TiO2‑modified Carbon Nanoparticles (CNPs@TiO2) Enabled Interfacial Charge Regulation.

Balancing polarization strength and electrical stability remains a fundamental challenge in carbon-based electrorheological (ER) fluids, where enhanced electronic mobility often leads to leakage current and voltage collapse at high electric fields. Here, we design TiO2-surface-modified carbon nanoparticles (CNPs@TiO2) to decouple polarization efficiency from conductive percolation. The graphene-like sp2-rich carbon nanoparticles provide strong intrinsic polarizability, while an amorphous TiO2 interface limits long-range charge transport and promotes interfacial polarization. Structural and chemical analyses suggest the formation of amorphous TiO2 on the surface of CNPs and Ti-O-C interfacial bonding. EIS and leakage-current measurements further support this interpretation, showing high impedance, low-frequency charge-relaxation behavior, and microampere-level leakage current up to 3000 V. Dispersed in low-viscosity (100 cSt) silicone oil at ultralow loadings of 1-3 wt % (≈0.6-1.9 vol %), the system exhibits stable operation up to 3000 V without voltage collapse. At 3 wt %, a viscosity enhancement of ∼1870% and a field-induced yield stress of 25.5 Pa are achieved, with subsecond dynamic switching and minimal hysteresis over multiple voltage cycles. A TiO2-only control prepared by the same route shows a lower viscosity enhancement of ≈298%, confirming the advantage of the CNP-TiO2 heterointerface over TiO2 alone. Power-law scaling of yield stress with electric field suggests a polarization-dominated mechanism, while density functional theory calculations reveal significant interfacial charge redistribution at the CNPs@TiO2 interface. These results support interfacial charge regulation as an effective strategy for achieving electrically stable, low-loading ER fluids with low baseline viscosity, providing a scalable pathway toward energy-efficient adaptive fluid systems.