Vapor Nanobubbles around Heated Nanoparticles: Wetting Dependence of the Local Fluid Thermodynamics and Kinetics of Nucleation.

Plasmonic nanobubbles are composite objects resulting from the interaction between light and metallic nanoparticles immersed in a fluid. Plasmonic nanobubbles have applications in photothermal therapies, drug delivery, microfluidic manipulations, and solar energy conversion. Their early formation is, however, barely characterized due to the short time and length scales relevant to the process. Here, we investigate, using molecular dynamics (MD) simulations, the effect of nanoparticle wettability on both the local fluid thermodynamics and the kinetics of nanobubble generation in water. We first show that the local onset temperature of vapor nucleation decreases with the nanoparticle/water interfacial energy and may be 100 K below the water spinodal temperature in the case of weak nanoparticle/water interactions. Second, we demonstrate that vapor nucleation may be slower in the case of weak water/nanoparticle interactions. This result, which is qualitatively at odds with the predictions of isothermal classical nucleation theory, may be explained by the competition between two antagonist effects: while, classically, hydrophobicity increases the vapor nucleation rate, it also penalizes interfacial thermal transfer, slowing down kinetics. The kinetics of heat transfer from the nanoparticle to water is controlled by the interfacial thermal conductance. This quantity turns out not only to decrease with the nanoparticle hydrophobicity but also drops down prior to phase change, yielding even longer nucleation times. Such conclusions were reached by considering the comparison between MD and continuous heat transfer models. These results put forward the role of nanoparticle wettability in the generation of plasmonic nanobubbles observed experimentally and open the path to the control of boiling using nanopatterned surfaces.

Size-dependent effects of the thermal transport at gold nanoparticle-water interfaces.

The transfer of heat from a plasmonic nanoparticle to its water environment has numerous applications in the fields of solar energy conversion and photothermal therapies. Here, we use nonequilibrium molecular dynamics to investigate the size dependence of the interfacial thermal conductance of gold nanoparticles immersed in water and with tunable wettability. The interfacial thermal conductance is found to increase when the nanoparticle size decreases. We rationalize such a behavior with a generalized acoustic model, where the interfacial bonding decreases with the nanoparticle size. The analysis of the interfacial thermal spectrum reveals the importance of the low frequency peak of the nanoparticle spectrum as it matches relatively well the oxygen peak in the vibrational spectrum. However, by reducing the nanoparticle size, the low frequency peak is exacerbated, explaining the enhanced heat transfer observed for small nanoparticles. Finally, we assess the accuracy of the continuum heat transfer equations to describe the thermal relaxation of small nanoparticles with initial high temperatures. We show that, before the nanoparticle loses its integrity, the continuum model succeeds in describing with small percentage deviations the molecular dynamics data. This work brings a simple methodology to understand, beyond the plasmonic nanoparticles, thermal boundary conductance between a nanoparticle and its environment.