The act of boiling water is an important part of contemporary cooling technologies and power-generation systems. Although boiling is a commonplace phenomenon, scientists are still working to understand its exact process, including how bubbles of water vapor form on surfaces. Clarifying the processes involved in the formation and persistence of vapor bubbles — improving our understanding of features like nanobubble appearance, size distribution, stability, shape, and growth — may help to design surfaces of new materials able to bear very high heat fluxes.
A study reported in Applied Physics Letters expands our understanding of surface boiling by reporting the first detection of vapor bubbles — generated by surface boiling — at the nanometer scale.
Researchers used small angle neutron scattering to probe water boiled on untreated aluminum and stainless steel samples. They made measurements in two boiling regimes per surface. The regimes were distinguished by the temperature difference between the sample and the liquid, which the researchers varied to see the effect on nanobubble size or shape. They used a polydisperse spherical model to calculate the size of the spheres that would produce the measured scattering: between 3 and 80 nanometers.
Based on measurements and modeling, the average nanobubble size detected was approximately 50 nanometers, although the instrumentation can detect features of smaller size. The nanobubbles persisted over the entire duration in which heat was applied to the metallic surfaces, and disappeared when heat was removed. The authors inferred from the signal detected that the population of nanobubbles on the metal surfaces was stable and considerable, a remarkable result which they hope will lead to further inquiry.
Source: “Detection of vapor nanobubbles by small angle neutron scattering (SANS),” by Emilian Popov, Lilin He, Elvis Dominguez-Ontiveros, and Yuri Melnichenko, Applied Physics Letters (2018). The article can be accessed at https://doi.org/10.1063/1.5023595.