Surfactant-Switched Positive/Negative Electrorheological Effect in Tungsten Oxide Suspensions

The electrorheological (ER) effect was experimentally observed in dielectric suspensions containing tungsten oxide (WO₃) modified with surfactant molecules (sodium dodecyl sulfate (SDS) and dodecylamine (DDA)) in electric fields up to several kilovolts per millimeter. The dielectric properties of WO₃ suspensions in silicone oil were analyzed, depending on the frequency of the electric field, in the range from 25 to 10⁶ Hz. Unmodified WO₃ suspensions, as well as suspensions modified with sodium dodecyl sulfate, were shown to exhibit a positive electrorheological effect, whereas suspensions modified with dodecylamine demonstrated a negative electrorheological effect. The quantitative characteristics of the negative electrorheological effect in the strain-compression and shear regimes were obtained for the first time. Visualization experiments were performed to see the chain structures formed by WO₃ particles modified with sodium dodecyl sulfate, as well as for dynamic electroconvection in electrorheological fluids containing WO₃ modified with dodecylamine. The negative electrorheological effect was shown to be associated with the processes of phase separation in the electric field, which led to a multiplicative effect and a strong electroconvection of the suspension at field strengths above 1 kV/mm.