  Most paints contain pigments that absorb light and fade over time. A robust
alternative can be found in nature, where structural coloration arises from the
interference of light with submicron features. Plant-derived, cellulose
nanocrystals (CNCs) mimic these features by self-assembling into a cholesteric
liquid crystal that exhibits structural coloration when dried. While much
research has been done on CNCs in aqueous solutions, less is known about
transferring CNCs to apolar solvents that are widely employed in paints. This
study uses a common surfactant in agricultural and industrial products to
suspend CNCs in toluene that are then dried into structurally colored films.
Surprisingly, a stable liquid crystal phase is formed within hours, even with
concentrations of up to 50 wt.-%. Evaporating the apolar CNC suspensions
results in photonic films with peak wavelengths ranging from 660 to 920 nm. The
resulting flexible films show increased mechanical strength, enabling a
blue-shift into the visible spectrum with applied force. The films also act as
humidity sensors, with increasing relative humidity yielding a red-shift. With
the addition of a single surfactant, CNCs can be made compatible with existing
production methods of industrial coatings, while improving the strength and
responsiveness of structurally-colored films to external stimuli.
