Researchers have confirmed that at distances of just a few nanometers, heat can transfer between objects at a rate up to 100 times higher than predicted by the classical laws of physics. This phenomenon, which defies long-standing theories of thermal radiation, was recently verified with high-precision measurements by a team at the University of Oldenburg. The findings could have significant implications for the design and engineering of nanoscale devices, such as computer chips and other electronics, where temperature control is a critical factor.
The established laws of physics, particularly Planck’s radiation law, have for over a century accurately described heat transfer between objects at macroscopic distances. However, at the nanoscale, these rules no longer seem to apply. The unexpected increase in heat transfer is thought to be caused by evanescent waves, which are electromagnetic waves that exist only very close to the surface of an object and do not radiate away like conventional thermal radiation. At extremely close proximity, these waves can “tunnel” from one object to another, creating an additional channel for heat to flow. While this effect has been theorized for some time, the latest research provides the most definitive confirmation to date of its magnitude.
Defying Classical Physics
For more than a century, our understanding of heat transfer between two objects not in physical contact has been governed by the laws of thermal radiation developed by Max Planck and Gustav Kirchhoff. These principles assert that the amount of heat radiated by an object is determined by its temperature and surface properties. However, these laws were formulated based on observations of objects at macroscopic distances, much larger than the wavelength of thermal radiation. At such distances, heat is transferred through propagating electromagnetic waves that travel through the vacuum of space.
The recent experiments conducted at the University of Oldenburg, and supported by earlier work at MIT and other institutions, have shown that at distances smaller than the wavelength of heat, the classical predictions break down. When two objects are brought within a few nanometers of each other, the heat transfer between them can be orders of magnitude greater than the blackbody limit predicted by Planck’s law. This discrepancy is not a minor adjustment to the existing theory, but a fundamental departure that opens up a new regime of heat transfer physics.
The Role of Evanescent Waves
The primary mechanism believed to be responsible for this enhanced heat transfer is the action of evanescent waves. These are non-propagating waves that form at the surface of any object that is emitting thermal radiation. In the macroscopic world, these waves decay exponentially with distance and their effect is negligible. However, when another object is brought into the “near-field” of the first object, meaning at a distance smaller than the wavelength of the thermal radiation, these evanescent waves can interact with the second object and transfer energy in the form of heat.
A New Channel for Heat Flow
This “tunneling” of evanescent waves creates a new and highly efficient channel for heat transfer that is not accounted for in classical radiation theory. This is why the measured heat transfer at the nanoscale is so much higher than predicted. The closer the objects are to each other, the more significant the contribution of evanescent waves, and the greater the enhancement of heat transfer. The research from the University of Oldenburg confirmed this effect with unprecedented accuracy, using a specially designed near-field scanning thermal microscope. This instrument allowed them to measure heat currents with nanometer resolution, providing a detailed picture of this phenomenon.
Experimental Confirmation
The team at the University of Oldenburg, led by Prof. Dr. Achim Kittel and PD Dr. Svend-Age Biehs, was not the first to observe this effect, but their recent work, published in the journal *Physical Review Letters*, has provided the most robust confirmation of it to date. Their experiments involved bringing a warm measuring probe to within a few nanometers of a colder sample surface and measuring the heat transfer between them. The results showed that the heat transfer values were consistently around 100 times higher than what classical theory would predict.
This work builds on previous research from the same group in 2017, which first provided evidence of this dramatic increase in heat transfer at distances of less than ten nanometers. The latest measurements have confirmed these earlier findings with a higher degree of precision and have helped to solidify the understanding of this phenomenon within the scientific community. The cause of this effect, however, is still not fully understood from a physical standpoint, and the researchers note that their findings cannot yet be fully explained by existing theories.
Implications for Nanotechnology
The confirmation of this unexpectedly high nanoscale heat transfer has significant implications for the future of nanotechnology. As electronic components continue to shrink in size, managing heat becomes an increasingly critical challenge. The ability to control and enhance heat transfer at the nanoscale could lead to the development of more efficient cooling systems for computer chips, preventing them from overheating and improving their performance and longevity.
New Avenues for Energy Conversion
Beyond cooling, this discovery could also open up new possibilities for energy conversion technologies. For example, thermophotovoltaic devices, which convert thermal energy into electrical energy, could be made much more efficient by harnessing this enhanced near-field heat transfer. By placing a heat source very close to a photovoltaic cell, it may be possible to transfer heat much more efficiently and generate more electricity from the same amount of heat. This could have applications in waste heat recovery and other areas of energy generation.
Furthermore, a better understanding of nanoscale heat transfer could lead to the development of new materials with tailored thermal properties. Researchers at MIT have already begun to explore which materials might be best suited for maximizing this effect, with aluminum showing significant promise. By carefully designing materials and structures at the nanoscale, it may be possible to control the flow of heat with unprecedented precision, opening up new frontiers in a wide range of fields, from electronics and optics to materials science and energy engineering.
Future Research Directions
While the recent experiments have provided a clear confirmation of this phenomenon, there is still much that is not understood. The underlying physical cause of this dramatic increase in heat transfer is still a subject of debate and further research is needed to develop a comprehensive theoretical framework that can accurately describe this effect. Scientists will also be working to explore the full range of materials and geometries over which this enhanced heat transfer can be observed and to develop practical applications that can take advantage of this new understanding of nanoscale physics.
The journey to fully unraveling the mysteries of nanoscale heat transfer is far from over, but the latest findings have provided a crucial piece of the puzzle. As researchers continue to push the boundaries of what is possible at the smallest scales, the discoveries they make will undoubtedly have a profound impact on the technologies of the future. The ability to control the flow of heat at the nanoscale could be a key enabling technology for the next generation of computing, energy, and materials science innovations.