No abstract

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 À1 , 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 impact of a liquid droplet on a solid surface is one of the most common phenomena in nature and frequently encountered in numerous technological processes. Despite the significant progress on understanding the droplet impact phenomenon over the past century, the impact dynamics, especially the coupling effects between the properties of a liquid and surface wettability on the impact process, is still poorly understood. In this work, we experimentally investigated the impact of viscous droplets on superamphiphobic surfaces, with the viscosity of liquids ranging from 0.89 to 150 mPa s. We showed that an increase in liquid viscosity will slow down the impact process and cause bouncing droplets to rebound lower and fewer times. The critical impact velocity, above which droplets can rebound from the superamphiphobic surface, was found to linearly increase with the liquid viscosity. We also showed that the maximum spreading factor increases with Weber number or Reynolds number but decreases with the liquid viscosity. Scaling analyses based on energy conservation were carried out to explain these findings, and they were found to be in good agreement with our experimental results.