Coalescence and movement of nanobubbles studied with tapping mode AFM andtip–bubble interaction analysis

Abstract

Imaging of a polystyrene (PS) coated silicon wafer immersed in deionized (DI) water wasconducted using atomic force microscopy (AFM) in the tapping mode (TMAFM). Asreported earlier, spherical cap-like domains, referred to as nanobubbles, were observed tobe distributed on the PS surface. Experiments reveal that, in addition to the well-knownparameter of scan load, scan speed is also an important parameter which affectsnanobubble coalescence. The process of nanobubble coalescence was studied. It was foundthat during coalescence, small nanobubbles were easily moved and merged into bigger ones.Based on the interaction between the AFM cantilever tip and a bubble in the so-calledforce modulation mode of TMAFM, bubble height and adhesive force information for agiven bubble was extracted. A viscoelastic model is used to obtain the interactionstiffness and damping coefficient, which provides a method to obtain the mechanicalproperties of nanobubbles. The model was further used to study the effect ofsurface tension force on attractive interaction force and contact angle hysteresis onthe changes of the interaction damping coefficient during tip–bubble interaction.