P. Kuzhir

This work is devoted to the detailed study of jet instability occurring in concentrated aqueous mixtures of calcium carbonate (CC) isotropic-shaped particles and rigid polyamide (PA) fibers. These mixtures exhibit very sharp discontinuous shear thickening (DST). The jets were subjected to a free fall under gravitational stretching at a constant flow rate. In the absence of PA fibers, we observed relatively strong lateral oscillations occurring for jet lengths L≳6cm and accompanied by small periodic undulations of the jet diameter. Two-dimensional Direct Fourier Transform analysis reveals approximately linear dispersion relations for propagation of lateral oscillations and diameter undulations with similar wave speeds ∼1m/s. This instability is ascribed to complex rheological behavior in an extensional flow above the DST transition. Theoretical modeling reveals abrupt jumps of the tensile stress along the jet likely leading to fluctuation of longitudinal and transverse velocity fields within the jet perceived through jet diameter and centerline undulations. The addition of PA fibers to CC suspension damps lateral oscillations but favors ruptures along the jet. This is tentatively explained by the interplay between growing lower and decreasing upper DST threshold stresses with increasing fiber volume fraction φf along with the thinning of the jet diameter down to the size of fiber flocs. Quantitatively, the stabilizing effect of PA fibers is manifested through an abrupt decrease in the lateral drift amplitude at φf≳0.75vol%.