the metasurfaces on and off to visualize real-time modulation of the sound field. They designed twin stacked composite metasurfaces to demonstrate and contain the focused beams. The sound pressure balance inside these enclosures could pinch objects in low sound pressure regions.
For experimental validation, the research team moved a lightweight polystyrene ball between the two compartments. Upon actuation, the bead did not fall, indicating how rapid modulation of the sound field could maintain acoustic levitation.
Outlook
In this way, Christabel Choi and her team introduced dynamic meta-bricks as a paradigm for designing dynamic acoustic metamaterials that have emerged at the forefront of innovation in sound manipulation technologies. Materials scientists have been carefully exploring this niche to improve complex wave shaping applications, including acoustic levitation, masking, beam steering, and holographic imaging.
By including a small dynamic magnetic flap, the scientists transformed a static meta-brick into a dynamic construct and combined the two to produce more than one output as a dynamic metasurface. The results could pave the way for more sophisticated designs.
The team explored the experimental results with a theoretical model and via COMSOL Multiphysics simulations to show their excellent agreement. Such actuators can be functionalized, structured or coated to provide additional functionality to fluidic systems and valves. These interdisciplinary approaches could pave the way for the development of the next generation of metamaterials.
More information:
Christabel Choi et al, A magnetically actuated dynamic labyrinthine transmissive ultrasonic metamaterial, Communication media (2024). DOI: 10.1038/s43246-023-00438-4
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