Capillary Flow Characterizations of Chiral Nematic Cellulose Nanocrystal Suspensions

Abstract

Studying the flow-induced alignment of anisotropic liquid crystalline materials is of major importance in the 3D printing of advanced architectures. However, in situ characterization and quantitative measurements of local orientations during the 3D printing process are challenging. Here, we report a microfluidic strategy integrated with polarized optical microscopy (POM) to perform the in situ characterization of the alignment of cellulose nanocrystals (CNCs) under the shear-flow condition of the 3D printer's nozzle in the direct ink writing process. To quantify the alignment, we exploited birefringence measurements under white and monochromatic light. We show that the flow-induced birefringence patterns are significantly influenced by the initial structure of the aqueous CNC suspensions. Depending on the CNC concentration and sonication treatment, various structures can form in the CNC suspensions, such as isotropic, chiral nematic (cholesteric), and nematic (gel-like) structures. In the chiral nematic phase, in particular, the shear flow in the microfluidic capillary has a distinct effect on the alignment of the CNC particles. Our experimental results, complemented by hydrodynamic simulations, reveal that at high flow rates (Er ≈ 1000), individual CNC particles align with the flow exhibiting a weak chiral structure. In contrast, at lower flow rates (Er ≈ 241), they display the double-twisted cylinder structure. Understanding the flow effect on the alignment of the chiral liquid crystal can pave the way to designing 3D printed architectures with internal chirality for advanced mechanical and smart photonic applications.

3D Printing‐Assisted Self‐Assembly to Bio‐Inspired Bouligand Nanostructures

“…[ 34 ] We have recently demonstrated that the biphasic structure, in which the isotropic and chiral nematic phases co‐exist, appears within the CNC concentration of 3–7 wt.%. [ 33 ] At these concentrations, however, the chiral nematic CNC suspensions exhibit very low viscosity (≈1 Pa.s) [ 33 ] and extreme liquid‐like behavior, making them impractical inks for printing self‐sustainable filaments. To increase the viscosity and induce solid‐like behavior in the printed constructs, we incorporated photo‐curable additives, including acrylamide monomer (AAm) and N,N'‐methylene‐bisacrylamide (Bis), to aqueous CNC suspensions which can be polymerized after printing the filament.…”

“…[ 31,32 ] We have recently studied the effect of Poiseuille shear flow on the alignment of the chiral CNC particles inside a capillary tube with the opening diameter of our DIW printer's nozzle. [ 33 ] Our measurements revealed that at high shear rates, CNC particles form a flow‐induced nematic structure. [ 33 ] The chiral nematic structure of the CNC suspensions, however, only forms within the CNC concentrations in which their extreme liquid‐like behavior makes producing self‐supporting filament challenging.…”

“…[ 33 ] Our measurements revealed that at high shear rates, CNC particles form a flow‐induced nematic structure. [ 33 ] The chiral nematic structure of the CNC suspensions, however, only forms within the CNC concentrations in which their extreme liquid‐like behavior makes producing self‐supporting filament challenging. [ 33 ] To remedy this issue, we employ an embedded printing approach where jammed microparticles of Carbopol microgels provide spatial support for the liquid‐like CNC‐based filaments.…”

“…[ 33 ] The chiral nematic structure of the CNC suspensions, however, only forms within the CNC concentrations in which their extreme liquid‐like behavior makes producing self‐supporting filament challenging. [ 33 ] To remedy this issue, we employ an embedded printing approach where jammed microparticles of Carbopol microgels provide spatial support for the liquid‐like CNC‐based filaments. A free‐standing 3D printed construct was achieved after photo‐polymerization of the reactive monomers incorporated into the chiral CNC ink.…”

Self-Healing Injectable Hydrogels for Tissue Regeneration

“…Hence, both oscillatory and steady shear rotational rheology fail to capture the flow profile and rheology of hydrogels during capillary extrusion and result in an over- or underestimation of their viscosity, respectively . The flow profile of hydrogels during capillary extrusion was investigated by tracking incorporated fluorescent beads, , small-angle X-ray or neutron scattering, ,, or polarization microscopy exploiting the alignment of anisotropic particles. , The reports all indicate that hydrogels exhibit a wide central plug flow region where the material experiences minimum shear rates, and a narrow shear zone near the capillary wall, as visualized in Figure E. In contrast, more liquid samples such as cell suspensions exhibit a parabolic flow profile with relative shear over the whole syringe radius, which is detrimental for cell survival .…”