By injecting metallic nanoparticles into a liquid, localized heating without physical contact is achieved, facilitated by steady-state or pulsed illumination that generates nanobubbles around the particles. This technique has applications [1] in optical hyperthermia for local therapy, microbiology, and solar thermal water heating. The complex dynamics of bubble formation involve vaporization, heat transfer by vapor molecules, and multiple scattering [1]. However, the thermal radiation between nanoparticles and the liquid has not been adequately studied due to the size of the nanoparticles being below the characteristic wavelength (Wien’s wavelength). To accurately assess the sub-wavelength thermal radiation emission, fluctuational electrodynamics calculations are necessary. These calculations reveal that the effective emissivity of small objects can exceed unity [2]. Furthermore, due to the small distance between the nanoparticle and the fluid, the radiative transfer occurs through photon tunneling, in the near field. Fluctuational electrodynamics provides also a suitable approach for considering this phenomenon. This work focuses on evaluating these radiative contributions. The analysis considers the dipole approximation, encompassing both electric and magnetic dipole cases within a cavity immersed in a dissipative medium. Multireflection scattering solutions account for wave-related effects in the vacuum cavity. The results obtained in spherical geometry, using the Mie theory, demonstrate the significance of both wave effects. The volumetric near-field absorption in water varies as almost a sixth power of the distance from the origin and as a third power of the cavity radius, exhibiting observable interferences when approaching the far-field regime. Spectral signature resonances of water dominate the radiative exchange in this scenario. This work will contribute to a more comprehensive understanding of radiative exchange in nanoparticle-liquid systems ultimately aiding the development of thermal engineering, bioengineering, and biomedical applications

Keywords: near-field thermal radiation, nanobubbles, nanoparticles.

Acknowledgments: The authors acknowledge the support of ANR research funding in the frame of the CASTEX project and valuable discussions with Dr. Samy Merabia.

References:
1. Yifan Zhang, Wei An, Chang Zhao, and Qingchun Dong. Radiation-induced plasmonic nanobubbles: fundamentals, applications, and prospects. AIMS Energy, 9(4):676–713, 2021.
2. KL Nguyen, Olivier Merchiers, and P-O Chapuis. Temperature-dependent and optimized thermal emission by spheres. Applied Physics Letters, 112(11):111906, 2018.