Dispersions of micrometric powders of molybdenum and alumina in liquid paraffin: role of interfacial phenomena on bulk rheological properties

The flow behavior of particles size about 10 μm of molybdenum and alumina powders in liquid paraffin at 20 °C has been evaluated under maximum shear rate D values of 100 (s−1), 200 (s−1) and 400 s−1. The molybdenum suspensions slowly deviate from the Newtonian behavior while the alumina ones obey the Herschel–Bulkley model. The viscosities of the suspensions increase with the volume fraction (Φ) of dispersed solid phase. The molybdenum/paraffin system obeys the Einstein equation, while the alumina/paraffin one obeys the Krieger–Dougherty equation. The intrinsic viscosity of the molybdenum suspensions is about 10, while the one calculated for the alumina/paraffin dispersion is 5. The spreading data give an apparent surface diffusion coefficient equal to 5.1×10−3 mm2 s−1 for the molybdenum/paraffin system, and equal to 2.6×10−2 mm2 s−1 for the alumina/paraffin one. The average advancing dynamic contact angle for the molybdenum/paraffin system is equal to 26 and equal to 17 for the alumina/paraffin one. The interfacial data have been discussed on the background of a new theory on nanometric film and drops on non-equilibrium solid surfaces. The average potential of interaction between the solid surfaces and the liquid paraffin has turned out to be equal to −60.9×10−3±10−2 J m−2 for molybdenum and −62.6×10−3±10−2 J m−2 for the alumina one. The difference among the interfacial solid–liquid data can explain the observed differences between the bulk rheological properties of the 10 μm particle sized dispersions. The results allow us to expand to these kinds of dispersions the tailoring methods adopted for the ceramic colloidal dispersions.