American researchers recently identified a fascinating phenomenon that appears to allow materials to defy the laws of physics. These extremely counterintuitive behaviors could lead to a small revolution in materials science, with very interesting real-world applications.
The vast majority of known materials follow a certain number of virtually immovable rules, rooted in the laws of thermodynamics. Pressure, for example, forces the constituents of matter closer together, within a certain limit; the more it increases, the more the volume is supposed to decrease. Temperature, on the other hand, has the opposite effect. As it increases, so does the internal energy of atoms, which begin to vibrate with an increasingly large amplitude: the volume of the material therefore tends to increase when it is heated.
But in a study published in the prestigious journal Nature, researchers from the Universities of Chicago and San Diego have demonstrated that these two rules are not, after all, set in stone. In fact, they can even reverse when the materials are in a so-called “metastable” state.
A question of energy
Over time, all physical systems tend to evolve towards a state of equilibrium where their energy level will be minimal (see the concept of entropie for more details): this is called a stable state. If we represent the system's energy on a curve, this stable state is represented by the lowest point — the global minimum.
To visualize this concept, you can imagine a ball spontaneously rolling toward the bottom of a valley over time: it stops moving when it stabilizes at the bottom of the slope, and thus reaches its stable state.
But the situation can also be more complex. Imagine, for example, that our ball is placed in a hollow located at the top of a peak. Under these conditions, it cannot reach the lowest point, the global minimum, which corresponds to the stable state. Instead, it finds itself stuck in what is called a local minimum from which it can only escape with the intervention of an external force, such as a person pushing it out of the hollow in which it is lodged.
In thermodynamics, this is called a metastable equilibrium. This phenomenon, for example, is why supercooled water (below zero degrees, but still liquid) does not spontaneously freeze if left alone. However, at the slightest disturbance, the entire volume quickly turns to ice.
Extremely counterintuitive effects
Returning to this study, the authors showed that these metastable states can significantly alter the behavior of materials. In their stable state, they perfectly follow the conventional rules of thermodynamics. But under these particular conditions, the situation changes radically, and particularly counterintuitive phenomena can emerge.
Certain materials in metastable states can, for example, contract when heated, and conversely, expand under pressure. The authors speak of “negative compressibility,” and this is something that has never been observed before.
Immense concrete potential
The most exciting thing is that this work isn't just promising for fundamental research. According to the authors, it might be possible to tune these metastable states, for example through redox reactions, to change how materials respond to heat and other forms of energy. In theory, this could lead to the design of materials with extremely useful properties.
The team cites a particularly relevant example in the field of construction. To build a structure, the effects of thermal expansion on the constituents must be rigorously controlled; if a building's structure expands or contracts beyond a critical threshold during a heatwave or cold spell, it could collapse with catastrophic consequences. If it were possible to design materials with a zero coefficient of thermal expansion, this would immediately remove a major constraint in structural engineering.
Another potential application lies in electric vehicles. Over the course of charge and discharge cycles, their batteries tend to lose capacity because the lithium ions used to store energy gradually become trapped at the anode. This chemical alteration is currently irreversible—but with a perfectly calibrated metastable material, it would theoretically be possible to reverse the reaction to restore the battery to its original state.
And this is only the tip of a huge iceberg of possibilities. This concept could pave the way for a host of materials, each more revolutionary than the last... on paper, at least. It should be noted that this study remains very exploratory. At present, there is no guarantee that researchers will one day succeed in taming these metastable states to bring out all these highly desirable properties.
But the potential of this new field of research is such that many specialists will undoubtedly take a close interest in it in the near future. It will be very interesting to follow their work, which could ultimately lead to a small theoretical revolution with a very concrete impact on many industries.
The text of the study is available here.
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