Evaluation of apparent viscosity of liquid-containing porous particles by the intrusion method using a conical rotor

The apparent viscosity of a wet powder consisting of porous silica particles and a viscous liquid was evaluated by means of a newly developed powder rheometer in which a rotating conical rotor with grooves on the surface intrudes semi-statically into the powder bed. Using this rheometer, the relationship between the shear torque and the depth of intrusion, i.e., the torque characteristic curve, was measured under various conditions. The shear force acting on a contact point between the particles was estimated from the torque characteristic curve. Above the critical liquid amount in which the pores of the particles were filled with the liquid, the shear force increased with an increase in the thickness of the liquid film formed on the particle surface regardless of the pore volume. From the change of shear force with the physical properties of the liquid, it was clear that the shear force is closely related to the liquid bridge force acting on the contact points between the particles. The apparent viscosity coefficient of the wet powder was determined from the shear rate dependence of the shear force. At the relatively high liquid amounts corresponding to funicular and capillary states, the apparent viscosity coefficient increased sharply with the thickness of the liquid film since the viscosity of the liquid strongly affected the shear flow of the wet powder. Subtle changes of the apparent viscosity due to the liquid amount and the physical properties of the liquid can be sensitively detected by using the rotary-intrusion method.