  Polymers are known to wet nanopores with high surface energy through an
atomically thin precursor film followed by slower capillary filling. We present
here light interference spectroscopy using a nanoporous membrane-based chip
that allows us to observe the dynamics of these phenomena in situ with
sub-nanometer spatial and milli- to microsecond temporal resolution. The device
consists of a mesoporous silicon film (average pore size 6 nm) with an
integrated photonic crystal, which permits to simultaneously measure the phase
shift of the thin-film interference and the resonance of the photonic crystal
upon imbibition. For a styrene dimer, we find a flat fluid front without a
precursor film, while the pentamer forms an expanding molecular thin film
moving in front of the menisci of the capillary filling. These different
behaviors are attributed to a significantly faster pore-surface diffusion
compared to the imbibition dynamics for the pentamer and vice versa for the
dimer. In addition, both oligomers exhibit anomalously slow imbibition
dynamics, which could be explained by apparent viscosities of six and eleven
times the bulk value, respectively. However, a more consistent description of
the dynamics is achieved by a constriction model that emphasizes the increasing
importance of local undulations in the pore radius with the molecular size and
includes a sub-nanometer hydrodynamic dead, immobile zone at the pore wall, but
otherwise uses bulk fluid parameters. Overall, our study illustrates that
interferometric, opto-fluidic experiments with nanoporous media allow for a
remarkably detailed exploration of the nano-rheology of polymeric liquids.
