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

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.