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What if particles don't slow down in a crowd, but move faster? Â鶹ÒùÔºicists from Leiden worked together and discovered a new state of matter, where particles pass on energy through collisions and create more movement when packed closely together.

We all know crowds of people, or cars in a traffic jam—when it gets too crowded, all you can do is stand still. Until now, scientists have mainly studied cases of large groups just like this, which slow down when they get too close to each other.

But what if the opposite happens? What if particles could start moving more when packed together? That question hadn't been studied much—until now. Â鶹ÒùÔºicists Marine Le Blay, Joshua Saldi and Alexandre Morin from Leiden University do research in the field of active matter physics—they observe and analyze the collective behaviors that emerge when large groups of particles are packed together.

The work is in the journal Nature Â鶹ÒùÔºics.

Metal beads and electrodes

In their experiments, Morin, Le Blay and Saldi worked with tiny, non-moving particles: one-millimeter metal beads, placed between two glass plates. "These two plates holding the beads are not just simple glass plates; they are also electrodes. Our beads can't move by themselves, but when we charge them with electricity, they start to jump up and down. They travel back and forth between the two glass plates extremely fast, around 100 times in 1 second. This way, we give energy to our system of beads," Saldi explains. The Fine Mechanical Department of the Science Faculty helped them create this creative lab setup.

Morin adds, "While the beads move around, we take 300 to 400 images per second with our high-speed camera. We then make slow-motion videos of these images to discover in detail what is happening. We link every particle from one image to the next and make precise statistics of the movements observed. One afternoon of experimenting can fill up an entire hard drive. We use a strong computer and an efficient analysis routine to do this work."

Moving as a group

With only a few beads in the experiment, the beads stay in place—nothing surprising yet. When the researchers increased the number of beads to hundreds and thousands, something very surprising happened: the beads started moving around wildly. They formed a very dynamic and disordered system, which we call an 'active gas' state of matter.

How is this possible? The researchers watched and analyzed the movement patterns and discovered that the particles bump into each other in a very special way. Their collisions are super elastic. Instead of losing energy like a bouncing ball eventually coming to rest, they pass energy on to each other during these bumps, which triggers more movement. So, the more particles there are, the more collisions happen—and the more active the whole system becomes.

Controlling the movement patterns

"Once we understood how these packed particles power up, we realized that we could even control the collective behavior of the metal beads," Morin explains. "To do so, instead of powering the particles continuously, we powered them intermittently by switching the electric field on and off. We observed that the faster the switching, the slower the movements. But more importantly, the structure of the group also changed.

"Overall, we could obtain structures analogous to the three well-known states of matter: gas, liquid, and crystal—by simply turning a knob on our power generator."

Why this discovery matter

Morin says, "This is an important discovery, because it reveals that there are still many unknown ways in which particles can organize themselves." This opens the door to new types of behavior in particle systems, with possible applications in technology, biology, and materials science.

Even though the research is still in an early stage, Morin thinks this discovery could help create new smart materials in the future. Living things can do many things—like remembering, growing, healing, and processing information—that normal materials like fabric or steel cannot do. This research shows how simple active materials can change their shape and patterns on their own. A step towards better designed and more advanced man-made materials.