Experimental study on reversible formation of 2D flocs from plate-like particles dispersed in Newtonian fluid under torsional flow

In this study the reversible aggregation and breakage of two dimensional (2D) flocs in the bulk phase are investigated experimentally in a suspension of platelike particles under a torsional flow between two parallel plates. The suspension was prepared by dispersing platelike particles in a Newtonian fluid. The Newtonian fluid has a viscosity of 1.27 Pa s and the average dimension of the particle face is approximately 1 μm with a thickness of 250 nm. Peclet number of the particle is over 100 and the aggregation is orthokinetic. The microstructure was examined under an optical microscope without disturbing the flow or particle motions. The result shows that 2D flocs of fractal types with the aspect ratio of over 20 between the average face diameter and thickness are formed in the bulk phase by the aggregation of platelike particles and the face of the planar floc is oriented in the same direction of the velocity gradient. The flocs have non-circular faces with size dependent fractal dimensions. The flocs tend to be oriented along the vorticity or the flow direction depending on shear rate and the orientation strongly affects floc breakage and floc size. The breakup of flocs occurs mostly at the middle of the floc after being aligned along the flow direction.