Tracking phase transitions of tactoids in sulfated cellulose nanocrystals using second harmonic generation microscopy

Cellulose nanocrystals (CNCs) have emerged as promising candidates for chiroptical functional materials due to their ability to form cholesteric liquid crystals with tunable periodicity. The quality of the final cholesteric phase is influenced by the nucleation, growth and coalescence mechanism of the initial droplets, known as tactoids. Current research focuses on understanding the size and morphological transformations of these tactoids, to gain deeper insights into their dynamic behavior and, in turn, to better control the final properties of novel photonic materials. For theoretical models to effectively describe these transitions, precise measurement of these geometric quantities is critical. While polarized microscopy is commonly used, it struggles to accurately measure volumes and aspect ratios. Here, we demonstrate how second-harmonic generation (SHG) microscopy can be used to visualize tactoids with unprecedented detail, offering three-dimensional representations of their size, morphology, and fusion processes in a single volumetric scan. Additionally, the fully cholesteric phase of the suspension can be imaged with SHG to identify potential defects and correlate them with the fusion process of tactoids; specifically, with how the tactoids fuse and merge along the chiral axis, thus linking early tactoid events to the overall quality of the final cholesteric phase.