Controlling Extrudate Volume Fraction through Poroelastic Extrusion of Entangled Fibers*

When a suspension of spherical or near-spherical particles passes through a constriction the volume fraction typically either remains the same or decreases. In this talk, in contrast to these particle suspensions, we observe that an entangled fiber suspension increases its volume fraction by 2 to 14 fold after passing through a constriction. We attribute this feature to the entanglements among the fibers that yield an elastic response when stretched, which allows the fiber network to move faster than the surrounding liquid in a converging channel. By changing the geometry of the fibers, we find that the entanglements may originate from interlocking shapes or high flexibility. Through direct imaging inside an extrusion channel, we quantify the velocity variations of the fibers during extrusion. A quantitative poroelastic model is used to explain the increase in velocity and extrudate volume fractions. The model-derived product of the permeability and Young's modulus of the fiber suspension demonstrates a universal relationship with the fiber volume fraction. These results provide opportunities to control suspension concentration, and so porosity, during its delivery through a needle or a catheter, as occurs in healthcare, three-dimensional printing, or material repair.