Modeling the Interaction between AFM Tips and Pinned Surface Nanobubbles

Abstract

Although the morphology of surface nanobubbles has been studied widely with different AFM modes, AFM images may not reflect the real shapes of the nanobubbles due to AFM tip-nanobubble interactions. In addition, the interplay between surface nanobubble deformation and induced capillary force has not been well understood in this context. In our work we used constraint lattice density functional theory to investigate the interaction between AFM tips and pinned surface nanobubbles systematically, especially concentrating on the effects of tip hydrophilicity and shape. For a hydrophilic tip contacting a nanobubble, its hydrophilic nature facilitates its departure from the bubble surface, displaying a weak and intermediate-range attraction. However, when the tip squeezes the nanobubble during the approach process, the nanobubble shows an elastic effect that prevents the tip from penetrating the bubble, leading to a strong nanobubble deformation and repulsive interactions. On the contrary, a hydrophobic tip can easily pierce the vapor-liquid interface of the nanobubble during the approach process, leading to the disappearance of the repulsive force. In the retraction process, however, the adhesion between the tip and the nanobubble leads to a much stronger lengthening effect on nanobubble deformation and a strong long-range attractive force. The trends of force evolution from our simulations agree qualitatively well with recent experimental AFM observations. This favorable agreement demonstrates that our model catches the main intergradient of tip-nanobubble interactions for pinned surface nanobubbles and may therefore provide important insight into how to design minimally invasive AFM experiments.

Formation, dissolution and properties of surface nanobubbles

“…There are many open questions worthy of numerical and experimental investigation. The current approach could be extended to the study of substrate wettability, contamination [21], and pinning [24][25][26], which deserve careful consideration in order to understand the behaviors of nanobubbles. Direct measurement of the formation and the properties of nanobubbles, such as density, surface tension, and contact angle, would be helpful.…”

“…For example, nanobubbles are known to be stable for days, which contrasts with the expectation of their rapid dissolution within the diffusive time scale (microseconds) due to the high Laplace pressure inside nanobubbles [20]. Some studies suggest that contamination on the substrate may play a role in this stability [21], whereas other studies explain this phenomenon through the balance between gas influx and outflux [22,23], pinning [24][25][26], and gas oversaturation [27,28]. Recently, molecular dynamics (MD) simulations of a coarse-grained model suggested that nanobubbles dissolve within less than a microsecond, claiming that the experimental nanobubbles are sta- bilized by a nonequilibrium mechanism [29].…”

Surface Nanobubbles Studied by Time-Resolved Fluorescence Microscopy Methods Combined with AFM: The Impact of Surface Treatment on Nanobubble Nucleation

“…22,23 Most reports on surface nanobubbles, nanodroplets 24 and "nanopancakes" 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 4 conditions, AFM may in fact be an invasive technique and this impact is similar to the well-known effects of tip shape convolution and longer range forces on nanobubble dimensions difficult to account for or to correct quantitatively. 20,27,28,29,30,31,32 Being essentially a chemically insensitive method (for exceptions in typical ambient conditions, see e.g. ref 33), conventional AFM may not provide conclusive evidence for the occurrence of surface nanobubbles (and e.g.…”

A review of recent theoretical and computational studies on pinned surface nanobubbles

“…LDFT calculations [69] indicated that the nanobubble showed an elastic deformation for the approach of a hydrophilic tip [ Fig.4 (a)]. The hydrophilic nature of the tip and a thin wetting film covering it prevented the tip from penetrating the bubble during an approach process, showing an elastic effect.…”

“…Capillary force between AFM tip and nanobubble and the morphology of the nanobubbles for different tip-substrate distances in the approach (red arrow) and retraction process (black arrow): (a) for a hydrophilic tip and (b) for a hydrophobic tip.The figure is reproduced from Ref [69]…”