Imagine discovering that within a flowing liquid metal, some atoms refuse to move—standing still amidst a sea of chaos. This surprising finding challenges our traditional understanding of how atoms behave in liquids, especially under extremely high temperatures. What's even more fascinating is that these immobile atoms aren't just passive spectators; they actively influence the transformation of liquid into solid, leading to the formation of a peculiar state known as a corralled supercooled liquid.
Understanding how materials transition from liquids to solids is vital across many natural and technological processes. For example, in nature, it explains how minerals form, how ice crystallizes, and how protein structures fold. In industry, this knowledge underpins manufacturing processes in fields like pharmaceuticals, metallurgy, aerospace, construction, and electronics, where controlling solidification is often key to product quality.
To probe these phenomena at the atomic level, scientists from the University of Nottingham and the University of Ulm employed advanced transmission electron microscopy to observe tiny molten metal droplets—or nano-sized pools of liquid metal—as they cooled and solidified. Their groundbreaking research was published on December 9 in the reputable journal ACS Nano.
Lead researcher Professor Andrei Khlobystov explained, "While we typically think of matter as existing in three basic states—gas, liquid, and solid—liquids remain the most elusive and enigmatic. The behavior of atoms in gases and solids is relatively straightforward to understand. However, liquids have a complex, crowded dance of atoms that is much harder to decipher, especially during the critical transition into a solid. This phase determines many of the material's ultimate properties."
The team uncovered that, despite the high-energy environment, some atoms in molten metal droplets stay fixed. Dr. Christopher Leist, who conducted the experiments using the low-voltage SALVE electron microscope at Ulm, shared, "We started by melting metal nanoparticles like platinum, gold, and palladium placed on an ultra-thin support—graphene. We used this graphene 'hob' to heat the particles, and as they melted, their atoms moved rapidly, as expected. But surprisingly, some atoms remained stationary, strongly anchored at specific defects in the support structure."
Further investigation revealed that these stationary atoms are bonded tightly at certain defect sites—called point defects—on the surface. These strong bonds persist even at very high temperatures. By deliberately focusing the electron beam on these areas, researchers could create additional defects, effectively controlling how many atoms remained trapped and inert within the liquid.
This discovery introduces a fascinating interplay between the wave-like and particle-like behaviors of electrons, as explained by Professor Ute Kaiser of Ulm University. "Our experiments vividly demonstrate the wave-particle duality of electrons: they behave like waves when visualized using electrons, yet they deliver discrete momentum 'bursts'—particles—that can either mobilize or even freeze atoms in place at the boundary of a liquid metal. This phenomenon led us to identify a new, exotic state of matter."
Their earlier work involved tracking chemical reactions at the single-molecule level, capturing real-time images of bonds breaking and reforming. Now, their latest research sheds light on how immobile atoms influence the process by which liquids turn into solids. When only a handful of atoms are pinned, crystals can form and grow progressively from the liquid. But when many atoms remain fixed, they interfere with crystal growth, preventing the formation of a regular crystal structure.
Khlobystov highlights a striking phenomenon: when stationary atoms form a ring-like barrier around the liquid, they create what the scientists call an 'atomic corral.' This corral traps the liquid metal beneath it, stabilizing it even at temperatures well below its usual freezing point—in platinum’s case, as low as 350°C, which is over 1,000°C lower than expected. Such a supercooled, corralled liquid can persist until the temperature drops enough further for it to eventually solidify—however, not into a traditional crystalline form. Instead, it becomes an amorphous metal—a non-crystalline, glass-like solid that is highly unstable and exists only temporarily before transforming back into a crystalline structure once the confinement weakens.
Interestingly, this research reveals a new hybrid state of metal that combines characteristics of both liquids and solids. Dr. Jesum Alves Fernandes from Nottingham explains, "Discovering this confined liquid state challenges traditional phase diagrams and might revolutionize our understanding of catalysts—especially platinum-based ones, which are among the most widely used globally. This phenomenon could lead to the creation of self-cleaning catalysts with enhanced activity and durability."
This breakthrough represents a significant step forward because, until now, nanoscale corral-like control was achieved only with photons or electrons. Now, researchers demonstrate for the first time that individual atoms can also be effectively "corralled" at the nanoscale. Khlobystov envisions a future where deliberately placing pinned atoms could lead to the design of more complex, larger atomic corrals, opening new pathways in the efficient use of precious metals—particularly in sustainable energy technologies such as energy harvesting and storage.
Funded by the EPSRC Program Grant 'Metal atoms on surfaces and interfaces (MASI) for sustainable future,' this research is not just a scientific milestone but also a potential game-changer for both fundamental physics and practical applications. It prompts us to ask: Could controlling atoms at this level soon become a routine part of manufacturing, and how might this reshape our approach to creating advanced materials? As we push the boundaries of what’s possible at the atomic scale, the big question remains—are we on the verge of a new era in materials science, or are there still surprises waiting in the quantum realm? Your thoughts and opinions are eagerly awaited—share your perspective in the comments below.
