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

TL;DR

This study explores how salt and sonication influence the self-assembly of cellulose nanocrystals in spherical droplets, revealing new kinetic regimes and enabling programmable structural colors in soft materials.

Contribution

It introduces droplet confinement as a platform to understand and control CNC self-assembly dynamics under out-of-equilibrium conditions.

Findings

Salt and sonication accelerate pre-arrest pitch decrease.

Droplet shrinkage governs post-arrest pitch behavior.

Tuning parameters shift gelation and cholesteric order onset.

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.