Marangoni Flow-Driven Angular Self-Assembly of Cellulose Nanocrystals: The Tale of Tilted Tactoids and Folded Domains

Abstract

Cellulose nanocrystals (CNCs) can spontaneously self-assemble into cholesteric photonic films with vibrant colors with multidomain structures and variations in cholesteric pitch. Herein, an angular deposition technique is employed to harness capillary and Marangoni flows to fabricate CNC photonic films with spatially tunable structural colors spanning from red to blue. A crucial relation between the substrate angle, the development of color zones, film coverage and film thickness is discovered. The color range of the photonic films can be shifted by tuning the size distribution of CNC particles is also demonstrated. As the CNC particles and tactoids are deposited on the substrate, a central deformation line emerged with tilted and folded domains, which is a consequence of Marangoni flow-induced deformation of the tactoids at the early stages of deposition. Further in the process, well-aligned domains emerged at the bottom of the substrates, indicating the simultaneous kinetic onset of multiple gelation processes which depend on size segregation across different color zones. Such insights allow us to tune the color domains using angular deposition and manipulate the kinetic arrest phase transition to produce more uniform and homogeneous films.

Keywords: angular deposition; cellulose nanocrystals; marangoni flow; tactoid domains.

Figure 1

Drop‐casted CNC films. a) The digital photograph of the drop‐casted CNC film and (a‐1, a‐2) POM images of the edge and the middle of the CNC film under cross‐polarizers. b) The lateral profilometry of the CNC film and (b‐1, b‐2) the reflectance spectra of the CNC film taken from the positions of images (a‐1) and (a‐2). c) Cross‐sectional SEM images taken from the different test points (i central region, ii and iii on both edge sides of coffee ring band) on the film, showing the distinct orientation and alignment of CNCs. d) Schematic diagram of flow dynamics within the droplet dominated by capillary and Marangoni flow. The scale bar in the SEM images is 10 µm.

Figure 2

Angularly self‐assembled CNC films. a) Digital photographs of CNC films imaged under the normal incidence of light and the films were prepared at angular set‐ups at 30°, 45°, 60°, and 90°; b) Color analysis on the CNC films using K‐means fitting algorithm, c) RGB color analysis on acquired CNC films, derived from the digital bulk images displaying the relative ratios of the color domain coverage, i.e. integrated area under red gets smaller as the deposition angle is moved from 30 degrees to 90 degrees; d) Height profile analysis on CNC films cleaved along the longitudinal direction. The scale bar is 7 mm.

Figure 3

High‐resolution POM images of CNC and their reflection spectrum. a) Reflection polarized optical photographs of CNC films imaged under normal light at different analysis points and the corresponding UV–vis spectroscopy of b) CNC‐30, c) CNC‐45, d) CNC‐60, and e) CNC‐90. The scale bar is 200 µm.

Figure 4

Lateral structural transition of CNC tactoid domains. a) Representative SEM images of multi‐hierarchical microstructure of CNC tactoid domains at different structural transition stages and b) Schematic diagram for the formation of CNC folding domains.

Figure 5

Digital photographs of CNC films at their multidomain region under circularly polarized microscopy and their resulting CD spectroscopy. a) Schematic diagram of the interaction of the tilted domains with incident light and the formulation for the reflective photonic bandgap. b) Circularly polarized microscopy of the tactoid domains at their transition region. c) CD spectroscopy analysis on the misaligned domain region of CNC films. The scale bar is 100 µm.

Figure 6

Size distribution analysis of CNC films flaked from each color zone. a) AFM images of CNCs from blue film pieces (CNC‐Blue), from green film pieces (CNC‐Green), and from the red film pieces (CNC‐Red) and b) Size distribution analysis on those images for length determination of long axis and short axis, c) Mean aspect ratio analysis on the extracted CNC particles.