An engineering team at the University of Colorado Boulder recently demonstrated a new type of material with a sci-fi feel: an entangled particle system composed of small particles with special shapes that can switch freely between "hard whole" and "loose fluid". The researchers were inspired by a pair of ordinary office staples. When a large number of staples are entangled into a ball, they will resist external forces like a whole when being pulled. However, when vibrated or shaken in a specific way, they will quickly loosen and collapse into a pile of separated metal strips.
This phenomenon has prompted researchers to rethink material design approaches: instead of using traditional monolithic solids or chemical bonding, starting from geometric shapes, using a large number of small particles that can be "connected" to each other to build an overall structure through physical entanglement, while at the same time being able to quickly disintegrate when needed. "We have been playing with configurations and geometries for many years, but only recently have we begun to seriously study interlocking, entangled particles," said Professor Francois Barthelat, leader of the project and director of the Laboratory of Advanced Materials and Bio-Inspiration. "This system can exhibit a very unique set of performance combinations, and we believe it has a lot of room for engineering imagination."
The study, published in the Journal of Applied Physics, calls this phenomenon "entanglement" - the process by which particles become entangled with each other and form structural connections. Similar principles are familiar in nature: bird nests rely on the interweaving of branches and fibers for strength, and bones rely on the coupling between rigid minerals and soft proteins to achieve a balance of mechanical properties. The engineering challenge lies in reproducing this "interlocking" effect in artificial materials in a controllable way.
Barthelat's team believes the key lies in the geometry of the particles. "Take sand as an example. The surface of the sand grains is smooth and the overall shape is convex. It is almost impossible to achieve true interlocking between the particles," explained doctoral student Youhan Sohn. "But if we change the shape of a 'grain of sand', its macroscopic behavior and mechanical properties will change drastically, including the ability to entangle and interlock with other particles."
After realizing that shape is a key factor, the researchers used Monte Carlo simulations, a computational method, to predict the interactions between particles of different shapes and find geometric designs that produce the highest degree of entanglement. They then validated the simulation results through a series of "pickup tests" to see how the newly designed particles behaved during actual assembly, lifting and vibration.
The experiment finally gave an unexpected but extremely simple answer: "two-legged" particles similar to staples showed the strongest tendency to interlock. After stacking a large number of particles in this shape, the system can be tightly entangled to form a whole, and can also be loosened and dispersed under certain conditions.
This design brings several important performance advantages, one of which is the rare combination of high strength and high toughness. In traditional materials, high strength is often accompanied by an increase in brittleness, while high toughness often means a decrease in strength; however, this entangled particle material composed of "staple particles" performs well in both tensile strength and toughness. Ph.D. student Saeed Pezeshki noted: "Our entangled particle material utilizes these staple particles to maintain high strength while exhibiting excellent toughness."
Another major advantage is the rapid assembly and reversible disassembly of the system. The research team fine-tuned the degree of interlocking between particles by changing the vibration mode applied to the particle pile: gentle, low-intensity vibrations help the particles slowly "drill" into the gaps between each other, forming tighter entanglements, and improving the overall strength; while stronger vibrations will disrupt the original contact state, causing the structure to disintegrate and the particles to return to a free-flowing granular state.
"This is a very strange material. It is obviously not a liquid, but it cannot be simply classified as a solid," Barthelat said. "This opens a new door for engineering design. When you actually manipulate such a ball of entangled particles with your hands, there will be a strange and surreal feeling."
Among potential application directions, sustainable architecture is an important scenario. The research team envisions that future buildings and bridges can partially use this entangled particle material as a structure or filling unit: during the service period, they have good load-bearing capacity; and when the construction task is completed or the structural life ends, they can be disassembled as a whole to realize the reuse and recycling of components or particles.
Robotics is another possible path. Pezeshki revealed that in discussions with other students, he believed that this material concept could be extended to "swarm robotics": a large number of small robots are entangled with each other through shape and mechanism design, and are combined into larger and more complex structures when performing tasks; after the task is completed, they are unentangled with each other and dispersed to execute new instructions.
Barthelat used a familiar science fiction image as a metaphor - similar to the liquid metal robot T-1000 in the movie "Terminator 2": it can "liquefy" into a fluid state in a small space and pass through obstacles, and can re-condensate into a complete form on the other side. "Of course, the cost of this technology is currently very high, and there are still many challenges to achieve large-scale application, but this is a direction that many researchers are paying attention to," he said.
Currently, the team is still continuing to optimize this material system and try more complex particle designs, such as adding extra protruding "legs" or "hooks" to make the particles somewhat similar to the thorny tribulus commonly found on clothing. This type of multi-protruding structure is expected to further enhance the entanglement effect and improve the stability and adjustability of the overall structure.