This review is written by non-expert and used for personal study only.
Mechanical nonreciprocity in a uniform composite material
- Metamaterials
Understanding Reciprocity and Nonreciprocity
Reciprocity is a fundamental concept in physics that ensures equal signal propagation in both directions within physical systems. In contrast, nonreciprocity occurs when signals propagate differently depending on the direction, which can be achieved by breaking time-reversal or material deformation symmetry. By incorporating mechanical nonreciprocity as a built-in property in a uniform material, it becomes possible to control and manipulate objects that interact with the material.
Developing Nonreciprocal Materials
To create a uniform material with nonreciprocal response to shear forces (forces that act parallel to a surface), scientists can introduce symmetry breaking within the material’s nanostructure. This can be done using a simple model that exploits buckling—a phenomenon where a structure bends under pressure. The process involves a tilted beam that buckles differently depending on the direction of the applied force, leading to an asymmetric response of the entire material to shear forces. This mechanism is beneficial for producing a uniform material with mechanical nonreciprocity.
Composite Hydrogel with Graphene Oxide Nanosheets
Researchers designed a composite hydrogel containing graphene oxide nanosheets that are unidirectionally oriented in a tilted configuration. Graphene oxide is a highly versatile material known for its exceptional mechanical properties. This hydrogel exhibited mechanical nonreciprocity, displaying a 10-times larger displacement when sheared to the left than to the right. It showed a highly asymmetric response, with the softer and harder directions undergoing larger and smaller shear deformation, respectively. The elastic modulus, which measures the material’s stiffness, was found to be 5.7 kPa for left-shear strain and 380 kPa for right-shear strain.
Characteristics and Potential Applications of Nonreciprocal Gels
When sheared, the graphene oxide nanosheets within the nonreciprocal (NR) gel exhibited direction-dependent disordering and alignment. Advanced imaging techniques, such as small-angle x-ray scattering and scanning electron microscopy, confirmed the buckling of the nanosheets when the gel was sheared to the left. The NR gel’s nonreciprocal behavior resulted from the direction-dependent reinforcement ability of the graphene oxide nanosheets, which was influenced by factors like tilt angle, size, and concentration of the nanosheets.
Nonreciprocal mechanical systems can direct mechanical energy in a specific manner. For instance, asymmetric vibrations generated by an NR gel film can efficiently transport objects in a particular direction and may be used to convert vibrational noise into rotational motion. The NR gel is a versatile material that can be fabricated in any shape or size, making it highly adaptable and promising for energy-harvesting applications.
When subjected to local deformation, the NR gel exhibits significant asymmetric deformation, which can induce directional motion of objects it interacts with, such as tiny particles or small living organisms. For example, the migration of Caenorhabditis elegans (a type of worm) was affected by the NR gel, causing all worms to move toward the harder side of the gel film. In contrast, a gel with randomly oriented nanosheets did not influence the worms’ movement direction.
This innovative uniform material demonstrates mechanical nonreciprocity and asymmetric deformation, with potential applications in mechanics, energy, and biology. This development could inspire the creation of nonreciprocal materials in other physical systems like optics and acoustics, broadening the range of possible applications.
Required Additional Study Materials
Introductory material
- “Metamaterials: Theory, Design, and Applications” by Tie Jun Cui, David Smith, and Ruopeng Liu
- “An Introduction to Metamaterials and Nanophotonics” by Constantin Simovski and Sergei Tretyakov
- “Metamaterials: Technology and Applications” by Pankaj K. Choudhury
Reference
SCIENCE 13 Apr 2023 Vol 380, Issue 6641 pp. 192-198 DOI: 10.1126/science.adf12
Leave a comment