Traditionally, heating and cooling – processes that are fundamental to thermodynamics and follow similar paths – have been viewed as analogous. A new study has discovered a fundamental asymmetry between warming and cooling: warming is faster than cooling. Thus, researchers introduced the concept of thermal kinematics. The study conducted by Spanish and German scientists was published in the journal Nature Physics.
At the microscopic level, heating is an input of energy to individual molecules, which increases their motion. Cooling involves the release of energy, which slows down its movement. But one question always remains: Why is heating more effective than cooling? Why is there no such device as a microwave oven for rapid cooling? The phenomenon of thermal relaxation has always been the subject of significant research in the research group (these are difficult problems in nonequilibrium physics). But specific questions about the asymmetry between heating and cooling initially stemmed from mathematical intuition. They didn't expect the answer to be so surprising.
Processes on a microscopic scale
At the microscopic level, heating and cooling within a system is the exchange and rearrangement of energy between individual molecules. In the context of the latest research, the focus is on understanding the dynamics of microscopic systems undergoing thermal relaxation – how these systems evolve when exposed to temperature changes.
Upon heating, energy is introduced to each particle in the system, resulting in increased particle motion. This makes them move more quickly. The higher the temperature, the stronger the Brownian motion of molecules due to increased collisions with surrounding water molecules. During cooling, energy is released from individual molecules at the microscopic level, slowing down their movement. This process corresponds to a loss of energy in the system, which leads to a decrease in the intensity of particle motion. In their work, the researchers analyze the evolution of a macroscopic system after the system drifts away from equilibrium. They look at the heating of the macroscopic system, that is, how the system at a given temperature evolves to the temperature of the thermal bath in which it is placed and with which it comes into contact.
For example, we take an object out of boiling water (its temperature is 100°C) and immerse it in a mixture of water and ice (its temperature is 0°C). They compared how quickly the system reached equilibrium in the reverse case, when the body is first placed in a cold bath and then heated in boiling water. They observed that heating on the microscopic scale is faster than cooling, which is explained theoretically by thermokinematics.
Thermokinematics combines the principles of stochastic thermodynamics—the generalization of classical thermodynamics to given random paths—with information geometry.
Optical tweezers and thermokinetics
The researchers used a sophisticated experimental setup to observe and measure the dynamics of microscopic systems undergoing thermal relaxation. Optical tweezers were used: a powerful technology that picks up individual microparticles made of silicone or plastic using laser light.
These small objects move randomly as a result of their collision with water molecules – this is Brownian motion – while the tweezers confine them to a small area. The higher the temperature of the water, the greater the intensity of Brownian motion due to more frequent and stronger collisions with water molecules. To create thermal changes, the researchers exposed the trapped microparticles to different temperatures. They carefully controlled the temperature of the surrounding environment with a noisy electrical signal, simulating a thermal bath. The experimental device allows the movement of particles to be tracked with extreme precision, thus providing access to these previously undiscovered dynamics.
By manipulating the temperature and observing the resulting movement, the researchers gathered essential data to understand the complexities of heating and cooling at the microscopic level.
By specifying distance and speed in a space of probability distributions, they proved mathematically that the effect is general.
Asymmetry and Brownian heat engines
The researchers found that the asymmetry extends beyond ranges between specific temperatures and holds true for heating and cooling between any two temperatures.
The application of asymmetry extends to Brownian heat engines. These are microscopic machines designed to generate useful work from temperature differences.
Understanding how to heat a system using different heat baths improves performance generation processes. Equilibration time is a key parameter for the precise design of a device operating protocol, and although it has no practical application yet, researchers believe it will be effective in micromotors, microcargo transport, and self-organizing or self-repairing materials.