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The size and the agglomeration state of nanoparticles (NPs) play a very important role in nanoparticle applications due to the effect of the dispersion level of NPs on their toxicity, pharmaceutical activity, and catalytic activity. In this study, a novel two-stage nanoparticle disperser system was proposed. We focused on the effects of the aspect ratio (L/D, length (L) and diameter (D)) of the round jet (a straight pipe) on the nanoparticle size distribution (PSD) and we attempted to explain how the jet wakes induced the NP deagglomeration based on the numerical results. The median diameters, mode diameters, and geometric mean diameters (GMD) of the PSDs that were obtained using the second-stage were significantly smaller than those achieved using the first-stage disperser alone. Overall, the GMD, mode diameters, and median diameters could all reach sub-70 nm, which was approaching close to the NP primary diameter (30–40 nm). The particle sizes decreased with the increasing dimensionless nozzle diameter (D* = D/Dp × 10−3, Dp represents the particle diameter). The increase in L/D for a constant D hardly affected the PSDs but led them to be closer to a standard lognormal distribution. Regarding the NP traveling path and its considerable inertia as it penetrated the Mach disc, these particular field distributions of the jet-wake might explain why the suspended sub-micro NP agglomerates could be highly deagglomerated. Namely, the nano NP agglomerates would suffer the severe shear stress around the Mach disc.

Particle sedimentation has widely existed in nature and engineering fields, and most carrier fluids are non-Newtonian. Recently, the manipulation of a settling particle in liquid has been a topic of high interest to those involved in engineered processes such as composite materials, pharmaceutical manufacture, chemistry and the petroleum industry. Compared with Newtonian fluid, the viscosity of non-Newtonian fluid is closely related to the shear rate, leading to a single settling particle having different dynamic behaviors. In this article, the trajectories and velocities of two side-by-side particles of different densities (heavy and light) settling in a shear-thinning fluid with viscoelastic property were studied, as well as that for the corresponding single settling particle. Regardless of the difference in the particle density, the results show the two-way coupling interaction between the two side-by-side settling particles. As opposed to a single settling particle, the wake of the heavier particle can clearly attract or rebound the light particle due to the shear-thinning or viscoelastic property of the fluid. Regarding the trajectories of the light particle, three basic path types were found: (i) the light particle is first attracted and then repelled by the wake of the heavy one, (ii) the light particle approaches and then largely traces within the path of the heavy one in the limited field of view, (iii) the light particle is first slightly shifted away from its original position and then returns to this initial position. In addition to this, due to the existence of a corridor of reduced viscosity and negative wake generated by the viscoelastic property, the settling velocity of a light particle can exceed the terminal velocity of a single particle of the same density. On the other hand, the sedimentation of the light particle can induce the distinguishable transverse migration of the heavy one.