Orientation in simple shear flow of semi-dilute fiber suspensions .2. Highly elastic fluids

The orientations of fibers suspended in strongly viscoelastic polymer solutions were observed. The suspending fluids consisted of a glycerine-polyethylene glycol mixture containing different amounts of dissolved polyacrylamide (PAAm) with concentrations ranging from 200 to 2000 ppm. In the highly elastic limit of 1000 and 2000 ppm PAAm, a single fiber oriented in a fixed stable orientation. For the 2000 ppm fluid, the fixed orientation was along the flow direction, whereas for the 1000 ppm fluid, the fixed orientation was either along the flow direction or at a 20 degrees angle from the flow direction in the flow-vorticity plane. The orientation distributions of semi-dilute fiber suspensions in these highly elastic fluids were insensitive to fiber concentration and exhibited sharp peaks at the stable orientations observed in the single fiber experiments. In contrast, the orientation distribution in the 200 and 500 ppm PAAm solutions were much broader. The distributions at the higher end of the semi-dilute fiber concentration regime in the 200 ppm solution resembled the distributions observed previously in suspensions with lower fiber concentrations in a weakly elastic fluid (100 ppm PAAm), although a single fiber in the 200 ppm fluid does not spiral towards the vorticity but aligns in a stable fixed orientation in the flow-vorticity plane.