Thermal diffusion mediated nucleation of vapor bubbles on metal microspheres

Due to their unique dynamic behaviors, vapor bubbles have shown great potential in numerous applications, including photothermal therapy, micro/nano manipulation and manufacturing. In this study, we report the observation of vapor bubble nucleation on water-immersed metal microspheres triggered by a continuous wave laser. Upon laser exposure, a significant amount of heat is produced, leading to the nucleation of a vapor bubble on the microsphere. We found that the nucleation dynamics of these vapor bubbles can be effectively tuned by adjusting the laser power and microsphere size. More importantly, we observed a distinct dependence of the maximum bubble volume V_max on the deposited laser energy E_d for microspheres made from different materials and in different media environments of water, ethanol, and methanol. Mathematical models were derived to describe the spatiotemporal evolution of temperature in microspheres and surrounding water. Analytical analysis indicates that laser irradiation-induced heat generation and the subsequent thermal diffusion inside the microspheres govern the nucleation dynamics of vapor bubbles. By tuning thermal diffusivity and microsphere size, the dependence of V_max on E_d can be controlled. Furthermore, the experimentally observed V_max on E_d dependence was theoretically interpreted by considering thermal energy loss to the supporting substrate and convective heat transfer in the surrounding liquid. This study provides a straightforward approach for producing vapor bubbles with tunable nucleation dynamics and offers insights into the detailed nucleation process of photothermal vapor bubbles, holding significant implications for a wide range of applications in micro/nanofluidics.