In nature, honeycomb materials are mechanically very efficient, especially in terms of various performance couplings. For example, some random honeycomb structures found in nature, such as teeth, bones, and beaks, have superior strength and toughness relative to their density. Some projects in materials science mimic this type of structure, such as polymers or metal foams with similar structures and functions.
In contrast, ordered honeycomb structures, including periodic structures that have evolved naturally in nature, often outperform random structures. For example, the mollusk's defensive carapace pearl-like inner layer is composed of a hard brick-like structure. Correspondingly, the mantis shrimp evolved an aggressive large claw to hit the shell of the mollusk at high speed, and its front cheek was composed of a spiral-shaped stack of resistant mineral fibers.
Periodic and hierarchical structures have been widely used in large buildings such as truss bridges and Eiffel towers. Now, new manufacturing and 3D printing technologies can also build honeycomb structures at the nano, micro, meso, and macro levels. Generally, these materials can show unique combinations of mechanical, functional, and thermal properties and become so-called "metamaterials."
Metamaterials refer to a class of man-made materials with special properties that are not found in nature, including lightweight but hard, high mechanical elasticity, negative Poisson's ratio, and multi-material layouts with negative thermal expansion coefficients. In the past, these materials and buildings tended to be fixed quickly after being built, which limited their usefulness.
In order to make materials with more sensitive and adaptable materials, "4D printing" has become a new research hotspot in the field of materials. Compared to 3D, the extra "D" represents time. 4D printing is to make materials in addition to the X, Y, Z axis to move around, because of changes in external conditions, changes in shape or function over time. Due to mechanical forces, temperature, expansion, and magnetic fields, 4D printed materials can reconfigure themselves to change color or shape.
Unfortunately, to date, existing 4D printing technologies either lack a high degree of precise control over mechanical properties, or require long response times due to transmission restrictions or the slowness of the chemical reaction itself. To this end, a group of materials scientists from Lawrence Livermore National Laboratory, Argonne National Laboratory, and the University of California proposed a new 4D printing solution-Magnetic Field Reactive Mechanical Metamaterial (FRMM), to demonstrate programmable Predictable and highly controlled changes in mechanical properties, with large dynamic range and fast reversible response, facilitate the application of remote magnetic fields.
To obtain a FRMM with dynamically adjustable stiffness, researchers introduced magnetorheological fluid suspension (MR) into the core of a three-dimensional printed polymer tube, a building block of honeycomb units and lattices. MR is composed of ferromagnetic particles suspended in a non-magnetic liquid. Under the action of a magnetic field, the viscosity of MR will change rapidly. In the absence of a magnetic field, the MR fluid behaves as a liquid with randomly distributed suspended particles. The suspended particles will flow freely to form a pool when they are deposited on a flat substrate.
When a magnetic field is applied, the suspended particles are arranged in chains along the magnetic field lines, forming a needle-like, leaf-like structure. When the ordered particles in the MR fluid are subjected to a magnetic field, the viscosity of the fluid monotonically increases until it saturates. At this time, further strengthening the magnetic field will not produce additional rheological effects.
After the theory was put forward, the research team conducted quite complicated tests and verifications, which are not listed here one by one. To put it simply, to make this 3D structure including pillars, honeycomb units and lattices, a photochemical scanning ultraviolet additive manufacturing technology is called large projection area micro stereolithography (LAPμSL). With this technique, a cured 2D layer is formed from a cured liquid resin, and then the substrate is placed in a resin bath, and subsequent images are placed in the scan stack to form the next layer. This process will continue until a 3D part is generated.
As a result of the experiment, the research team created an adjustable FRMM with a large dynamic range and a fast and reversible mechanical response to remotely applied magnetic fields. At the same time, through the fabrication and testing of a single magnetorheological rod, they also developed an empirically calibrated model to predict the magneto-mechanical response of the FRMM grid and provide support for future design optimization work.
In addition, they have created a new production process based on 3D printing technology and controlled fluid delivery methods. In the future, FRMM may consist of actively addressed microfluidic networks, where MR fluid composition can be performed in space and time. Tweaked to further expand the design and accessible property space. In addition, the magnetic field adjustment can enhance the direction control, which is suitable for a wider range of deformation modes and application environments. Eventually, FRMM may be widely used in a range of emerging applications, including software robots, rapid adaptation helmets, and smart wearables with vibration damping capabilities.