Scientists report fundamental asymmetry between heating and cooling

A new study by Spanish and German scientists has discovered a fundamental asymmetry showing that heating is systematically faster than cooling, challenging conventional expectations and introducing the concept of “thermal kinematics” to explain this phenomenon. The results are published in Natural physics.

Traditionally, heating and cooling, fundamental processes in thermodynamics, have been viewed as symmetrical, following similar paths.

At the microscopic level, heating involves injecting energy into individual particles, thereby intensifying their movement. On the other hand, cooling causes the release of energy, dampening their movement. However, one question always remained: why is heating more efficient than cooling?

To answer these questions, researchers led by Associate Professor Raúl A. Rica Alarcón of the University of Granada in Spain and Dr. Aljaz Godec of the Max Planck Institute for Multidisciplinary Sciences in Germany introduced a new framework: thermal kinematics.

Speaking about their motivation to explore such a fundamental topic, Professor Alarcón told Phys.org: “Since my childhood, I have been intrigued by why heating is more efficient than cooling. a device like a microwave for rapid cooling? »

Dr. Godec added: “Thermal relaxation phenomena have always been an important research topic in the group (these are difficult problems in non-equilibrium physics). However, specific questions about heating and cooling asymmetry were initially provoked by mathematical intuition. I don’t expect the answer to be so striking.

Processes at microscopic scales

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

In the context of recent research, emphasis is placed on understanding the dynamics of microscopic systems undergoing thermal relaxation, that is, how these systems evolve when subjected to temperature changes.

When heated, energy is injected into each particle in a system, causing particle motion to intensify. This causes them to move more vigorously. The higher the temperature, the more intense the Brownian (or random) motion of these particles 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 damping of their motion. This process corresponds to a loss of energy from the system, leading to a reduction in the intensity of particle movement.

“Our work is devoted to the analysis of the evolution of a microscopic system after it has been moved away from equilibrium. We consider the thermalization of a microscopic system, that is to say how a system at a given temperature evolves up to the temperature of a thermal bath. is brought into contact,” explains Dr Godec.

Professor Alarcón. “A clear example would be taking an object out of a boiling water bath (at 100 degrees Celsius) and immersing it in a mixture of water and ice (at 0 degrees Celsius).”

“We compare the speed at which the system equilibrates with the inverse protocol when the object is initially in the cold bath and heated in boiling water. We observe that on the microscopic scale, the heating is faster than cooling, and we explain this theoretically by developing a new framework that we call thermal kinematics.

Optical tweezers and thermal kinematics

The researchers used a sophisticated experimental setup to observe and quantify the dynamics of microscopic systems undergoing thermal relaxation. At the heart of their experimentation were optical tweezers, a powerful technique using laser light to capture single microparticles of silica or plastic.

“These small objects move in a seemingly random manner due to collisions with water molecules, performing what is called Brownian motion while they are confined to a small region by tweezers. The higher the temperature of the “The higher the water, the more intense the Brownian motion. will be due to more frequent and more intense collisions with the water molecules,” explained Professor Alarcón.

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

“Our experimental setup allows us to follow the movement of the particle with exquisite precision, thus providing access to these previously unexplored dynamics,” said Dr. Godec.

By manipulating temperature and observing the resulting movements, the team gathered data crucial to understanding the intricacies of heating and cooling on a microscopic scale.

The development of the theoretical framework (thermal kinematics) played a central role in the explanation of the observed phenomena. This framework combined the principles of stochastic thermodynamics (a generalization of classical thermodynamics to individual stochastic trajectories) with information geometry.

“By defining distance and speed in the space of probability distributions, we performed mathematical proofs using analytical methods to show that the effect is general,” explained Dr. Godec.

Thermal kinematics provided a quantitative means to elucidate the observed asymmetry between heating and cooling processes. This allowed the researchers not only to validate theoretical predictions, but also to explore the dynamics between any two temperatures, revealing a consistent pattern of faster heating than cooling.

Asymmetry and Brownian heat engines

Professor Alarcón and Dr Godec discovered an unexpected asymmetry in the heating and cooling processes. Initially aiming to experimentally verify a theory proposed by their colleagues at the Max Planck Institute, the researchers found that the asymmetry extended beyond specific temperature ranges, which was true for heating and cooling between two any temperatures.

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

“Understanding how a system thermalizes with different thermal baths can optimize the electricity production process. The equilibration time becomes a key parameter to accurately design the operational protocols of the device,” explained Professor Alarcón.

Although there are no immediate practical applications, researchers envision increased efficiency in micromotors, microscale freight transportation, and materials that can self-assemble or self-repair.

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

“We expect that the effect will not be limited to thermal disturbances, composition quenches, etc., and will likely exhibit analogous asymmetries. At this stage, it is too early to make any statements on these situations, but We’re certainly already thinking about it.” “added Dr. Godec.

Professor Alarcón concluded by saying: “We aim to expand our findings to various protocols and systems, by conducting experiments involving small groups of interacting particles and systems with broken time reversal symmetry. Advancing the theoretical understanding and mathematical control of non-self-adjoint stochastics. “Systems are crucial to this direction. Our ongoing strategy involves the simultaneous development of experiments and theories.”

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