Nanobubbles around plasmonic nanoparticles: Thermodynamic analysis.

We describe the dynamics of vapor nanobubbles in water, on the basis of simulations of a hydrodynamics phase-field model. This situation is relevant to recent experiments, where a water nanobubble is generated around a nanoparticle immersed in water, and heated by an intense laser pulse. We emphasize the importance of nanoscale effects in the dynamics of the nanobubble. We first analyze the evolution of the temperature inside the bubble. We show that the temperature drops by hundredths of kelvins in a few picoseconds, just after nanobubble formation. This is the result of the huge drop of the thermal boundary conductance between the nanoparticle and the fluid accompanying vaporization. Subsequently, the temperature inside the vapor is almost homogeneous and the temperature gradient is concentrated in the liquid, whose thermodynamic state locally follows the saturation line. We discuss also the evolution of the pressure inside the vapor nanobubble. We show that nanobubble generation is accompanied by a pressure wave propagating in the liquid at a velocity close to the liquid speed of sound. The internal pressure inside the vapor just after its formation largely exceeds Laplace pressure and quickly relaxes as a result of the damping generated by the viscous forces. All these considerations shed light on the thermodynamics of the nanobubbles generated experimentally.