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Dimensions and the Profile of Surface Nanobubbles: Tip–Nanobubble Interactions and Nanobubble Deformation in Atomic Force Microscopy
journal contribution
posted on 2014-10-14, 00:00 authored by Wiktoria Walczyk, Holger SchönherrThe interactions between argon surface
nanobubbles and AFM tips
on HOPG (highly oriented pyrolitic graphite) in water and the concomitant
nanobubble deformation were analyzed as a function of position on
the nanobubbles in a combined tapping mode and force–volume
mode AFM study with hydrophilic and hydrophobic AFM tips. On the basis
of the detailed analysis of force–distance curves acquired
on the bubbles, we found that for hydrophobic tips the bubble interface
may jump toward the tip and that the tip–bubble interaction
strength and the magnitude of the bubble deformation were functions
of vertical and horizontal position of the tip on
the bubble and depended on the bubble size and tip size and functionality.
The spatial variation is attributed to long-range attractive forces
originating from the substrate under the bubbles, which dominate the
interaction at the bubble rim. The nonuniform bubble deformation leads
to a nonuniform underestimation of the bubble height, width, and contact
angle in conventional AFM height data. In particular, scanning with
a hydrophobic tip resulted in severe bubble deformation and distorted
information in the AFM height image. For a typical nanobubble, the
upward deformation may extend up to tens of nanometers above the unperturbed
bubble height, and the lateral deformation may constitute 20% of the
bubble width. Therefore, only scanning with a hydrophilic tip and
no direct contact between the tip and the bubble may reduce nanobubble
deformation and provide reliable AFM images that can be used to estimate
adequately the unperturbed nanobubble dimensions. The deformation
of the bubble shape and underestimation of the bubble size lead to
the conclusion that the profile of surface nanobubbles is much closer
than previously thought to a nearly flat bubble profile and hence
that the Laplace pressure is much closer to the atmospheric pressure.
Together with line pinning, this may explain the long nanobubble lifetimes
observed previously. The findings presented in this report hold independently
of the material that constitutes the interrogated nanoscale surface
features.