Effect of surface properties on the electrorheological response of hematite/silicone oil dispersions

► Bare and treated hematite particles are used as base of an electrorheological (ER) fluid. ► Dehydration by heating and hydrophobization by oleic acid coating were the treatments tested. ► Oleic acid treated hematite (CH) showed negative ER behavior above a critical field. ► Charging by electrochemical reactions produces electrophoretic migration of the particles. ► Migration leaves the inter-electrode area empty of particles and leads to negligible yield stress.