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

Growth of Floc Structure and Subsequence Compaction into Smaller Granules through Breakup and Rearrangement of Aluminum Flocs in a Constant Laminar Shear Flow

We have constructed an outer-cylinder-rotating Couette device for high-speed shear flow in laminar flow conditions and visualized the structure formation and subsequent rearrangement of PACl (flocculant made of aluminum hydroxide gel) and kaolinite flocs by visible light imaging. In a previous report, we analyzed the case of relatively low shear rate (G-value = 29 1/s) and confirmed that the flocculation process could be separated into two stages: a floc growth stage and a breakup/rearrangement stage. Once the large bulky flocs that reached the maximum size appeared, they rearranged and densified through structural fracture and rearrangement. In this report, this process was further investigated by conducting experiments under two different high shear rates (58 and 78 1/s) at which breakup and rearrangement became more pronounced, and three different aluminum kaolinite ratios (ALT ratios) that were over and under the optimum dosage (neutralization point by Zeta potential). Visualization results confirmed that, during the growth stage, the flocculation rate could be approximated by a scaling relationship between floc size and elapsed time, which depended on the ALT ratio. After reaching the maximum size, the floc rapidly became compact and dense following adsorption of the gel, incorporating fine fragments from erosion breakup. The over and under dosages created a lot of fragments of erosion breakup, but less so in the optimum dosage. In the optimum ALT ratio, fragments did not remain because they were incorporated into the flocs and densified, and the floc size was thought to be maintained. The floc circularity distribution peaked at around 0.6 and 1, suggesting that the flocs were spherical in shape due to erosion breakup.

Study on Coagulation Kinetics of Disk-like Particles under Simple Shear Flow

In this study, a theoretical study is carried out on the collision of two disk-like particles to understand the coagulation of disk-like particles suspended in liquid under a shear flow. The diameter of the particle is fixed at 2 μm while the length is varied so that the aspect ratio (length/diameter) varies from 0.1 to 0.4. The liquid viscosity is changed from 0.01 to 1 Pa s. The minimum Peclet number is 10, and the Brownian motion is considered to be negligible. Both hydrodynamic and van der Waals interactions are included in tracking the position and the orientation of each particle. The Hamaker constant is fixed at 1.06 × 10^-20 J. The boundary integral formulation is used to calculate the hydrodynamic interaction. To obtain the kinetic constant of coagulation, the time-independent orientation distribution function for a particle is obtained under the noninteracting condition. The kinetic constant of coagulation is obtained by considering the presence of collision between two particles initially separated by a long distance, the orientations of two particles, and the flux of the liquid flow. The result shows that the kinetic constant of coagulation is reduced to approximately 1/3 of the value for the noninteracting particles when the viscosity is 1 Pa·s. As collision modes, side-side, face-edge, side-edge, and edge-edge are considered. The edge-edge mode is frequently observed in the given range of the aspect ratio. The collision mode does not change much from the noninteracting case except for the side-side mode.