On the shape of surface nanobubbles

Area of Science:

• Surface science
• Nanotechnology
• Physical chemistry

Background:

• Previous atomic force microscopy (AFM) studies suggested anomalously large contact angles for surface nanobubbles, often around 160°.
• A potential dependence of contact angle on nanobubble size (theta(R)) was also hypothesized.

Purpose of the Study:

• To accurately determine the contact angle (theta) of surface nanobubbles as a function of their radius (R).
• To investigate the influence of substrate smoothness and cantilever properties on nanobubble contact angles.
• To clarify the set-point dependence in AFM measurements of nanobubbles.

Main Methods:

• Atomic Force Microscopy (AFM) was employed to image and measure surface nanobubbles.
• Experiments were conducted on smooth, highly oriented pyrolytic graphite (HOPG) substrates.
• A variety of different AFM cantilevers were utilized to assess reproducibility and identify potential artifacts.

Main Results:

• A constant contact angle of 119 ± 4° was measured for nanobubbles down to a radius of 20 nm on smooth HOPG.
• This observed contact angle is the lowest reported for surface nanobubbles and is reproducible.
• Surface roughness induced by cantilever debris led to larger apparent contact angles (~150°).
• Nanobubble shape and measured contact angle showed dependence on AFM set-point ratio below ~95% and cantilever properties.

Conclusions:

• The contact angle of surface nanobubbles on smooth HOPG is size-independent down to 20 nm.
• Clean cantilevers and smooth substrates are crucial for accurate nanobubble characterization.
• AFM measurement parameters, particularly the set-point ratio, significantly influence nanobubble morphology and apparent contact angles.