We have a new article out in the journal Cellulose, describing the results from a collaboration with Sweden and Slovenia. The article, entitled "Correlation between structural properties and iridescent colors of cellulose nanocrystalline films", describes an optical and electron microscopy study of dried films of cellulose nanocrystals (CNC), prepared with or without circular shear flow. This is a follow-up study of our previous study on the influence of shear flow when drying CNC samples to make iridescent films, and the new component is a quantitative spectrophotometric study as a function of location in the films, together with a high-resolution electron microscopy characterization of the fractured films. It turns out that the reflection spectra are surprisingly similar between the two types of films, but the film cross section is very different. Whereas films dried with shear flow have a clearly periodic helical structure throughout the film, the ones dried without shear flow are disordered near the air interface. Because there is still a sufficiently thick internal regime, close to the bottom substrate, that is helical, the reflection spectra are similar, but the structural difference at the film top can have a strong effect on birefringence. The reflection spectra are much broader than what is expected for a cholesteric liquid crystal with well-defined pitch, indicating a variation in pitch within the film, which may be related to the polydispersity of CNC samples.

Read the article on the Cellulose website.

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