The iridescence displayed by films made from cellulose nanocrystals (CNC) have been the subject of vast fundamental research. It has expanded our understanding of colloidal self-assembly towards the development of advanced materials. However, the application of such findings is less reported for visual designs that exploit structural color. Such realization is already in reach but requires input from trend setters in the design and art industries. In this realm, a clear benefit of CNC-based iridescence over other iridescent materials is the development of broadband, multi-colored reflections due to the “coffee ring” effect upon evaporation induced self-assembly (EISA). Although such effect has been thoroughly studied in the context of axisymmetric patterns (e.g. spheroidal sessile drops), complex geometries remain to be evaluated for large-scale implementation. This is central to the present effort where we study the effect of sessile drops of CNC suspensions assembled by EISA onto non-circular surfaces. We used orientation-dependent contact angle measurements, profilometry and fixed-light source photography to unveil the effect of asymmetric drying flux arising from sharp angles, between 30 and 90°, on CNC particle surface concentration and resultant color patterns. We also demonstrate the causality of increased capillary fluxes with the help of modelling via energy minimization using the diffusion equation for the concentration of water vapor upon drying. Lastly, we provide an additional insight on the effect of exposition to UV-light and heat on the properties of CNC-based reflectors, important for any deployment. The results from this multidisciplinary effort, involving applied design, art and colloid chemistry, point to the excellent prospects of CNC films for the development of structured and chromatic patterns and also as a tool to explore further the realm of drying-induced colloidal self-assembly.

Effect of drying flux inhomogeneities on the development of coffee rings from cellulose nanocrystals and visual designs perspective.

2019 Spring National Meeting

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