Rheology and structure of flocculated iron oxide suspensions

This paper focuses on the relation between structure and rheology of flocculated suspensions. Rheological measurements were taken both in shear and nearly extensional flows. The structures of magnetic iron oxide suspensions were visualized using cryogenic scanning electron microscopy (Cryo-SEM) and video-enhanced light microscopy (VELM) under shear, elongational, and rotational flows. Shear fields were generated by using a miniature Couette geometry, whereas elongational and rotational fields were generated by an innovative technique, which takes advantage of a surface tension gradient on the interface of an air bubble, and were visualized in real time under a light microscope. In Couette flow, the particle networks formed at high concentrations (9.4% by volume) do not uniformly break up into smaller units of particles, as at lower concentrations, but rather break up at the center of the gap, segregating particles toward the walls. Real-time light microscopy observations of a spinning floc show that the deformation and breakup of flocs is a process that gradually occurs with time. In extensional flow, flocs elongate and break up, explaining the apparent extensional thinning behavior measured. Apparent extensional viscosities were one to two orders of magnitude larger than shear viscosities at the same applied stress.