Tuning cholesteric cellulose nanocrystal self-assembly in spherical confinement via salt and sonication

TL;DR

This work probes how salt concentration [NaCl] and tip sonication dose $u_s$ steer nonequilibrium self-assembly of cellulose nanocrystals into cholesteric order inside spherical water-in-oil droplets. Using real-time polarized optical microscopy, the authors show a universal post-arrest pitch scaling $p \propto V^{1/3}$ (equivalently $p \propto C^{-1/3}$) driven by droplet shrinkage, and identify a pre-arrest regime where pitch evolution accelerates with higher $[\text{NaCl}]$ and $u_s$, shifting the onset of cholesteric order and gelation. Salt screens electrostatic repulsion, promoting earlier kinetic arrest, while sonication fragments aggregates and weakens chiral interactions, delaying order but increasing final pitch beforehand. The droplet confinement thus provides a quantitative platform to dissect out-of-equilibrium CNC self-assembly and to kinetically program structurally colored soft materials. Overall, the study links confinement geometry to assembly pathways and offers a framework for designing CNC-based photonic materials under nonequilibrium conditions, with implications for controlled tactoid coalescence and hierarchical ordering.

Abstract

Cellulose nanocrystals (CNCs) self-assemble into cholesteric liquid crystals that produce structural color upon solvent removal. Although most studies examine this process in planar films, confinement within micron-sized water-in-oil droplets provides a powerful platform for resolving self-assembly dynamics in real time. Here, we investigate how two common pitch-tuning strategies, sodium chloride addition and tip sonication, govern the kinetics and structure of CNC self-assembly under spherical confinement. Polarized optical microscopy timelapses capture the evolution from isotropic suspension through tactoid nucleation and annealing to kinetic arrest and final buckling. Consistent with prior work, pitch-concentration analysis reveals a universal post-arrest regime governed by droplet shrinkage. Beyond this established behavior, we identify a pre-arrest regime in which pitch decreases rapidly and whose kinetics accelerate with increasing salt concentration and sonication dose. These parameters shift the onset of cholesteric order and gelation, thereby tuning the concentration window for tactoid coalescence. Together, these results establish droplet confinement as a quantitative platform for probing out-of-equilibrium CNC self-assembly and for kinetically programming structurally colored soft materials.

Figures (5)

• Figure 1: (A) Schematic of CNC self-assembly within a water droplet. The emulsion droplets consist of CNCs in water (blue), dispersed in a hexadecane/Span-80 continuous phase (yellow). When deposited onto a fluorophobic glass slide, the CNC droplets rest on the surface while largely maintaining their spherical geometry. (B) Log-log plot showing the evolution of cholesteric pitch size (y-axis) as a function of CNC volume concentration ($\phi$, x-axis) for a [8 J/mL, 150 mmol/kg] CNC droplet. The self-assembly pathway is divided into four stages, color-coded on the plot: tactoid stage (red circles, D), radial alignment stage (yellow squares, E), post-kinetic-arrest stage (green pentagons, F), and buckling stage (blue triangles, G). (C)-(G) Crossed-polarized micrographs illustrating these stages for the same sample: (C) isotropic phase, showing no birefringence; (D) nucleation and growth of tactoids; (E) coalescence of tactoids and anchoring of cholesteric layers to the droplet perimeter. The inset in (E) provides a clearer view of the radially aligned cholesteric pitch; (F) onset of kinetic arrest; (G) fully buckled droplet after complete water diffusion. Scale bars for (C)-(G): 50 µm. Scale bar for inset of (E): 5 µm.
• Figure 2: Evolution of the cholesteric pitch size (y-axis) as the CNC volume fraction increases (x-axis). Colored diamonds on the x-axis mark the volume fraction at which each sample undergoes kinetic arrest. (A) Three samples with increasing [NaCl] values at a fixed $u_s= 8$ J/mL; (B) Three samples with increasing $u_s$ values at a fixed [NaCl] of 150 mmol/kg.
• Figure 3: Values of ${\alpha_1}$ and ${\alpha_2}$ values for (A) samples of increasing $u_s$ at a fixed [NaCl] = 150 mmol/kg, and (B) samples of increasing [NaCl] at a fixed $u_s$ = 8 J/mL. (C), (D) Pitch-size data before kinetic arrest for increasing $u_s$ and increasing [NaCl], respectively, with trend lines of the form $p = k\phi^{\alpha_1}$. (E), (F) Pitch-size data after kinetic arrest for the same sample sets, fitted with $p = k\phi^{\alpha_2}$.
• Figure 4: Droplet surface area plotted over time for droplets with varying salt and sonication levels. Linear fits (dashed lines) follow the $D^2$ law. Shaded regions indicate the propagated uncertainty from a $\pm$5 µm error in measured droplet diameter.
• Figure 5: Progress bars showing the different stages of self-assembly. All samples begin at a CNC volume fraction ($\phi$) of 2.2 vol.%. The isotropic phase is shown in gray; the tactoid nucleation and annealing stage in red; and the gelled, kinetically arrested stage in blue. Each progress bar ends at $\phi = 100$ vol.%, corresponding to complete water loss.