Cooling happens in the first water layers.
A hot silver nanoparticle disturbs the nearby water for only a few picoseconds. Resolving the first shells reveals both how heat escapes and why a continuum description needs special treatment at the surface.
The Journal of chemical physics · 2019 · A Rajabpour, R Seif, S Arabha, MM Heyhat, S Merabia, A Hassanali
Cooling starts in the water closest to the surface
A heated nanoparticle cools through the surrounding liquid, but the first few molecular layers are structured and may not behave like bulk water. Continuum descriptions need a defensible way to represent this nanoscale region.
How quickly does heat leave a silver nanoparticle, how far does the temperature disturbance reach, and can a continuum model capture that transient?
Measure the interface, then resolve the surrounding liquid
The authors compared four molecular-dynamics methods for estimating nanoparticle–water conductance. They followed temperature relaxation in concentric water shells and tested a continuum heat-conduction model against those shell temperatures.
The model uses a silver particle about 1.7 nm in diameter surrounded by water. Cooling runs start with the particle at 400 K and water at 300 K. Four methods estimate the interfacial conductance. The water-shell analysis resolves 0.2 nm shells and averages 2000 cooling trajectories to identify a small signal amid molecular fluctuations. A finite-element conduction model then tests whether the shell temperatures can be reproduced without tracking individual molecules.
Key findings
Localized cooling
The nearest shell rises by roughly 5 K; the signal falls below about 1 K near r ≈ 2 nm from the particle center.
Method comparison
The two transient interpretations give different conductances, revealing how transport assumptions affect the inferred interface value.
Molecular-to-continuum bridge
Conduction reproduces the shell-temperature evolution, with the best first-shell fit near 1.4 W/m·K.
Keep the interface when changing modeling scale
The cooling event is localized and fast, so representing the first water shells separately can improve continuum descriptions of nanoparticle heat transfer.
The study follows one localized cooling event through two descriptions. Its central lesson is to preserve the distinct near-surface region when translating molecular heat transfer into continuum behavior.