Submicron thermal imaging of a nucleate boiling process using fluorescence microscopy

The submicron characterization of transient heat-transfer processes at solid-liquid interfaces is of great importance in many areas of science and engineering. This paper reports on a technique that allows for the transient thermal imaging of the temperature field underneath a growing bubble during nucleate boiling with submicron spatial resolution. The boiling experiments were performed on a temperature-sensitive, erbium-doped, heavy-metal glass, Er:ZBLALiP, used as a robust sensing material for the non-invasive, transient temperature measurements. These measurements were made by analyzing the intensity variations of the fluorescence emission. The thermal imaging of an active nucleation site was performed by utilizing high-resolution, fluorescence microscopy, which enabled a maximum spatial resolution of 370 nm/pixel. The high-speed acquisition above 400 fps ensured sampling of individual bubble-nucleation events. Our transient measurements clearly revealed temperature variations underneath the growing bubble, as well as a measurable bubble-departure frequency under saturated conditions. These encouraging results suggest the need for a systematic use of the corresponding fluorescence technique on enhanced boiling surfaces in order to define the local heat-transfer characteristics and to gain a better understanding of the underlying physical processes.