Research Reveals Surprising Behaviors of Nanoparticles

A collaboration between scientists from the University of New Mexico, Los Alamos National Laboratory, and the Institute of Optics (Spain) has developed a framework that enables the efficient and simple description of the thermalization dynamics of systems that are made up of even thousands of nanoparticles. The work delivers insight into the way that collections of nanoparticles radiatively exchange heat with one another and their environment.

Radiative heat transfer occurs as the sun emits light (electromagnetic radiation) that travels to Earth and heats an object that absorbs it. Radiative heat transfer is also the mechanism behind thermal cameras; every hot object, including humans, emits light, allowing the object to release heat and thermalize to the environment. The specific wavelengths that are emitted depend on the temperature of the object — with the sun, for instance, hot enough to produce visible light, and human bodies emitting light that is not visible to the eye but can be detected by infrared sensors.

Planck’s law of black-body radiation, which describes the spectral density of electromagnetic radiation emitted by a black body at a given temperature and in the absence of net flow of matter or energy between the body and the environment, applies to objects that the human eye can perceive (macroscopic objects). When the size of an object approaches the nanoscale, however, Planck’s law no longer applies. At the nanoscale, the radiative exchange of heat can be many times more efficient.

Artistic rendering of the thermalization of an ensemble of nanoparticles mediated by radiative heat transfer. Courtesy of UNM Newsroom.

The ability to control nanoscale radiative heat transfer can enable the development of a wide range of applications, such as thermovoltaics and the cooling down of electronic components in microchips (the distinct components of which are sized at the nanoscale). Effective thermal management for such devices can help prevent computers from overheating and support the development of chips with an increased number of transistors.

In their framework, the collaborators aimed to break down the dynamics of radiative heat transfer. With simple mathematical techniques, they studied the thermalization of large and complicated systems and investigated the physical arrangement of nanoparticles in relation to temperature and environment dynamics.

When an arrangement of nanoparticles has some amount of heat initially stored in it, the researchers found, the system will approach the temperature of its environment in the same way, regardless of which particles are actually hot. If the total heat initially in a system is zero, as happens when one nanoparticle is hotter than the environment and another is colder than the environment, the system achieves thermal equilibrium more quickly than any temperature distribution with some amount of initial heat. The team determined this was true even if the latter case requires a smaller change in temperature than the former case.

The researchers also described an oscillatory evolution of the temperature of a nanoparticle as it thermalizes to the environment. Over the course of thermalization, the nanoparticle cools down and heats back up several times — even though the environment maintains the same temperature.

The research was published in Physical Review Letters (www.doi.org/10.1103/PhysRevLett.126.193601).