Cellulose nanocrystals (CNCs) are intrinsically birefringent with two distinct refractive indices along the transverse () and longitudinal () directions, respectively. [1, 2] When an isotropic CNC suspension evaporates, it is known to undergo evaporation-induced self-assembly (EISA) to form a left-handedly chiral cholesteric phase, whose structure is then preserved as helical domains in the dried CNC film due to kinetic arrest. The periodic variation in the refractive index along the helical axis causes angular-dependant structural colouration on the film <i>via </i>Bragg reflection. Due to the polydispersity of the CNC suspensions and pre-agglomerations formed during the concentration of the suspensions, the CNC films form multi-domains with varying colours via the EISA process [3]. In order to use photonic cellulose films in emerging applications such as in optical sensors, diagnosis and security features, full control over the domain formation and the droplet drying mechanisms should be achieved. In this work, we reveal new information on the directional folding of helical domains that causes an inversion in the structure due to the flow fields created during the evaporation of the sessile droplet. Our observations provide direct evidence that suggests Marangoni flow to be the fundamental cause of reflection in both polarisation channels in the coffee-ring area of the droplets. Furthermore, our work provides new insights into the particle shape and the substrate curvature in relation to suppressing the Marangoni flow.

Marangoni flow assisted assembly of cellulose nanocrystals in sessile droplets: Effect of droplet shape and substrate

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