Negative thixotropy in ferric-oxide suspensions

Negative thixotropy was observed in suspensions of ferric-oxide particles in mineral oil, in that viscosity increased with time under shear. The enhanced viscosity under the shear was retained under rest, and it decreased gradually by application of shear at lower shear-rate.The ferric-oxide powders used are acicular submicron maghemite and hematite. The dispersing medium is a heavy mineral oil. The suspensions were prepared with a ball mill in 33% by particle weight using a dispersing agent.A qualitative interpretation was made for the development of the phenomenon with a floc model in which suspensions of acicular particles form bulky structures with larger volume fraction of flocs phase at higher rate of shear, accompanied with increase of viscosity. The expanded structures, then, shrink again at lower shear-rate due to the inter-particle attractions.