When we think about heat travelling through a material, we typically picture diffusive transport, a process that transfers heat from high-temperature to low-temperature as particles and molecules bump into each other, losing kinetic energy in the process. But in some materials heat can travel in a different way, flowing like water in a pipeline that – at least in principle – can be forced to move in a direction of choice. This second regime is called hydrodynamic heat transport.
Heat conduction is mediated by movement of phonons, which are collective excitations of atoms in solids, and when phonons spread in a material without losing their momentum in the process you have phonon hydrodynamics. The phenomenon has been studied theoretically and experimentally for decades, but is becoming more interesting than ever to experimentalists because it features prominently in materials like graphene, and could be exploited to guide heat flow in electronics and energy storage devices.
In a new article in Physical Review Letters, MARVEL scientists from the THEOS lab at EPFL have made a leap forward in modelling and explaining phonon hydrodynamics. Their brand new mathematical description makes the phenomenon easier to test experimentally and clarifies the physics behind it. It also points to a bizarre phenomenon that can emerge with hydrodynamic transport and by which heat can flow in reverse, from a colder region towards a hotter one.
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