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Anomalous negative Poisson's ratio materials: why does compression make them wider?
In the world of materials science, mechanical metamaterials are gradually becoming a popular research field. These artificial materials are designed with precise geometric arrangements and exhibit unconventional physical and mechanical properties that are often derived from their unique internal structure rather than the materials they are made of. Researchers are inspired by many factors, including biological materials in nature (such as honeycombs and cells), molecular and crystal unit structures, and origami and kirigami techniques in the arts.
Mechanical metamaterials have mechanical properties not found in nature, such as negative stiffness, negative Poisson's ratio, negative compressibility and vanishing shear modulus.
Negative Poisson’s ratio materials (Auxetics)
Poisson's ratio defines the extent to which a material expands or contracts laterally when it is compressed longitudinally. Most natural materials have a positive Poisson's ratio, meaning they expand vertically when compressed. The exception is negative Poisson's ratio materials (Auxetics). The Poisson's ratio of this type of material is less than zero, which means that when compressed, these materials will shrink further in the transverse direction. This phenomenon has long been proposed in some simple designs. , especially in 1985, relevant negative Poisson's ratio composite materials have been published in literature.
Negative stiffness material
Negative stiffness materials are designed to exhibit the exact opposite properties of traditional materials: when an external force is applied, the material deforms in a manner that causes the applied force to decrease, not increase. The material is composed of periodically arranged elements that create elastic instabilities when a load is applied, thus exhibiting negative stiffness behavior, which makes it stand out for its energy absorption and other mechanical properties.
Negative thermal expansion
Negative thermal expansion materials, whose coefficient of thermal expansion can be positive, negative or zero, often contain large amounts of voids, allowing them to exhibit considerable adjustability in the face of temperature changes. This feature is very helpful for improving optical, acoustic or vibration control systems.
High strength to low density ratio
The carefully designed internal microstructure of this type of mechanical metamaterial gives it outstanding performance in terms of weight. The material usually exhibits an extremely high strength to low density ratio, making it useful in a variety of applications. Excellent performance.
Negative compressibility and negative bulk modulus
Compared with ordinary materials that expand in the direction of force when stretched, some mechanical metamaterials can be designed to shrink in the opposite direction when stretched. This is simply contrary to people's basic understanding of mechanical materials. Additionally, these negative bulk modulus materials choose to expand when subjected to stress, making their applications in audio or sound wave propagation endlessly possible.
Programmable mechanical metamaterials
As research advances, more and more mechanical metamaterials not only have purely mechanical properties, but also combine electronic components to give them programmable response capabilities. This allows mechanical metamaterials to not only exhibit unique properties in static states, but also respond intelligently in dynamic situations.
Future Development Direction
With the continuous in-depth exploration of the properties and applications of mechanical metamaterials, the future of this research field is full of possibilities, especially in terms of automatic sensing and energy harvesting. These materials will be able to interact with their environment, adjust their responses to optimize their performance, and perhaps even incorporate their own intelligence into the mix.
Future mechanical metamaterials will likely have cognitive abilities similar to those of complex organisms, capable of sensing, self-powering, and even information processing in environmental changes.
The ecosystem of mechanical metamaterials is rapidly evolving, so how will these materials change the way we interact with materials in the future?
