Electrorheological behavior of nanofluids based on used cooking oil and cellulose nanocrystals for smart lubrication

This study examines the electrorheological (ER) behavior of nanofluids based on used cooking oil (UCO) with dispersed cellulose nanocrystals (CNCs) to assess their potential as smart lubricants. The ER effect results from electric field‑induced particle alignment into chain structures that increase flow resistance and allow reversible modulation of viscosity to adapt frictional performance.

Nanofluids containing 0.5–4 wt.% CNCs were tested at 25 °C under electric fields up to 4 kV/mm. Without an electric field, all samples behaved as Newtonian fluids. When voltage was applied, shear stress developed a low‑shear plateau, evidencing nanoparticle structuring, with stronger effects at higher fields and concentrations. At high shear rates, the fluids returned to a Newtonian regime due to disruption of ER structures. Optical microscopy confirmed chain formation between electrodes in the absence of flow.

Leak‑current measurements suggested structural reconfiguration under shear, especially at moderate‑to‑high fields and low CNC contents. Overall, the results demonstrate that CNCs can impart ER behavior to waste cooking oil, supporting its use in smart lubrication while promoting circular‑economy approaches.