Scientists Discover Fundamental Asymmetry in Heating and Cooling Efficiency

The date is January 17, 2024.

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The article was written by Tejasri Gururaj from Phys.org.

A recent study led by scientists from Spain and Germany revealed an essential asymmetry illustrating that heating consistently occurs at a quicker pace than cooling. This discovery challenges traditional beliefs and introduces a new term, 'thermal kinematics,' to elucidate this phenomenon. The study's results are published in Nature Physics.

Thermodynamics' core procedures, heating and cooling, are generally viewed as symmetric, following comparable paths.

At a microscopic level, heating involves putting energy into individual particles, thereby increasing their movement. Conversely, cooling involves energy release, which reduces their action. But one question has always persisted: Why is heating more efficient than cooling?

To address this, researchers led by Associate Prof. Raúl A. Rica Alarcón from the Universidad de Granada in Spain, and Dr. Aljaz Godec from the Max Planck Institute for Multidisciplinary Sciences in Germany, have proposed a new theoretical framework: thermal kinematics.

During a conversation with Phys.org, Prof. Alarcón said that he has been intrigued since his youth about why heating is more efficient than cooling. He also wondered why a device analogous to a microwave oven for rapid cooling did not exist.

Dr. Godec also noted that thermal relaxation phenomena have consistently been a crucial research area in their group. They had initially asked specific questions about heating and cooling asymmetry based on mathematical intuition, but the answer turned out to be more surprising than they'd anticipated.

At its most basic, heating and cooling at the microscopic level involve energy exchange and reallocation among a system's individual particles.

The recent study primarily strives to understand the dynamics of microscopic systems undergoing thermal relaxation- how these systems react to temperature fluctuations.

In the heating process, energy is injected into each constituent particle, leading to a heightening of their motion, which makes them more active. An increase in temperature intensifies the random movement of these particles due to a higher number of collisions with water molecules surrounding them.

Meanwhile, cooling at a microscopic level involves releasing energy from these particles, which dampens their movement. This process signifies the system losing energy, leading to a reduced intensity in particle motion.

Dr. Godec explains, 'Our study is dedicated to analysing the evolution of a microscopic system after it is driven beyond equilibrium. We consider a system at a specific temperature and how it adjusts to the temperature of a thermal bath it comes in contact with.'

Prof. Alarcón further explained by providing an example: taking an object from a boiling water bath (at 100 degrees Celsius) and plunging it into a mixture of water and ice (at 0 degrees Celsius). They wanted to compare the speed at which the system comes to equilibrium when initially in a cold bath and heated in boiling water. The observation was that on a microscopic level, heating happens more quickly than cooling, which they theoretically explain using the new framework of thermal kinematics.

The researchers used an advanced experimental setup to observe and measure the dynamics of microscopic systems undergoing thermal relaxation. They utilised optical tweezers, which use laser light to trap single microparticles made of silica or plastic, for their experiments.

"These minuscule objects move in seemingly random patterns due to collisions with water molecules, leading to Brownian motion as they are confined to a small area by the tweezers. The water's temperature determines the intensity of the Brownian motion- the hotter the water, the more intense the Brownian motion due to a higher frequency and intensity of collisions with water molecules," Prof. Alarcón explained.

To instigate thermal changes, the researchers exposed the captured microparticles to varying temperatures. They maintained the surrounding environment's temperature carefully using a fluctuating electrical signal, acting as a thermal bath.

'Our experimental device allows us to monitor the particle's movements with incredible precision, providing access to these previously unexplored dynamics,' said Dr. Godec.

By manipulating the temperature and observing the resulting movements, the team gathered crucial data to understand the intricacies of heating and cooling at the microscale level.

The development of the theoretical framework (thermal kinematics) played a pivotal 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.

'Defining distance and speed in the space of probability distributions, we conducted mathematical proofs using methods from analysis 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 heating being faster than cooling.

Prof. Alarcón and Dr. Godec discovered an unexpected asymmetry in the heating and cooling processes. Initially aiming to experimentally verify a proposed theory by their colleagues at the Max Planck Institute, the researchers found that the asymmetry extended beyond specific 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 generate useful work from temperature differences.

'Understanding how a system thermalizes with different thermal baths can optimize the power generation process. The equilibration time becomes a key parameter for precisely designing the device's operational protocols,' explained Prof. Alarcón.

While no immediate practical applications exist, the researchers envision enhanced efficiency in micromotors, microscale cargo transport, 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 driven far from equilibrium.

'We expect that the effect is not limited to thermal perturbations, quenches in composition, etc., and will likely display analogous asymmetries. At this point, it is too early to make statements about these situations, but we are certainly already thinking about it,' added Dr. Godec.

Prof. Alarcón concluded, saying, 'We aim to broaden our findings to various protocols and systems, conducting experiments involving small groups of interacting particles and systems with broken time-reversal symmetry. Advancing theoretical understanding and mathematical control of non-self-adjoint stochastic systems is crucial for this direction. Our ongoing strategy involves concurrent development of experiments and theories.'

Journal information: Nature Physics

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