Capillary deformation of ultrathin glassy polymer films by air nanobubbles

Confined glasses and their anomalous interfacial rheology raise important questions in fundamental research and numerous practical applications. In this work, we study the influence of interfacial air nanobubbles on the free surface of ultrathin high-molecular-weight glassy polystyrene films immersed in water, in ambient conditions. In particular, we reveal the counterintuitive fact that a soft nanobubble is able to deform the surface of a rigid glass, forming a nanocrater with a depth that increases with time. By combining in situ atomic-force-microscopy measurements and a modified lubrication model for the liquidlike layer at the free surface of the glass, we demonstrate that the capillary pressure in the nanobubble together with the liquidlike layer at the free surface of the glass determine the spatiotemporal growth of the nanocraters. Finally, from the excellent agreement between the experimental profiles and the numerical solutions of the governing glassy thin-film equation, we are able to precisely extract the surface mobility of the glass. In addition to revealing and quantifying how surface nanobubbles deform immersed glasses, until the latter eventually dewet from their substrates, our work provides a novel, precise, and simple measurement of the surface nanorheology of glasses.