Nano Bubbles on a Hydrophobic Surface in Water Observed by Tapping-Mode Atomic Force Microscopy

TLDR

The study used a silicon wafer hydrophobized with OTS, immersed in water, and examined in situ with tapping‑mode AFM, characterizing bubble shapes via height imaging and analyzing surface property differences with phase imaging and force‑curve measurements. The AFM images revealed numerous nano‑size domains that were identified as nanoscopic bubbles attached to the surface, a conclusion supported by their absence when the surface was hydrophobized without air exposure and by the unexpectedly small apparent contact angles that likely enable their stable adhesion.

Abstract

The silicon wafer hydrophobized with OTS was immersed into water to observe the surface in-situ by tapping-mode AFM. A large number of nano-size domain images were found on the surface. Their shapes were characterized by the height image procedure of AFM, and the differences of the properties compared to those of the bare surface were analyzed using the phase image procedure and the interaction force curves. All the results consistently implied that the domains represent the nanoscopic bubbles attached on the surface. This was confirmed by the fact that no domain was observed in the case of the surfaces hydrophobized in the AFM fluid cell without exposure to air. The apparent contact angle of the bubbles was much smaller than that expected macroscopically, which was postulated to be the reason bubbles were able to sit stably on the surface.