A ‘Solid–Liquid Hybrid’ at the Nanoscale: Why Scientists Say the Line Between Phases Is Blurring

Scientists from Germany and the U.K. have reported evidence of an unusual new state of matter — one that behaves like a solid and a liquid at the same time, but only at the nanoscale. Observed inside individual metal nanoparticles, this solid–liquid hybrid challenges long-held assumptions about how matter changes phase and could have important implications for catalysts used in fuel cells and clean-energy technologies.

What did the scientists discover?

The study, carried out by researchers from Ulm University and the University of Nottingham, reports a previously unknown phase behaviour in nanoparticles of platinum, palladium and gold. Their findings were published in ACS Nano.

Rather than being entirely solid or entirely liquid, individual nanoparticles were found to exist in a hybrid state: parts of the same particle behaved like a solid, while other parts behaved like a liquid. Importantly, this is not a macroscopic mixture like slush or gel, but a single nanoparticle with different atomic regions simultaneously occupying different phases.

How is this different from how solids and liquids are usually understood?

In conventional physics, the distinction is clear. In a solid, atoms occupy fixed positions in a crystal lattice, vibrating only slightly. In a liquid, atoms move rapidly and randomly, constantly rearranging their positions.

At very small scales, however, those assumptions begin to fray. The researchers set out to study what happens at the boundary between solid and liquid phases when matter is reduced to nanoparticles — objects only a few billionths of a metre across — and placed under physical confinement.

How did researchers observe this hybrid state?

Using high-resolution transmission electron (HRTE) microscopy, the team observed metal nanoparticles deposited on graphene, a one-atom-thick sheet of carbon atoms arranged in a honeycomb lattice.

The images revealed something striking. Even when the nanoparticles were heated enough to be considered liquid, some metal atoms remained stationary. These atoms were effectively trapped in the tiny gaps of graphene’s carbon network. Under the microscope, the stationary atoms appeared sharp and clearly defined, while the liquid region appeared blurry or transparent because those atoms moved faster than the imaging timescale.

When many such stationary atoms lined up around the edge of a nanodroplet, they formed a rigid perimeter that “corralled” the liquid core — creating a solid–liquid hybrid within a single particle.

Why can these nanoparticles stay liquid at unusually low temperatures?

One of the most surprising results was how stable the liquid state became under confinement. The corralled nanoparticles remained liquid at temperatures between 200–300°C. By contrast, similar but unconfined particles typically crystallised only at around 500°C.

This physical confinement also altered how the particles froze. Instead of forming an orderly crystal lattice when cooled, the nanoparticles solidified into a disordered, amorphous solid — chemically identical to the metal, but structurally distinct from its natural crystalline form.

These findings suggest that, at the nanoscale, the transition between solid and liquid is not a sharp boundary but a blurred continuum shaped by geometry, confinement and atomic motion.

Why does this matter beyond fundamental physics?

The implications extend well beyond phase theory. The behaviour is especially relevant for heterogeneous catalysts, such as platinum on carbon — a material widely used in proton exchange membrane fuel cells and direct methanol fuel cells.

In real-world applications, platinum nanoparticles tend to clump together or become chemically “poisoned”, reducing their effectiveness over time. The new findings suggest a possible way around this problem. Corralling effects could pin nanoparticles in place, preventing clumping while keeping them in highly active liquid or amorphous states.

Such catalysts could remain effective for longer periods instead of becoming structurally unavailable — a potentially significant advance for hydrogen vehicles and stationary power generators.

What does this mean for our understanding of matter?

At its core, the study challenges a basic assumption taught in textbooks: that solid and liquid are cleanly separable phases. At sufficiently small scales, the distinction becomes fuzzy, and new hybrid behaviours emerge.

For physicists, this opens a window into how phase transitions really work when atoms are few, motion is constrained, and surfaces dominate behaviour. For engineers, it hints at new ways to design materials whose properties can be tuned not just by composition, but by structure and confinement at the atomic level.

The solid–liquid hybrid nanoparticle may not rewrite thermodynamics — but it strongly suggests that, at the nanoscale, matter still has many surprises left.