  Gaining a detailed understanding of water transport behavior through
ultra-thin polymer membranes is increasingly becoming necessary due to the
recent interest in exploring applications such as water desalination using
nanoporous membranes. Current techniques only measure bulk water transport
rates and do not offer direct visualization of water transport which can
provide insights into the microscopic mechanisms affecting bulk behavior such
as the role of defects. We describe the use of a technique, referred here as
Bright-Field Nanoscopy (BFN) to directly image the transport of water across
thin polymer films using a regular bright-field microscope. The technique
exploits the strong thickness dependent color response of an optical stack
consisting of a thin (~25 nm) germanium film deposited over a gold substrate.
Using this technique, we were able to observe the strong influence of the
terminal layer and ambient conditions on the bulk water transport rates in thin
(~ 20 nm) layer-by-layer deposited multilayer films of weak polyelectrolytes
(PEMs).
