Coupled chemical reactions in dynamic nanometric confinement: Ag2O membrane formation during ion track etching

In this study, continuous swift heavy ion tracks in thin polymer foils were etched from both sides to create two conical nanopores opposing each other. Shortly before both cones merged, one of the nanopores was filled with a silver salt solution, whereas etching of the other cone continued. At the moment of track breakthrough, the etchant reacted with the silver salt solution by forming an impermeable and insulating membrane. Continued etching around the thus-created obstacle led to repetitive {etchant - silver salt solution} interactions. The coupling of the two chemical reactions, {etchant - polymer} and {etchant silver salt solution}, within the confinement of etched tracks, with continuously changing shapes, showed a highly dynamic nature as recorded by measuring both the electrical current and the optical transmission across the foils. At low etching speeds, a central membrane that grew in radius and thickness with time until, at a critical thickness, the membrane became rather impermeable was formed. However, at high etching speeds, the emerging reaction products exhibited a sponge-like consistency, which allowed for their infinite growth. This precipitation was accompanied by a pronounced current spike formation. A simple theoretical model explains, at a minimum, the basic features.