Experimental results on the flow rheology of fiber-laden aqueous foams

We report here the first experimental results on the rheology of fiber-laden aqueous foams. The measurements were carried out in a laboratory-scale environment with a glass pipe of diameter 15 mm. The slip velocity at the pipe wall was measured with high-speed video imaging. Plain aqueous foam was generated from 8.5 mM aqueous solution of sodium dodecyl sulphate (SDS). Foam generation was realized as a combination of tank mixing and injection of compressed air in a special inline generation block (turbulence generator) installed into the flow loop. Fiber-laden foam was prepared by dispersing hardwood fibers into the SDS solution at consistency of 20 g/kg. In the measurements, an absolute slip velocity was observed that increased with the wall shear stress. On the other hand, the relative slip velocity decreased with the wall shear stress. At highest shear stresses relative slip values of ca. 10% were observed, i.e. considerable shearing took place inside the foam. At low wall shear stress relative slip velocities up to 40% were measured. The addition of wood fibers decreased the absolute slip by ca. 25% while the relative slip increased by a factor close to four. The real wall shear rate in foam was calculated with the Weissenberg-Rabinowitsch correction. All the studied foams could be modeled with Herschel-Bulkley law with flow behavior index n = 0.5. The viscosity of the fiber-laden foam was ca. 100% larger than that of the plain aqueous foam at same density and temperature. This increase in viscosity is much less than in the case of plain aqueous fiber suspension, where the viscosity increases by a factor five or more due to fibers being in continuous contact in shearing. Thus the current results imply that in aqueous foams fibers do not interact or flocculate to the same extent as in plain aqueous suspensions. By applying the methodology described here on the data measured with one pipe diameter, one can calculate real material properties that are independent of boundary effects like slip velocity.