Very Small Bubble Formation at the Solid−Water Interface

Abstract

The formation of very small gas bubbles (so-called "nanobubbles") at structured solid-water interfaces has been studied using the tapping mode atomic force microscopy (TMAFM) imaging technique. Silicon oxide wafer surfaces were prepared with different degrees of nanometer scale surface roughness and hydrophobicity. Small bubbles do not form on smooth, hydrophilic, or dehydroxylated silicon oxide wafer surfaces immersed in aqueous solutions under known levels of gas supersaturation. Randomly distributed small bubbles were observed over the whole surface of observation on methylated surfaces of controlled roughness. Bubbles formed on rough, methylated surfaces were larger and less-densely distributed than those on a smooth surface of similar hydrophobicity. The process of bubble coalescence was observed as a function of time. The macroscopic contact angle, measured with respect to the aqueous or gas phase, is very different from the microscopic contact angle detected by TMAFM and appears to be due to the influence of line tension at the pinned three-phase contact line. The latter has a value of -3 × 10 -10 N and acts to stabilize the small bubbles, flattening them and thereby reducing the Laplace pressure.

Surface Nanobubbles as a Function of Gas Type

“…Measurement of a negative line tension for nanobubbles/nanodroplets is expected [44]. Values in the literature include −3 nN [27], ∼ −2.3 pN [45], −0.2 nN [21], and ≈ −1 pN [44].…”

Force measurements with the atomic force microscope: Technique, interpretation and applications

“…In deaerated water, i.e., water in which dissolved gases have been taken out as much as possible, the formation of nanobubbles is very much reduced, and the force is of shorter range [507,511,514,537,539,540]. Nanobubbles on hydrophobic surface could be imaged with the AFM in tapping mode [541][542][543][544][545] (Fig. 23).…”

“…Being able to image nanobubbles confirms that they can be stable on hydrophobic surfaces on the time scale of an AFM experiment. It is, however, not clear whether they form spontaneously under normal conditions or only if the dissolved gas is supersaturated [545], the samples were in contact with air before being immersed in water [542], or the AFM tip itself induced bubble formation [546].…”

Controlling the Location and Spatial Extent of Nanobubbles Using Hydrophobically Nanopatterned Surfaces

“…Strong experimental evidences have been shown to support the conclusions that nanobubbles form as expected from earlier studies. [1][2][3][4][5][6][7][8] Nanobubbles did not form on patterned PMMA surfaces. However, they also did not form on patterned PS surfaces in which the lateral area of PS available for bubble formation was too small to allow nucleation.…”