Scientists report a fundamental asymmetry between heating and cooling

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(a) Experimental setup: Charged microparticles are trapped using a laser beam in a parabolic trap. Temperature control is achieved through a noiseless electrical signal that simulates a heat bath. (b) Evolutionary dynamics: Evolutionary dynamics is analyzed between two initial states—one hotter and cooler than an intermediate target state, equidistant from both. The results show that heating is faster than cooling under these conditions. (c) Temperature Asymmetry: The initially observed asymmetry focuses on two temperatures, comparing the cooling and heating processes between them. Credit: Professor Raul A. Riga Alarcon/Dr. Aljaz Codec

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(a) Experimental setup: Charged microparticles are trapped using a laser beam in a parabolic trap. Temperature control is achieved through a noiseless electrical signal that simulates a heat bath. (b) Evolutionary Dynamics: Evolutionary dynamics is analyzed between two initial states—one hotter and an intermediate one cooler than the target state, equal to both. The results show that heating is faster than cooling under these conditions. (c) Temperature asymmetry: The initially observed asymmetry focuses on two temperatures, comparing the cooling and heating processes between them. Credit: Professor Raul A. Riga Alarcon/Dr. Aljaz Codec

A new study led by scientists from Spain and Germany has found a fundamental asymmetry that shows that warming is consistently faster than cooling, challenging conventional expectations and introducing the concept of „thermodynamics” to explain this phenomenon. The findings are published Natural Physics.

Traditionally, in thermodynamics the fundamental processes of heating and cooling have been perceived as symmetric and follow similar paths.

At the microscopic level, heat imparts energy to individual particles, intensifying their motion. On the other hand, cooling releases energy, slowing their movement. However, one question always remains: Why is heating more efficient than cooling?

To answer these questions, Associate Professor Raul A. of the Universidad de Granada in Spain Researchers led by Dr. Aljaz Codec from Riga Allergan and the Max Planck Institute for Multidisciplinary Science in Germany have introduced a new framework: thermodynamics.

Speaking about their motivation behind researching such a fundamental topic, Prof. Allergan told Phys.org, „Since childhood, I've been interested in why heating is more efficient than cooling. And questions like: 'Why don't we have? A microwave oven-like device for rapid cooling?'”

Dr. Kodek added, „Thermal relaxation phenomena have always been a big research topic in the group (these are hard problems in non-equilibrium physics). However, specific questions about heating and cooling asymmetries were initially motivated by mathematical intuition. We did. We didn't expect the answer to be so surprising.”

Processes at microscopic scales

At the microscopic level, heating and cooling are processes that involve the transfer and redistribution of energy between individual particles in a system.

A recent research context focuses on understanding the dynamics of microstructures undergoing thermal relaxation – how these structures evolve when subjected to temperature changes.

In heating, energy is injected into each particle of a system, which leads to an intensification of the motion of the particles. This makes them move more vigorously. At higher temperatures, the Brownian (or random) motion of these particles becomes more intense due to increased collisions with surrounding water molecules.

On the other hand, cooling at the microscopic level involves the release of energy from individual particles, resulting in a reduction in their motion. This process corresponds to an energy-losing system, which leads to a decrease in the intensity of particle motion.

„Our work is devoted to analyzing the evolution of a microscopic system after driving it far from equilibrium. We consider the thermalization of a microscopic system, that is, how a system at a given temperature evolves to the temperature of the heat bath. Interact with,” explained Dr. Codek.

Professor Alarcon. And explained, „An obvious example is taking an object from a boiling water bath (at 100°C) and immersing it in water and ice (at 0°C).”

„We compare how fast the system equilibrates when the material is initially heated in a cold bath and in boiling water. At the microscopic level, we observe that heating is faster than cooling, and we explain this theoretically by creating a new. structure we call thermodynamics.”

Optical Tweezers and Thermodynamics

The researchers used a sophisticated experimental setup to monitor and measure the dynamics of microscopic structures undergoing thermal relaxation. At the heart of their experiment was optical tweezers—a powerful technique that uses laser light to capture single microscopic particles made of silica or plastic.

„These small objects move apparently randomly due to collisions with water molecules, so-called Brownian motion, when they are confined to a small region by tweezers. The higher the temperature of water, the more intense the Brownian motion is, due to frequent and intense collisions with water molecules,” Professor Alarcon explained.

To induce thermal changes, the researchers subjected the defined microparticles to varying temperatures. They carefully controlled the temperature of the surrounding environment using a noiseless electrical signal, simulating a thermal bath.

„Our experimental device allows us to monitor the movement of particles with exquisite precision, providing access to these previously unexplored dynamics,” said Dr Kodek.

By manipulating the temperature and observing the resulting movements, the team gathered critical data to understand the subtleties of heating and cooling at a microscopic level.

The development of a theoretical framework (thermodynamics) played an important role in explaining the observed phenomena. This framework combined principles from stochastic thermodynamics—a generalization of classical thermodynamics to individual stochastic trajectories—with information geometry.

„We carried out mathematical proofs using analytical methods to define distance and speed in intervals of probability distributions to show that the effect is general,” explained Dr Kodek.

Thermodynamics provided a quantitative way to elucidate the observed asymmetry between the heating and cooling processes. This allowed the researchers not only to verify theoretical predictions, but also to examine the dynamics between any two temperatures, revealing a consistent pattern of faster heating than cooling.

Asymmetric and Brownian heat engines

Professor Alarcon and Dr Kodek discovered an unexpected asymmetry in the heating and cooling processes. Aiming to experimentally verify the theory proposed by their colleagues at the Max Planck Institute, the researchers found that it extended beyond certain temperature ranges, holding true for heating and cooling between any two temperatures.

The implications of this asymmetry extend to Brownian heat engines—microscopic machines designed to produce useful work from temperature differences.

„Understanding how a system heats up with different heat baths can improve the power generation process. The equilibration time becomes an important parameter in accurately designing the device's operating protocols,” Professor Alarcon explained.

Although there are no immediate practical applications, researchers envision improved performance in micromotors, microscale freight transport, and self-assembling or self-repairing materials.

Broader implications suggest contributions to the development of new general theories for the dynamics of Brownian systems driven far from equilibrium.

„The effect is not limited to thermal perturbations, quenching in mixing, and we expect to show similar asymmetries. At this point, it is too early to make statements about these scenarios, but we are certainly already thinking about it,” Dr. Codek added.

Prof. Alarcon concluded, „We aim to extend our findings to different protocols and systems, conducting experiments involving small groups of interacting particles and systems with broken time-reversal symmetry. Improving the theoretical understanding and mathematical control of self-adherent randomness. Systems are very important for this direction. Our current strategy is experiments and involves the simultaneous development of theories.”

More information:
M. Ibáñez et al, heating and cooling are fundamentally asymmetric and evolve along distinct paths, Natural Physics (2024) DOI: 10.1038/s41567-023-02269-z

Press Information:
Natural Physics


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