Cellulose nanofibers (CNFs) are promising environmentally friendly biomass materials. The orientation of the CNFs along the single-filament axis is essential for manufacturing strong CNF-based single filaments. In this study, the effects of the electric field and the flow rate ratio between the upstream and downstream sheath flows on the material properties of CNF filaments are clarified for a double-flow-focusing channel with 45 \(^\circ\) sheath flow inlets. The CNF orientation is effectively improved by applying an electric field at any sheath flow rate ratio, leading to outstanding material properties of the fabricated cellulose filament. Furthermore, biased sheath flow rate ratios can improve the material properties of the fabricated filaments. Increasing the upstream sheath flow rate compared to that of the downstream results in thinner filaments and improves the CNF orientation near the filament surface owing to the high shear rate at the interface between the CNFs core flow and sheath flow. As a result, the tensile strength and elastic modulus of the filament improves with the close packing of the CNFs inside. A higher downstream sheath flow rate is found to improve the tensile strength and toughness of the filaments. These results indicate that filaments with different mechanical properties can be selectively obtained by controlling the sheath flow rate ratio.
