The mechanical behavior of nanobubbles represents their physical essence and has been thought to be closely related to their mysteriously long lifetimes. However, it is difficult to measure the mechanical properties of nanobubbles by conventional atomic force microscopy (AFM). In this paper, nanobubbles were investigated via a novel AFM imaging mode, PeakForce Quantitative Nano-Mechanics (PF-QNM), at the interface of water and highly oriented pyrolytic graphite (HOPG). High resolution images of the nanobubbles in true-contact were achieved by PF-QNM and compared with those obtained by tapping mode AFM (TM-AFM) in the same area. From the force curves simultaneously captured during the PF-QNM imaging processes, the stiffness of the nanobubbles was derived and mapped, ranging usually from 60 to 120 pN nm⁻¹, indicating that the gas–water interface of nanobubbles has similar mechanical properties to those of microbubbles. Interestingly, a size dependence of the stiffness was found and the small nanobubbles had a higher stiffness.

The nucleation and stability of nanoscale gas bubbles located at a solid/liquid interface are attracting significant research interest. It is known that the physical and chemical properties of the solid surface are crucial for the formation and properties of the surface nanobubbles. Herein, we experimentally and numerically investigated the formation of nanobubbles on nanostructured substrates. Two kinds of nanopatterned surfaces, namely, nanotrenches and nanopores, were fabricated using an electron beam lithography technique and used as substrates for the formation of nanobubbles. Atomic force microscopy images showed that all nanobubbles were selectively located on the hydrophobic domains but not on the hydrophilic domains. The sizes and contact angles of the nanobubbles became smaller with a decrease in the size of the hydrophobic domains. The results indicated that the formation and stability of the nanobubbles could be controlled by regulating the sizes and periods of confinement of the hydrophobic nanopatterns. The experimental results were also supported by molecular dynamics simulations. The present study will be very helpful for understanding the effects of surface features on the nucleation and stability of nanobubbles/nanodroplets at a solid/liquid interface.

Previous studies revealed that the perpendicular impact of low-viscosity droplets on sufficiently lyophobic surfaces would stimulate a liquid jet during droplet recoiling, and in some cases, it is accompanied with the entrapment of an air bubble. However, whether such free-surface flow phenomena occur in oblique droplet impact and how surface inclinations influence the dynamics remain open questions. Herein, we perform a comparative investigation on the perpendicular and oblique impact dynamics of water droplets on superhydrophobic surfaces. We show that the inclinations of the target surface do not influence the jet emission and the bubble entrapment in droplet impact. The jetting phenomena, which are triggered by the collapse of the air cavity, can be classified into three types of scenarios, and the jet velocity scales with its size according to two power laws as suggested by the scaling analyses in the previous studies. On the other hand, the air bubble entrapment is caused by the asymmetric cavity collapse when the recoiling speed of the droplet top is close to that of the bottom, and the bubble size can be reduced by the rising jet from the cavity bottom. We also show that some entrapped bubbles on inclined surfaces would move to the droplet surface and eventually burst, shooting out another thin jet. The correlation between the jet velocity and the bubble size is compared with different empirical scaling laws in the literature.