Usually, chaotic behavior is observed in large systems, such as weather, coupled swinging pendulums or asteroids in space that are concurrently drawn to several massive celestial bodies. On the atomic scale, chaos is uncommon; instead, other effects are more prominent.
Yuri Suchorski (top), Keita Tokuda (bottom), Johannes Zeininger, Maximilian Raab, Günther Rupprechter (l.t.r.). Image Credit: Vienna University of Technology
Through chemical reactions on microscopic rhodium crystals, researchers at the Vienna University of Technology (TU Wien) have now, for the first time, clearly identified signs of chaos on the nanometer scale. Nature Communications published the findings.
From Inactive to Active—And Back Again
The chemical reaction under study is rather simple: oxygen mixes with hydrogen to generate water with the aid of a precious metal catalyst, which is also the fundamental working principle of a fuel cell.
The reaction rate is affected by external factors (pressure, temperature). Yet, even when the external circumstances are consistent, this reaction exhibits fluctuating behavior under some circumstances.
Similar to the way a pendulum swings from left to right and back again, the reaction rate oscillates between barely perceptible and high, and thus the catalytic system oscillates back and forth between inactive and active states.
Günther Rupprechter, Professor, Institute of Materials Chemistry, Vienna University of Technology
A pendulum is a prominent example of something predictable since it operates consistently even when disturbed slightly or put in motion twice in slightly different ways. In this sense, it differs significantly from a chaotic system, in which slight variations in the initial conditions produce drastically different outcomes in the long-term behavior. Several pendulums connected by elastic bands serve as an excellent illustration of this behavior.
Setting the Same Initial Conditions Twice Is Impossible
Setting exactly the same initial conditions twice is impossible. If we could start such a coupled system of pendulums in exactly the same way twice, the pendulums would move exactly the same way both times.
Yuri Suchorski, Professor, Vienna University of Technology
However, in reality, this is impossible because it is extremely difficult to duplicate the initial conditions exactly as they happened the first time. In fact, even a small difference in the initial conditions will cause the system to behave entirely differently than it did the first time.
This phenomenon is known as the “butterfly effect,” which indicates that small variations in the initial conditions have a significant impact on the state at a later time.
A rhodium nanocrystal’s chemical oscillations have recently shown something quite similar.
Maximilian Raab and Johannes Zeininger, who performed the experiments, stated, “The crystal consists of many different surface nanofacets, like a polished diamond, but much smaller, on the order of nanometers. On each of these facets, the chemical reaction oscillates, but the reactions on neighboring facets are coupled.”
Switching—From Order to Chaos
A fascinating new method of controlling coupling behavior involves altering the hydrogen content. At first, one element predominates and keeps things moving along like a pacemaker. Every other aspect joins in and moves to the same rhythm.
The situation becomes increasingly challenging as the hydrogen concentration rises. Various facets oscillate at various frequencies, but they all exhibit periodic and well-predictable behavior.
The order, however, abruptly deviates if the hydrogen concentration is raised. Chaos triumphs, the oscillations turn unpredictable, and slight adjustments to the starting circumstances result in radically distinct oscillation patterns.
Suchorski stated, “This is remarkable because you wouldn't really expect chaotic behavior in nanometer-sized structures. The smaller the system, the greater the contribution of stochastic noise. In fact, the noise, which is something completely different from chaos, should dominate the behavior of the system: it is even more interesting that it was possible to “extract” indications of chaos.”
Prof. Keita Tokuda (University Tsukuba) created a theoretical model that proved to be extremely helpful.
Chaos Research Applied to Nano-Chemistry
Rupprechter further stated, “Research on chaos theory has been going on for decades, and it has already been successfully applied to chemical reactions in larger (macroscopic) systems, but our study is the first attempt to transfer the extensive knowledge from this field to the nanometer scale.”
“Small deviations in the symmetry of the crystal can determine whether the catalyst behaves in an ordered and predictable way or in a disordered and chaotic way. This is important for different chemical reactions—and perhaps even for biological systems,” he concluded.
The Austrian Science Fund (FWF; P32772-N and SFB TACO F81-P08) provided funding for the study.
Journal Reference:
Raab, M., et al. (2023) Emergence of chaos in a compartmentalized catalytic reaction nanosystem. Nature Communications. doi:10.1038/s41467-023-36434-y