American scientists invented the new "super material" with 4D printing
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In nature, honeycomb materials are mechanically very efficient, especially in a variety of different performance couplings. For example, some random honeycomb structures found in nature, such as teeth, bones, and guanine, have excellent strength and toughness relative to their density. Some of the materials science projects mimic such structures, such as polymers and metal foams that are structurally and functionally similar.
In comparison, ordered honeycomb structures, including periodic structures that naturally evolved in nature, tend to outperform random structures. For example, the mollusk's defensive shell-like pearly inner layer is composed of a hard brick-like structure. Correspondingly, the prawn evolved an aggressive large-claw hook for high-speed impact on the outer shell of the mollusc, while its anterior stalk portion consisted of a spiral stack of fracture-resistant mineralized fibers.
Cyclical and hierarchical structures have been widely used in large buildings such as the truss bridge and the Eiffel Tower. Nowadays, new manufacturing and 3D printing technologies can also be used to build honeycomb structures at the nano, micro, medium, and macro levels. Typically, these materials exhibit a unique combination of mechanical, functional, and thermal properties that become so-called "supermaterials."
Metamaterial refers to a class of man-made materials with special properties that are not found in nature, including lightweight but hard, high mechanical elasticity, a negative Poisson's ratio, and a multi-material layout with a negative thermal expansion coefficient. In the past, these materials and buildings were often fixed quickly after they were formed, which limited their usefulness.
In order to create a more sensitive and adaptable material, "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 change the shape or function of the material over time, in addition to the X, Y, and Z axes. Due to mechanical forces, temperature, expansion and magnetic fields, 4D printed materials can be self-reconfigured to change color or shape.
Unfortunately, to date, existing 4D printing techniques either lack highly precise control of mechanical properties or require long reaction times due to transmission limitations or slow chemical reactions themselves. To this end, a group of materials scientists from Lawrence Livermore National Laboratory, Argonne National Laboratory, and the University of California presented 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 for easy application of remote magnetic fields.
To obtain FRMM with dynamically adjustable stiffness, the researchers introduced a magnetorheological fluid suspension (MR) into the core of a three-dimensional printed polymer tube, a building block for honeycomb cells and lattices. MR is composed of ferromagnetic particles suspended in a non-magnetic liquid, and the viscosity of MR changes rapidly under the action of a magnetic field. In the absence of a magnetic field, the MR fluid appears as a liquid in which the suspended particles are randomly distributed, and the suspended particles flow freely to form a pool when deposited on a planar substrate.
When a magnetic field is applied, the suspended particles are arranged in a chain along the magnetic field lines to form a needle-like, blade-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 is saturated. At this point, further strengthening the magnetic field does not create additional rheological effects.
After the theory was put forward, the research team conducted quite complicated tests and check calculations. This article does not list them one by one. To put it simply, to make such a 3D structure including pillars, honeycomb cells and crystal lattices, a photochemical scanning ultraviolet additive manufacturing technique called a large projection area micro-stereolithography (LAPμSL) is used. By this technique, a cured 2D layer is formed with a solidified liquid resin, and the substrate is placed in a resin bath, and a subsequent image is placed in the scanning 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 individual magnetorheological rods, they also developed an empirically calibrated model to predict the magnetic response of the FRMM grid to support future design optimization efforts.
In addition, they have created a new production process based on 3D printing technology and controlled fluid delivery methods. Future FRMMs may consist of actively addressed microfluidic networks, where MR fluid composition can be spatially and temporally Tuning to further extend the design and accessible property spaces. In addition, magnetic field adjustment enhances directional control for a wider range of deformation modes and applications. Ultimately, FRMM may be used in a wide range of emerging applications, including software robots, quick-fit helmets, and smart wearables with vibration absorbing performance.