EXTENSIONAL FLOW OF POLYDISPERSE FIBER SUSPENSIONS IN FREE-FALLING LIQUID JETS

Experimental results for axial velocity profiles in free-falling jets of a Newtonian liquid were in close agreement with numerical solutions of a mathematical model of the jet. The model was fitted successfully to experimental results for jets of fibre suspension by adjustment of an effective suspension extensional viscosity. The occurrence of polydispersity in fibre suspensions is discussed, and theoretical results for viscosity of aligned fibre suspensions in uniaxial extension are re-written to take into account distributions of fibre length and diameter. These distributions were determined experimentally, and predictions based upon them compared with experimental results. It is found that, over a 20-fold range of concentration (0.04-0.9 vol%) and a 6-fold change in fibre aspect ratio (0.5 and 3 mm × 10 μm), experimental values closely follow the form of concentration dependence exhibited by the close particles theory, but lie 15% below its predictions. They also lie 50% below the values given by an interpolation formula suggested for use in the intermediate concentration range occupied by the experiments. Irregularity in the jet flow was encountered which increased in severity with concentration and ratio of fibre length to jet diameter. This is believed to be due to fibre clumping resulting from the impossibility of complete fibre dispersion and alignment as concentration approaches 10% of the limiting value for a random three-dimensional aggregation of rigid rods. This conclusion means that it may, in practice, be very difficult to achieve the conditions upon which the "close particles" theory is based. © 1990.