Dynamic Processes of Nanobubbles: Growth, Collapse, and Coalescence

Abstract

Nanobubbles are typically classified as gas/vapor phase cavities in an aqueous solution with a characteristic length of approximately 100 nanometers (nm). The theoretical lifetime of these nanobubbles has been estimated to be less than ~1 microsecond at a diameter of 100 nm based upon the Young-Laplace pressure, but experimental observations have been reported that indicate that they may exist for many hours, or even days. These nanobubbles can be generated by a number of different methods, such as solvent exchange, pressure and/or temperature variations, chemical reactions, or through the electron beam radiolysis of water. The imaging methods utilized to observe these nanobubbles have evolved from low temporal resolution/high spatial resolution, using atomic force microscopy (AFM); or low spatial resolution/high temporal resolution, using optical microscopy (x-rays); or finally, high spatial/high temporal resolution using more recent electron microscopy techniques. A review of the various methods utilized in the nucleation of nanobubbles and the different imaging technologies utilized, along with a summary of the most recent experimental and theoretical investigations of the dynamic behavior and processes of these nanobubbles, including nanobubble growth, nanobubble collapse, and nanobubble coalescence, are presented, discussed and summarized.

Diffusively Controlled Growth of Nanobubbles under Contact Line Pinning: Implication of Nanoscale Flow and Heat Controls by Nanobubbles

“…First of all, the contact angle values measured in this study agree with the other previously reported values of water on graphene layers in TEM studies, i.e., in the range of 60°to 90°a t the nanoscale. 13,27 It is worthwhile to point out that some earlier reported nanoscale contact angles by atomic force microscopy (AFM) are larger than 90°, 37,38 which needs to be further clarified in future studies of the intrusive impacts from both TEM and AFM mechanisms in measuring the nanoscale contact angle. Nevertheless, the impact of contact line pinning on nanoscale contact angle is observed in both AFM and TEM studies, 39−41 and the causes of the pinning can result from both chemical and/or geometrical heterogeneities of 2−10 nm in diameter on the graphene sheets, and a small pinning force between 5 and 70 mN/m can pin the contact line of nanobubbles.…”

“…As illustrated in previous reports of in situ TEM experiments of nanoscale contact angles and nanobubbles, , a graphene liquid cell is assembled by two graphene membranes on TEM grids where distilled water is sandwiched in between, Figure a. The liquid cell is then mounted on a TEM sample holder with a temperature control mechanism and is imaged in a Hitachi H7600 TEM system at a 100 kV electron beam voltage.…”

“…For another example, Lohse and Zhang have further introduced a mechanism of contact line pinning to the dynamic balance of gas mass transfer across the nanobubble interface. The experimental imaging of nanobubbles conducted by AFM has a high spatial resolution but a very low temporal resolution of tens of seconds or longer, which is unable to image dynamics process of nanobubbles. , Therefore, in situ transmission electron microscopy (TEM) stands out in recent experimental explorations to investigate the dynamic process of nanobubbles by offering both high spatial resolution (∼1 nm) and high temporal resolution (∼0.01 s) images, which enables quantitative investigations of nanobubble growth or collapse, − including nanobubble nucleation, growth rate, contact line pinning and depinning, contact angle hysteresis, etc.…”