Tactoid Annealing Improves Order in Self-Assembled Cellulose Nanocrystal Films with Chiral Nematic Structures

Tran, Andy; Hamad, Wadood Y.; MacLachlan, Mark J.

doi:10.1021/acs.langmuir.7b03920

Cited by 103 publications

(112 citation statements)

References 30 publications

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“…The films in this study were all dried at 50% relative humidity to improve reproducibility, and to ensure a longer evaporation time and, therefore, to favor chiral nematic assembly as well as coffee ring formation via EISA (Klockars et al 2018;Tran et al 2018). Otherwise, at high enough drying rates, ''kinetic trapping or arrest'' would occur for particles, limiting the formation of the coffee ring (Li et al 2016).…”

Section: Coffee Ring Development In Square Filmsmentioning

confidence: 99%

Asymmetrical coffee rings from cellulose nanocrystals and prospects in art and design

et al. 2018

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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 CNCbased iridescence uniquely offers broadband, multicolored 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 noncircular 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.

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Section: Coffee Ring Development In Square Filmsmentioning

confidence: 99%

Asymmetrical coffee rings from cellulose nanocrystals and prospects in art and design

et al. 2018

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“…As the suspension evolves from biphasic to fully cholesteric upon solvent evaporation, the cholesteric domains are no longer separated by isotropic suspension but by grain boundaries with a relatively complex cholesteric reorganization and trapping of topological defects. [34][35][36][37][38] Locally, the evaporation conditions can cause additional flow in the drying suspension and lead to macroscopic accumulation of CNCs near the contact line, an example of coffee-ring effect. [18,19] At the end of that complex stage, the orientation of the cholesteric domains depends on many factors, and can present good vertical alignment through anchoring at the interfaces, or a rather disoriented distribution inherited from the initial random tactoid orientation, e.g., for thicker samples or faster solvent evaporation.…”

Section: Introductionmentioning

confidence: 99%

Angular optical response of cellulose nanocrystal films explained by the distortion of the arrested suspension upon drying

Frka‐Petesic

Kamita

Guidetti

et al. 2019

Phys. Rev. Materials

109

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Cellulose nanocrystals (CNCs) are bio-sourced chiral nanorods that can form stable colloidal suspensions able to spontaneously assemble above a critical concentration into a cholesteric liquid crystal, with a cholesteric pitch usually in the micron range. When these suspensions are dried on a substrate, solid films with a pitch of the order of few hundreds of nanometers can be produced, leading to intense reflection in the visible range. However, the resulting cholesteric nanostructure is usually not homogeneous within a sample and comports important variations of the cholesteric domain orientation and pitch, which affect the photonic properties. In this work, we first propose a model accounting for the formation of the photonic structure from the vertical compression of the cholesteric suspension upon solvent evaporation, starting at the onset of the kinetic arrest of the drying suspension and ending when solvent evaporation is complete. From that assumption, various structural features of the films can be derived, such as the variation of the cholesteric pitch with the domain tilt, the orientation distribution density of the final cholesteric domains and the distortion of the helix from the unperturbed cholesteric case. The angular-resolved optical response of such films is then derived, including the iridescence and the generation of higher order reflection bands, and a simulation of the angular optical response is provided, including its tailoring under external magnetic fields. Second, we conducted an experimental investigation of CNC films covering a structural and optical analysis of the films. The macroscopic appearance of the films is discussed and complemented with angular-resolved optical spectroscopy, optical and electron microscopy, and our quantitative analysis shows an excellent agreement with the proposed model. This allows us to access the precise composition and the pitch of the suspension when it transited into a kinetically arrested phase directly from the optical analysis of the film. This work highlights the key role that the anisotropic compression of the kinetically arrested state plays in the formation of CNC films and is relevant to the broader case of structure formation in cast dispersions and colloidal self-assembly upon solvent evaporation.

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“…With our chosen SOAK1 and SOAK2 conditions, we then varied the conditions for the preparation of the g‐CNC in our quest to achieve a CNC‐E with visible color. Recent work has shown that by slowing down the rate of water evaporation from the CNC suspension, a more ordered and blue‐shifted film is obtained due to a decrease in the helical pitch of the chiral nematic structure . We therefore hypothesized that we could change the pitch of the resulting CNC‐E composite by changing the helical pitch of the starting g‐CNC film.…”

Section: Methodsmentioning

confidence: 99%

“…With our chosen SOAK1 and SOAK2 conditions,wethen varied the conditions for the preparation of the g-CNC in our quest to achieve aCNC-E with visible color.Recent work has shown that by slowing down the rate of water evaporation from the CNC suspension, am ore ordered and blue-shifted film is obtained due to ad ecrease in the helical pitch of the chiral nematic structure. [20,32] We therefore hypothesized that we could change the pitch of the resulting CNC-E composite by changing the helical pitch of the starting g-CNC film. To test this hypothesis,w eu sed slow evaporation conditions to prepare the most blue-shifted g-CNC film possible and characterized it with CD spectroscopy.S low evaporation conditions produced af ilm with ar eflectance wavelength maximum of 494 nm ( Figure S6) and ah alf helical pitch of 336 nm ( Figure S7).…”

mentioning

confidence: 99%

Cellulose Nanocrystal Elastomers with Reversible Visible Color

et al. 2019

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Responsive photonic crystals have potential applications in mechanical sensors and soft displays; however, new materials are constantly desired to provide new innovations and improve on existing technologies. To address this, we report stretchable chiral nematic cellulose nanocrystal (CNC) elastomer composites that exhibit reversible visible color upon the application of mechanical stress. When stretched (or compressed) the colorless materials maintain their chiral nematic structure but the helical pitch is reduced into the visible region, resulting in coloration of the CNC‐elastomer composite. By increasing the percentage elongation of the material (ca. 50–300 %), the structural color can be tuned from red to blue. The color of the materials was characterized by reflectance optical microscopy and reflectance circular dichroism to confirm the wavelength and polarization of the reflected light. We also probed the mechanism of the structural color using 2D‐X‐ray diffraction. Finally, by either water‐patterning the starting CNC film, or by forming a CNC film with gradient color, through masked evaporation, we were able to prepare encoded stretchable chiral nematic CNC‐elastomers.

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Tactoid Annealing Improves Order in Self-Assembled Cellulose Nanocrystal Films with Chiral Nematic Structures

Cited by 103 publications

References 30 publications

Asymmetrical coffee rings from cellulose nanocrystals and prospects in art and design

Asymmetrical coffee rings from cellulose nanocrystals and prospects in art and design

Angular optical response of cellulose nanocrystal films explained by the distortion of the arrested suspension upon drying

Cellulose Nanocrystal Elastomers with Reversible Visible Color

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