Wetting-induced electrorheological effect

We demonstrate the role of particle-fluid wetting in inducing the electrorheological (ER) effect. Nanoparticles of barium titanyl oxalate coated with urea (BTRU) were suspended in either silicone oil or hydrocarbon oil. In the former, a high yield stress of more than 250 kPa can be attained at an electric field of 5 kVmm, while for the latter no measurable ER effect can be obtained. The two suspensions exhibit completely different appearances. Whereas the silicone oil suspension has the consistency of light cream, in the case of hydrocarbon oil with the same solid concentration the appearance is that of a lumpy paste. This clear visual distinction is attributed to the wetting characteristic between the solid BTRU particles and the suspending oil. We found that by adding a controlled, small amount of surfactant additive to the hydrocarbon oil suspension (oleic acid), the consistency can change drastically to resemble that of the silicone oil suspension, with a yield stress of 260 kPa attained at an electric field of 3 kVmm. We give a simple physical picture of wetting-induced giant electrorheological effect.