Identification of surface nanobubbles and resolving their size-dependent stiffness

We report a comparative investigation of the topographic features and nanomechanical responses of surface nanobubbles, polymeric nanodrops, and solid microparticles submerged in water and probed by atomic force microscopy in different operating modes. We show that these microscopic objects exhibit similar topographies, either hemispherical or hemiellipsoidal, in the standard tapping mode, and thus are difficult to distinguish. However, distinct differences, caused not only by their different mechanical properties but also by different cantilever tip-sample mechanical interactions that are affected by tip wettability, were observed in successive topographic imaging with controlled scanning forces and the nanoindentation tests, allowing for the identification of surface nanobubbles. Based on the indentation force-distance curves, we further extrapolated the stiffness of surface nanobubbles spanning a wide range of sizes and then developed a simple theoretical model to explain this size dependence. We also demonstrate how size-dependent stiffness can be used to determine the surface tension of nanobubbles, which was found to be much lower than the bulk value of water.