Metastable nanobubbles at the solid-liquid interface due to contact angle hysteresis

Area of Science:

• Surface science
• Nanotechnology
• Physical chemistry

Background:

• Nanobubbles are stable entities at hydrophobic solid-liquid interfaces, with observed lifetimes up to several days.
• Various shapes, including semispherical and micropancake, have been reported for surface nanobubbles.
• The fundamental nature of nanobubbles as gas-phase entities has been questioned.

Purpose of the Study:

• To investigate the nature and stability of surface nanobubbles at a pure water-highly ordered pyrolytic graphite (HOPG) interface.
• To explore the factors contributing to the long-term existence of nanobubbles.
• To provide evidence for the gas-phase nature of nanobubbles.

Main Methods:

• Utilized peak force quantitative nanomechanics (PF-QNM) to study surface nanobubbles.
• Employed the solvent-exchange method for nanobubble generation.
• Observed nanobubble behavior, including coalescence, on HOPG surfaces.

Main Results:

• Isolated nanobubbles were generated on terraced areas of the HOPG surface, avoiding steps.
• Adjacent nanobubbles coalesced, forming metastable structures, with coalescence enhanced by PF-QNM measurements.
• Nanobubble longevity was attributed to nanoscale contact angle hysteresis at the water-HOPG interface.

Conclusions:

• The study provides evidence that surface nanobubbles are indeed gas-phase entities.
• Nanobubbles avoid hydrophilic step edges on HOPG surfaces during coalescence, supporting their gas nature.
• Nanoscale contact angle hysteresis is a key factor in the prolonged stability of these nanobubbles.