# Thermal Conductance of the Gold–Water Interface: Implications for Cooling Rates, Melting, and Solidification in Laser Processing of Colloidal Nanoparticles

## TL;DR

This study uses simulations to understand how heat moves between gold nanoparticles and water during laser processing, revealing how this affects nanoparticle melting and solidification.

## Contribution

The study provides new insights into thermal conductance at the Au–supercritical water interface during laser processing of nanoparticles.

## Key findings

- Supercritical water layers strongly affect heat transfer through a planar interface.
- High interfacial curvature enhances thermal conductance and suppresses nanobubble formation.
- Laser melting and resolidification of nanoparticles produce nanocrystalline structures with planar defects.

## Abstract

Thermal conductance
at the nanoparticle–liquid interface
plays an important role in the heat transfer from colloidal nanoparticles
rapidly heated by short-pulse laser irradiation to the surrounding
liquid environment. In this study, interfacial thermal conductance
is investigated in nonequilibrium molecular dynamics simulations performed
for conditions characteristic of laser processing involving transient
melting and resolidification of Au nanoparticles in water. The dependence
of the Au–water interfacial thermal conductance on the nanoparticle
temperature, pressure in the surrounding water, and curvature of the
interface is systematically investigated, with a particular focus
on the regime in which water adjacent to the hot Au surface is heated
up to or above its critical temperature. The formation of a layer
of supercritical water strongly affects the heat transfer through
a planar interface, while high interfacial curvature enhances conductance,
suppresses nanobubble formation, and maintains efficient heat transfer,
even at high temperatures. The results of the atomistic simulations
are incorporated into a continuum model that couples laser-induced
electronic excitation and electron–phonon equilibration in
the nanoparticles with heat diffusion in water. Validation of the
continuum model against atomistic simulations demonstrates a reliable
prediction of nanoparticle temperature evolution and melting onset,
while a hybrid atomistic–continuum approach with “implicit”
water representation further improves efficiency and avoids finite-size
artifacts. Simulations of laser melting and resolidification of 7
and 20-nm nanoparticles predict quenching of the transiently melted
nanoparticles within 100 ps (7 nm) to several hundred ps (20 nm),
with solidification under deep undercooling producing nanocrystalline
structures with a high density of planar defects (twins, stacking
faults, and grain boundaries). Thus, beyond advancing the understanding
of thermal conductance at the Au–supercritical water interface,
the results of this study provide insights into the fundamental mechanisms
of the laser-induced modification of nanoparticles in a liquid environment.