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Cellulose nanocrystals: physics, physical chemistry & applications

Cellulose nanocrystals (CNCs) are rod-shaped nanoparticles that are derived from cellulose-rich materials like wood and cotton. From the point of view of material scientists, CNCs constitute an attractive yet sustainable biomaterial, due to their exceptional mechanical and optical properties. From a fundamental perspective, aqueous CNC suspensions are intriguing because they form a cholesteric phase above a critical concentration.

This lyotropic liquid crystal phase developed by suspensions of CNCs has come increasingly into focus from numerous directions over the last few years. One research direction of particular interest is the formation of solid CNC films with interesting photonic properties, created by drying a cholesteric-forming suspension. However, the pathway along which these films are realized, starting from a CNC suspension that may have low enough concentration to be fully isotropic, is more complex than often appreciated, leading to reproducibility problems and confusion.

Our recent review article is to focus primarily on the physics and physical chemistry of CNC suspensions and the process of drying them, by addressing a broad audience of physicists, chemists, materials scientists and engineers. We put emphasis on explaining the key colloid and liquid crystal science concepts that must be mastered in order to understand the

behavior of CNC suspensions, and we present some interesting analyses, arguments and data for the first time.

We go through the development of cholesteric nuclei from the isotropic phase and their potential impact on the final dry films; the spontaneous CNC fractionation that takes place in the phase coexistence window; the kinetic arrest that sets in when the CNC mass fraction reaches a critical value, preserving the cholesteric helical order until the film has dried; the ’coffee-ring effect’ active prior to kinetic arrest, which often affects uniformity in the dried films; and the compression of the helix during the final water evaporation, giving rise to visible structural color in the films.

You may find the full article (open access) here.

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