  Nanometer thin single-walled carbon nanotube (CNT) films collected from the
aerosol chemical deposition reactors have gathered attention for their
promising applications. Densification of these pristine films provides an
important way to manipulate the mechanical, electronic, and optical properties.
To elucidate the underlying microstructural level restructuring, which is
ultimately responsible for the change in properties, we perform large scale
vector-based mesoscopic distinct element method simulations in conjunction with
electron microscopy and spectroscopic ellipsometry characterization of pristine
and densified films by drop-cast volatile liquid processing. Matching the
microscopy observations, pristine CNT films with finite thickness are modeled
as self-assembled CNT networks comprising entangled dendritic bundles with
branches extending down to individual CNTs. Simulations of the film under
uniaxial compression uncover an ultra-soft densification regime extending to a
~75% strain, which is likely accessible with the surface tensional forces
arising from liquid surface tension during the evaporation. When removing the
loads, the pre-compressed samples evolve into homogeneously densified films
with thickness values depending on both the pre-compression level and the
sample microstructure. The significant reduction in thickness, confirmed by our
spectroscopic ellipsometry, is attributed to the underlying structural changes
occurring at the 100 nm scale, including the zipping of the thinnest dendritic
branches.
