The Physics & Astronomy Colloquium Series presents Jianjun Dong of Auburn University, discussing "Unifying Heat Conduction Across Time and Length Scales: A Time-Domain Approach to Transient Heat Conduction" on January 31.

Abstract: Understanding ultrafast and nanoscale heat conduction is crucial for advancing thermal management in nanotechnology. Traditional approaches, such as Fourier's law and heat diffusion equation, fail to capture the non-diffusive behavior observed at these scales.

This talk introduces a time-domain theoretical framework based on Zwanzig’s statistical theory of irreversible processes, bridging macroscopic transport equations with microscopic material properties. Central to this theory is the time-domain transport function, which captures equilibrium time-correlation functions of heat fluxes and generalizes the concept of transport coefficients, such as thermal conductivity.

A key outcome of this framework is that heat conduction is no longer equivalent to heat diffusion. Instead, conduction encompasses both onset wave dynamics and long-time diffusion dynamics, with the transition governed by the decay timescale of the transport functions. This approach provides a seamless description of heat conduction, from wave-like second sound propagation to Fourier diffusion, while avoiding mesoscopic constructs like phonon drift velocity in conventional phonon hydrodynamics.

The theory also offers a robust foundation for interpreting transient thermal techniques, such as time-domain thermoreflectance (TDTR), and modeling heat dissipation in advanced materials. By integrating atomistic insights with continuum-scale equations, this framework enables the design of materials and interfaces with optimized thermal properties, paving the way for breakthroughs in nanotechnology and energy systems.