Preparation of iridescent films and reflective pigments from cellulose
The chiral nematic
order in the suspension is partially retained when the water evaporates,
leaving iridescent chiral nematic films of cellulose
I. The reflected colour can be controlled by changing
the electrolyte concentration in the suspension. We end up with an iridescent
film that is essentially pure cellulose.
["Solidified Liquid Crystals of Cellulose with
Optically Variable Properties", J.-F. Revol, L.
Godbout and D.G. Gray, United States Patent 5,629,055 May 13, 1997 (to Paprican)].
Confetti-sized
discs of chiral nematic cellulose film. Each colour was cast from suspensions with different NaCl contents
The factors governing the final iridescent properties are
complex. The source of the chiral interactions that govern the reflection
wavelength is controversial, and it has recently become clear that other phenomena
such as gel and glass formation during the drying process also govern the final
colour.
[“Formation of chiral nematic
films from cellulose nanocrystal suspensions is a two-stage process”, X. Mu and
D.G. Gray, Langmuir, 30(31),
9256–9260 (2014)]
Shear deformation and mass
transfer during evaporation may also influence the structure of films prepared
by evaporation of water from cellulose nanocrystal suspensions.
[“Twist–Bend Stage in the
Relaxation of Sheared Chiral Nematic Suspensions of
Cellulose Nanocrystals”, D.G. Gray and X. Mu, ACS Omega, 1(2),
212–219 (2016)]
[“Droplets of cellulose nanocrystal suspensions
on drying give iridescent 3-D “coffee-stain” rings”,X.
Mu and D.G. Gray, Cellulose, 22(2), 1103-1107 (2015)]