Bubbles and droplets at the initial stage of nucleation: Recent advances in experimental techniques

Recent microfabrication techniques have exhibited tremendous opportunities to improve the phase-change heat transfer by tailoring the surface structure and wettability, which indicates that microscopic understanding of liquid-gas phase change is vital for further improvement of heat transfer devices. Boiling and condensation have been studied by numerous researchers for more than a half century and are known to be a successive process of nucleation, growth and departure of bubbles and droplets. Fluid dynamical modeling has been extensively developed for their growth and departure but the nucleation is still incompletely understood because of the lack of imaging techniques of two-phase phenomena smaller than the resolution limit of optical microscopy. This paper introduces new trends to investigate nanoscale bubbles and droplets experimentally, using AFM, SEM and TEM. AFM is of the highest spatial resolution and its feedback control of tip tapping enables us to obtain the accurate shape of nanobubbles at the solid-liquid interface. A new mode of AFM gives force data of approaching and retracting tips, which unveils the strong interaction between nanobubble and AFM tip. Environmental SEM is a useful tool for observing water condensation with droplets of micrometer-order diameter but there are several concerns including the contamination due to the electron beam irradiation. TEM requires ultra-high vacuum environment but utilization of nano liquid cell enables us to image the liquid-gas interface in nanoscale. By using these techniques, some key issues for generation and stability of interfacial nanobubbles and condensed nanodroplets have been understood, which should result in novel techniques to control the initial stage of phase change heat transfer.