Weak Coupling of Diffusional and Phonon-like Modes in Liquids Revealed by Dynamic Kapitza Length

TL;DR

This study reveals that interfacial thermal conductance in liquids varies with heating frequency due to weak coupling between diffusional and phonon-like modes, challenging previous assumptions of full mode equilibrium.

Contribution

It introduces a broadband, time-resolved thermoreflectance method to probe frequency-dependent interfacial conductance and proposes a two-channel liquid model with weak mode coupling.

Findings

Interfacial thermal conductance increases with modulation frequency in liquids.

No frequency dependence observed at the Al-silica interface.

Identification of three distinct transport regimes based on thermal penetration depth and nonequilibrium length.

Abstract

Understanding heat transfer across solid-liquid interfaces is central to thermal management and energy technologies, yet whether the interfacial thermal conductance (ITC) depends on the timescale of heating remains unclear. Here we use square-pulsed source thermoreflectance, which combines time-resolved detection with broadband modulation, to probe Al-water and Al-octane interfaces. We observe a reproducible increase of the apparent ITC with modulation frequency. A control Al-silica interface shows no measurable frequency dependence, indicating that the effect is specific to liquids rather than a generic feature shared by all amorphous materials. We explain the data with a two-channel liquid picture in which diffusional and phonon-like modes exchange energy weakly over a finite nonequilibrium length. From the relative magnitude of the thermal penetration depth and the nonequilibrium length, we identify three transport regimes. These findings challenge the common assumption of fully equilibrated liquid modes and provide experimental constraints for modeling dynamic energy exchange at liquid interfaces.