Relationship between Tribology and Optics in Thin Films of Mechanically Oriented Nanocrystals

Many crystalline dyes, when rubbed unidirectionally with
cotton
on glass slides, can be organized as thin films of highly aligned
nanocrystals. Commonly, the linear birefringence and linear dichroism
of these films resemble the optical properties of single crystals,
indicating precisely oriented particles. Of 186 colored compounds,
122 showed sharp extinction and 50 were distinctly linearly dichroic.
Of the latter 50 compounds, 88% were more optically dense when linearly
polarized light was aligned with the rubbing axis. The mechanical
properties of crystals that underlie the nonstatistical correlation
between tribological processes and the direction of electron oscillations
in absorption bands are discussed. The features that give rise to
the orientation of dye crystallites naturally extend to colorless
molecular crystals.