Study of Nucleation of Microbubbles at a Solid/Water Interface with Optical Microscopy and Atomic Force Microscopy

Abstract

Listen

Translate article

Microbubbles (MBs) or air bubbles have been explored for various applications in many fields of science and technology, such as water treatment, biomedical engineering and nanomaterials. The so‐called nanobubble was first proposed in 1994 and also confirmed their existence at the interfaces between water and hydrophobic solids [1]. In a previous study, microbubbles may evolve spontaneously on smooth modified hydrophobic Si surfaces following the decompression [2]. It was proposed that nanobubbles may be the gas micronuclei responsible for the microbubble formation. However, direct observation of microbubble nucleation sites is not yet reported. In this work, we investigated the formation process of microbubbles at a solid/water interface using pe‐degassed and N 2 ‐saturated water. Optical microscopy was first used to observe the nucleation site of microbubbles and atomic force microscopy (AFM) was utilized to obtain high‐resolution images of the nucleation sites. In figures 1(a)‐(c), optical images show that the formation process of microbubbles on graphene‐coated mica surface in N 2 ‐saturated water. Microbubbles tend to form on hydrophobic graphite or graphene‐coated areas but not on pure mica areas. Interestingly, there were certain sites on graphene‐coated areas that microbubbles prefer to form when we inject N 2 ‐saturated water. We zoomed in an area with the preferential gas nucleation sites [figure 1(d)] and acquired high‐resolution AFM images [figure 1(e)] after injecting pre‐degassed water to make the microbubbles disappear. The preferential bubble nucleation sites appeared dark in optical images, but AFM images revealed that these sites were local protrusions in the height images. In addition, AFM images showed presence of nanobubbles on flat graphene‐covered areas outside the protrusions [figure 1(f)]. The relation between microbubble and nanobubble will be discussed.