Decoupling the effects of shear and extensional flows on the alignment of colloidal rods

TL;DR

This study investigates how shear and extensional flows influence the alignment of cellulose nanocrystals, revealing that extension requires less deformation for alignment and that orientation depends on flow type and sign, informing soft material fabrication.

Contribution

It provides a quantitative comparison of shear and extensional flow effects on CNC alignment, decoupling their roles and offering guidelines for material structuring.

Findings

Alignment requires 4 times less deformation in extension than in shear.

In extension, CNC alignment is either 0° or 90°, depending on flow sign.

In shear, CNC gradually aligns towards 0° with increasing deformation rate.

Abstract

Cellulose nanocrystals (CNC) can be considered as model colloidal rods and have practical applications in the formation of soft materials with tailored anisotropy. Here, we employ two contrasting microfluidic devices to quantitatively elucidate the role of shearing and extensional flows on the alignment of a dilute CNC dispersion. Characterization of the flow field by micro-particle image velocimetry is coupled to flow-induced birefringence analysis to quantify the deformation rate--alignment relationship. The deformation rate required for CNC alignment is 4$\times$ smaller in extension than in shear. Alignment in extension is independent of the deformation rate magnitude, but is either 0$^\circ$ or 90$^\circ$ to the flow, depending on its sign. In shear flow the colloidal rods orientate progressively towards 0$^\circ$ as the deformation rate magnitude increases. Our results decouple the effects of shearing and extensional kinematics at aligning colloidal rods, establishing coherent guidelines for the manufacture of structured soft materials.