Cellulose nanocrystals (CNC) are rod-shaped nanoparticles produced by the acid hydrolysis of cellulose-rich materials. Due to their exceptional mechanical and optical properties, CNC constitute a sustainable biomaterial for numerous practical applications. From a fundamental viewpoint, aqueous CNC suspensions are fascinating because of their ability to form a cholesteric phase above a critical concentration. A particularly interesting case, combining both the scientific and practical interests, is that of films produced by drying CNC suspensions. The chiral nematic order is retained during water evaporation, however the pitch is decreased to sub-micrometer values. As a result, the films display iridescent colors due the selective reflection of impinging white light [1].<br/><br/>Despite the evident potential of CNC films for photonic applications, changing their optical features on demand has proved to be intricate. Numerous parameters have been reported to affect the morphology and therefore the optical properties of CNC films. However, in the vast majority of investigations, the effects of those parameters have only been addressed by characterizing the dry films [2]. In this contribution, we focus on the poorly understood effect of relative humidity on the film formation from drying sessile drops of CNC suspensions. By means of optical microscopy, we investigate the behavior of isotropic drops during drying at two extreme evaporation conditions, fast and slow drying. More specifically, we utilize Confocal Microscopy in drops containing a small amount of fluorescent microparticles to visualize the hydrodynamic flow patterns during evaporation. In parallel, Polarized Optical Microscopy is employed to study the emergence of an anisotropic phase, because of the evaporation-induced CNC concentration increase.<br/><br/>We aim at relating the flow behavior in the drying drop, to the morphology and the optical properties of the CNC deposits. We hypothesize that this link is the key for understanding the film formation process, which in turn is a prerequisite for rationally designing films with the desired photonic response.

Influence of hydrodynamic flows on the formation of cellulose nanocrystal films from sessile drops drying at different humidities

256th National Meeting (2018)

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