APA 7: TWs Editor. (2023, November 16). New Technique Boosts the Progress of Acoustic Lenses, Impact-Resistant Films, and More. PerEXP Teamworks. [News Link]
Simulations have their limits in design, requiring physical testing to validate the effectiveness of metamaterials. However, assessing how these materials respond to microscale forces without direct contact, which could potentially damage their intricate structures, has posed a significant challenge.
A novel laser-based technique has emerged as a rapid and secure solution, presenting the potential to accelerate the identification of promising metamaterials for practical applications.
Engineers at MIT have devised a novel technique employing a dual-laser system to investigate metamaterials. One laser rapidly applies energy to the structure, while the other assesses the resulting vibrations—similar to striking a bell and recording its reverberations. Notably, the lasers don’t make physical contact, yet they induce vibrations in the metamaterial’s microscale beams and struts, simulating physical impacts like stretching or shearing.
Subsequent to inducing vibrations, engineers can extract valuable data on the metamaterial’s dynamic characteristics using the laser-based technique. This information encompasses the material’s responses to impacts, as well as its capacity to absorb or scatter sound. The use of an ultrafast laser pulse facilitates the swift excitation and measurement of hundreds of minute structures within minutes, rendering this approach a secure, efficient, and high-throughput means of dynamically characterizing microscale metamaterials—a breakthrough in the field.
Carlos Portela, the Brit and Alex d’Arbeloff Career Development Professor in Mechanical Engineering at MIT, emphasizes the need for expedited methods to test, optimize, and refine metamaterials. He highlights that the laser-based technique developed by MIT allows for the acceleration of the discovery process for optimal materials based on desired properties.
Portela and his team introduce their novel system, dubbed LIRAS (laser-induced resonant acoustic spectroscopy), in a forthcoming paper in Nature. The MIT collaborators on the paper include lead author Yun Kai, along with Somayajulu Dhulipala, Rachel Sun, Jet Lem, and Thomas Pezeril. Additionally, Washington DeLima from the Department of Energy’s Kansas City National Security Campus contributed to the research.
A gradual pointer
Portela focuses on metamaterials crafted from everyday polymers, intricately 3D-printed into miniature scaffold-like structures comprising microscopic struts and beams. These structures, composed of repeated geometric units, like an eight-pointed configuration of interconnected beams, imbue the entire polymer with unique properties that transcend its inherent characteristics.
Engineers face significant constraints when it comes to physically testing and confirming the properties of these metamaterials. The conventional approach involves nanoindentation, a method that delicately employs a micrometer-scale tip to gradually exert pressure on a structure. Simultaneously, it measures the minute displacements and forces acting on the structure during compression.
Carlos Portela highlights the limitations of the nanoindentation technique, emphasizing its constrained speed and potential for structural damage. The aim was to devise a method that could dynamically assess how these structures respond, especially in the face of a powerful impact, without causing their destruction.
In the realm of (Meta) materials
The researchers adopted laser ultrasonics, a nondestructive approach utilizing a precisely tuned laser pulse to ultrasound frequencies. This technique allowed them to excite thin materials, like gold films, without direct physical contact. The ultrasound waves induced by the laser could make a thin film vibrate at a specific frequency, enabling scientists to accurately measure its thickness down to the nanometer level. Additionally, this method could identify any defects present in the thin film.
Portela and his team recognized that ultrasonic lasers could safely stimulate vibrations in their 3D metamaterial towers. The towers, with heights ranging from 50 to 200 micrometers, are on a similar microscopic scale to thin films.
In an effort to validate this concept, Yun Kai, a member of Portela’s team specializing in laser optics, constructed a compact experimental arrangement. This tabletop configuration involved two ultrasonic lasers: a “pulse” laser responsible for stimulating metamaterial samples, and a “probe” laser dedicated to gauging the ensuing vibrations.
The team fabricated hundreds of minuscule towers, each with distinct height and architecture, onto a compact chip no larger than a fingernail. Subsequently, they introduced this diminutive array of metamaterials into the dual-laser arrangement. Utilizing repeated ultrashort pulses, the towers were stimulated, and the resulting vibrations from each tower were measured by the second laser. Following this, the team collected and analyzed the data, searching for discernible patterns within the vibrations.
Portela explains that they stimulate all these structures using a laser, likening the process to hitting them with a hammer. Subsequently, they record the oscillations from numerous towers, each exhibiting slightly different movements. The collected data on these oscillations is then analyzed to extract the dynamic characteristics of each structure. This includes parameters like their stiffness in response to impact and the speed at which ultrasound travels through them.
Utilizing the identical approach, the team employed the technique to examine towers for imperfections. They manufactured several towers without defects and replicated the same structures with differing levels of flaws, such as absent struts and beams, each smaller than the dimensions of a red blood cell.
Portela elaborates that because each tower possesses a distinct vibrational signature, they observed a noticeable shift in this signature with an increase in defects within the same structure. The analogy provided is akin to scanning an assembly line of structures; detecting one with a slightly altered signature indicates imperfections in that particular structure.
According to Portela, researchers can readily replicate the laser setup in their laboratories. He anticipates a surge in the identification and development of practical metamaterials for real-world applications. Portela expresses his own interest in producing and evaluating metamaterials designed to concentrate ultrasound waves, potentially enhancing the sensitivity of ultrasound probes. Additionally, he is investigating the feasibility of impact-resistant metamaterials, such as those that could be employed to line the interiors of bicycle helmets.
Kai emphasizes the significance of developing materials to alleviate the effects of shock and impacts. He notes that their study represents a breakthrough, enabling, for the first time, the characterization of the dynamic behavior of metamaterials. This newfound understanding opens avenues for exploring the capabilities of metamaterials to their fullest extent.
Resources
- NEWSPAPER Massachusetts Institute of Technology. (2023, November 15). New technique could speed up the development of acoustic lenses, impact-resistant films and other futuristic materials. Phys.org. [Phys.org]
- JOURNAL Kai, Y., Dhulipala, S., Sun, R., Lem, J., DeLima, W. J., Pézeril, T., & Portela, C. (2023). Dynamic diagnosis of metamaterials through laser-induced vibrational signatures. Nature, 623(7987), 514–521. [Nature]
