Abstract
The iridescence displayed by films made from cellulose nanocrystals (CNCs) has long been the subject of fundamental research. This 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. Aesthetic outputs are already in reach, but requires input from trend setters in the design and art industries. In this realm, the CNC-based iridescence uniquely offers broadband, multi-colored reflections through the “coffee ring” effect, which arises upon evaporation-induced self-assembly (EISA). Although this effect has been thoroughly studied in the context of axisymmetric patterns, complex geometries remain to be evaluated for large-scale implementation. This is central to the present efforts, where EISA of CNC suspensions occurred onto non-circular surfaces. We used orientation-dependent contact angle measurements, profilometry and fixed-light source photography to unveil the effect of asymmetric drying fluxes at sharp angles, between 30° and 90°, on CNC particle deposition and resulting color patterns. We also demonstrate the causality between increased capillary fluxes and deposition with the help of modelling via energy minimization of the suspension volume onto a given surface and using the diffusion equation to obtain the local concentration of water vapor during EISA. Lastly, we study the effect of background reflections as well as light and temperature resistance of CNC-based reflectors, both 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.Graphical abstract
Highlights
It has been argued that interest in glittering objects originates from social reasons and from evolutionary biases, some of which relate to the effects perceived when looking at fresh water
Previous studies related to coffee rings from CNC suspensions consider circular shapes of fairly small areas (\ 10 mm diameter), where the characteristics of the coffee ring is identical in all radial directions (Mu and Gray 2015; Gencer et al 2018)
We studied the effect of the angle amplitude at the corners of non-circular films on the coffee ring development by using geometries with increased sharpness, from 90° to 75°, 30° and 27.5°
Summary
It has been argued that interest in glittering objects originates from social reasons and from evolutionary biases, some of which relate to the effects perceived when looking at fresh water. The drying of the CNC suspension results in the well-known ‘‘coffee ring’’ effect, whereby individual CNCs move preferentially towards the edges (Mu and Gray 2015) (see Fig. 1c, d) This is a consequence of capillary flow induced by the gradient of solvent evaporation rates across the drying film (Deegan et al 1997). In the context of art and design applications, this effect could be of great value for highlighting material and edges from a single ‘‘paint/colorant’’ suspension, especially if it yields iridescence spanning the whole color spectrum. Additional experiments were performed by using 4 ml of the CNC suspension in the presence of 35% PEG based on CNC dry mass These samples were exposed to UV light and are denoted CNCthin, CNCthick and CNCPEG, respectively. The widths of the coffee rings were obtained as the distance measured orthogonally (side) or diagonally (corner) from the film edge to the masked area
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