A tightly compressed bundle of office staples can behave in a surprising way. Even though it is made of many separate pieces, the tangled mass can be difficult to pull apart and can act almost like a single solid object.
Yet that same bundle can quickly come undone. With the right vibration or movement, the staples can separate and return to a loose collection of individual pieces.
Researchers at the Paul M. Rady Department of Mechanical Engineering at CU Boulder believe this unusual combination of strength and reversibility could help inspire a new generation of engineered materials. By designing particles that interlock in a similar way to staples, they hope to create materials that are strong, adaptable, and potentially recyclable.
“We’ve been playing around with the idea of building blocks and geometry for many years, but we started looking at interlocking, entangled particles only recently,” said Professor Francois Barthelat, the leader of the Laboratory for Advanced Materials & Bioinspiration. “We are excited about the combination of properties we can get out of these systems and we believe this technology has the potential to go in many directions.”
The findings were recently published in the Journal of Applied Physics.
How Entangled Particles Create Strength
The research centers on a phenomenon known as entanglement, which occurs when particles become intertwined and form connections with one another.
Entanglement is common throughout nature. Bird nests, for example, rely on a network of interwoven twigs and fibers to maintain their structure. Bones also gain strength through the interaction of hard mineral components and softer proteins.
The CU Boulder team wanted to understand how similar principles could be used to create manufactured materials. Their work pointed to one crucial factor: the shape of the particles themselves.
“Let’s take sand as an example. Sand is smooth and convex-shaped, meaning it cannot interlock from grain to grain,” PhD student Youhan Sohn said. “However, we found that if we change the shape of a grain of sand, we can drastically affect its behavior and mechanical properties, including the particle’s ability to link with other particles.”
To investigate further, the researchers used Monte Carlo simulations, a computational technique that allowed them to study how different particle shapes interact. Their objective was to identify a geometry that would maximize entanglement.
Why Staple-Shaped Particles Stand Out
After identifying promising designs through simulation, the team conducted pickup tests to observe how the particles behaved in real-world conditions.
The results revealed that a “two-legged” particle, resembling a staple, produced the highest degree of entanglement. The researchers also found that this shape offered several unexpected benefits.
One of the most notable was its ability to combine tensile strength and toughness, two properties that are often difficult to achieve together in conventional materials.
“Our entangled granular material using the staple-like particle demonstrates both high strength and toughness at the same time,” said PhD student Saeed Pezeshki.
The staple-like particles also displayed another unusual characteristic. They could rapidly come together into a stronger structure and then just as quickly separate again.
By applying different vibration patterns, the researchers were able to control how strongly the particles became entangled. Gentle vibrations encouraged the particles to interlock and strengthen the material, while stronger vibrations caused the network to unravel.
“It’s a strange material because it’s obviously not a liquid. However, it’s also not quite solid. This opens new and intriguing engineering possibilities,” Barthelat said. “Handling a bundle of these entangled particles feels very remote and exotic.”
Potential Uses in Construction and Robotics
The researchers believe the technology could eventually support more sustainable approaches to construction.
In the future, bridges, buildings, and other large structures might be built using entangled materials that can later be taken apart rather than demolished. Such materials could potentially be reused or fully recycled at the end of their service life.
The concept may also have applications in robotics.
“I was talking with other students who believe this technology can be used in swarm robotics — where small robots can entangle, do a task and then disentangle when they are done,” said Pezeshki.
“Yes, kind of like that liquid metal T-1000 in Terminator 2 who can change shape to slide under a door and then transform back to a human’s size on the other side,” added Barthelat. “It’s expensive and scaling up is a challenge, but it’s something that’s on everybody’s mind.”
Testing Even Stronger Particle Designs
The team is now moving into the next stage of the research.
Their latest experiments focus on a new particle design that includes additional protruding “legs.” The researchers compare the shape to the spiky burrs that cling stubbornly to shoes and clothing outdoors. They believe these added features could create even stronger entanglement effects and unlock new possibilities for future materials.