The internal structure of suspensions in uniaxial elongation

Self-thinning of a tiny thread of suspension of glass beads or rods is explored using high-speed X-ray phase-contrast imaging. The overall fluid mechanical pattern is provided by the thread-thinning rate, while the X-ray imaging of the evolution of the suspension internal structure sheds light onto the interplay between fluid mechanics and rheology. In particular, we measured in real time the motion and re-orientation of the individual glass rods in self-thinning liquid threads for the first time, and established the link between these micromechanical phenomena and macroscopic elongational suspension flow. The results with the rod-like particles raise questions on the validity of the common assumption of the affinity of the macroscopic flow kinematics and particle motion.