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

The electrorheological (ER) effect was experimentally observed in dielectric suspensions containing tungsten oxide (WO) modified with surfactant molecules (sodium dodecyl sulfate (SDS) and dodecylamine (DDA)) in electric fields up to several kilovolts per millimeter. The dielectric properties of WO suspensions in silicone oil were analyzed, depending on the frequency of the electric field, in the range from 25 to 10 Hz. Unmodified WO 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 WO particles modified with sodium dodecyl sulfate, as well as for dynamic electroconvection in electrorheological fluids containing WO 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.