Rheology of a Dilute Suspension of Aggregates in Shear-Thinning Fluids

The prediction of the viscosity of suspensions is of fundamental importance in several fields. Most of the available studies have been focused on particles with simple shapes, for example, spheres or spheroids. In this work, we study the viscosity of a dilute suspension of fractal-shape aggregates suspended in a shear-thinning fluid by direct numerical simulations. The suspending fluid is modeled by the power-law constitutive equation. For each morphology, a map of particle angular velocities is obtained by solving the governing equations for several particle orientations. The map is used to integrate the kinematic equation for the orientation vectors and reconstruct the aggregate orientational dynamics. The intrinsic viscosity is computed by a homogenization procedure along the particle orbits. In agreement with previous results on Newtonian suspensions, the intrinsic viscosity, averaged over different initial orientations and aggregate morphologies characterized by the same fractal parameters, decreases by increasing the fractal dimension, that is, from rod-like to spherical-like aggregates. Shear-thinning further reduces the intrinsic viscosity showing a linear dependence with the flow index in the investigated range. The intrinsic viscosity can be properly scaled with respect to the number of primary particles and the flow index to obtain a single curve as a function of the fractal dimension

Particle dynamics in microfluidics: slip and deformability

In the last decades there has been a growing interest in microfluidics, i.e. a technology characterized by the engineered manipulation of fluids at the sub-millimeter scale, due to the advantages of a fine tuning of flow and transport conditions and the capability to handle micrometric particles, which are fundamental in both synthesis and analysis. More recently, an increasing interest in using viscoelastic fluids in microfluidic devices is observed. Indeed, it has been recently proven that the non-Newtonian rheological properties of the fluid allow to perform several operations by using simpler apparatuses as compared to the case with a Newtonian liquid. However, at small scale and with complex fluids some peculiar phenomena can occur, such as the boundary slip, which can affect both the fluid velocity profile and the fluid-particle interactions. In this thesis, we study the effects of boundary slip on the dynamics of a spherical particle suspended in an inertialess Newtonian or viscoelastic shear-thinning fluid, under shear or Poiseuille flow, by means of 3D Arbitrary Lagrangian Eulerian (ALE) Finite Element Method (FEM) direct numerical simulations. In particular, we investigate separately on the effect of wall and particle slip on the cross-stream migration induced by fluid viscoelasticity. Furthermore, many particles that are treated in microfluidics are deformable, especially in biological and medical applications (from microgels to cells). Hence, in the second part of this thesis, we address the dynamics of an elastic particle suspended in an inertialess Newtonian fluid flowing through a channel with an orthogonal side branch (asymmetric T-shaped bifurcation) by means of 2D ALE FEM simulations. The effects of the elastic capillary number and the confinement ratio on the particle trajectory and deformation near the bifurcation are investigated. We discuss how this device can be exploited for separating particles based on their elasticity

Discrete Element Simulations of Nanoparticles Synthesis in Wet-operating Stirred Media: Effect of the Particle Material

An increasing number of applications from every branch of science and engineering are relying on the use of nanomaterials due to their peculiar properties with respect to the bulk counterpart and the potential of their employment, for example in the environmental field. Hence, there is a growing need to develop alternative strategies to produce such materials, providing first-rate performances and a fine control over the product specifications through safer and more sustainable processes. Here we focus on a low-energy magnetically driven wet milling technique for the synthesis of metal nanoparticles starting from a bulky solid, as a simple, cheap, and sustainable approach providing numerous advantages, including the minimization of nanoparticle air dispersion and greater control over the final product. We exploit discrete element method simulations to investigate the interactions among the grinding beads and the magnetic stirrer, providing information on the frequency and energy of collisions under various operating conditions, in an attempt to highlight the role of source material in the dynamics of the system. The relation of such data with the properties of the produced nanoparticles allows a fine tuning of the process parameters

Numerical Investigation of a Novel Grinding Device for the One-Pot Production of Ferromagnetic Nanoparticles

The use of nanoparticles (NPs) in industrial applications is consistently increasing given their peculiar properties compared to bulk precursor materials. As a result, there is a growing need to develop alternative technical strategies for the synthesis of such NPs using processes that are not only environmentally friendly but also easy and inexpensive to implement on an industrial scale. In this regard, a novel approach has recently been proposed for the safe and sustainable production of metal NPs directly from a bulky solid by magnetically driven low-energy wet milling, which overcomes the limits of applicability to ferromagnetic materials through a unique device configuration. In the present contribution, the understanding of this alternative configuration is deepened by computational investigation. Discrete Element Method (DEM) simulations were used to model the dynamics of the system, highlighting the role of the various parameters involved in the setup and operation of the process. The collisions between grinding and primary particles are analyzed in terms of frequency, impact angle, and energy. Comparing the results with the standard device configuration, the general trend is preserved, though collisions at higher impact angle and energy are also detected

Sedimentation of Fractal Aggregates in Shear-Thinning Fluids

Solid–liquid separation is a key unit operation in the wastewater treatment, generally consisting of coagulation and flocculation steps to promote aggregation and increase the particle size, followed by sedimentation, where the particles settle due to the effect of gravity. The sedimentation efficiency is related to the hydrodynamic behavior of the suspended particles that, in turn, depends on the aggregate morphology. In addition, the non-Newtonian rheology of sludges strongly affects the drag coefficient of the suspended particles, leading to deviations from the known settling behavior in Newtonian fluids. In this work, we use direct numerical simulations to study the hydrodynamic drag of fractal-shaped particles suspended in a shear-thinning fluid modeled by the power-law constitutive equation. The fluid dynamics governing equations are solved for an applied force with different orientations uniformly distributed over the unit sphere. The resulting particle velocities are interpolated to compute the aggregate dynamics and the drag correction coefficient. A remarkable effect of the detailed microstructure of the aggregate on the sedimentation process is observed. The orientational dynamics shows a rich behavior characterized by steady-state, bistable, and periodic regimes. In qualitative agreement with spherical particles, shear-thinning increases the drag correction coefficient. Elongated aggregates sediment more slowly than sphere-like particles, with a lower terminal velocity as the aspect ratio increases.</jats:p

Rheology of a Dilute Suspension of Aggregates in Shear-Thinning Fluids

The prediction of the viscosity of suspensions is of fundamental importance in several fields. Most of the available studies have been focused on particles with simple shapes, for example, spheres or spheroids. In this work, we study the viscosity of a dilute suspension of fractal-shape aggregates suspended in a shear-thinning fluid by direct numerical simulations. The suspending fluid is modeled by the power-law constitutive equation. For each morphology, a map of particle angular velocities is obtained by solving the governing equations for several particle orientations. The map is used to integrate the kinematic equation for the orientation vectors and reconstruct the aggregate orientational dynamics. The intrinsic viscosity is computed by a homogenization procedure along the particle orbits. In agreement with previous results on Newtonian suspensions, the intrinsic viscosity, averaged over different initial orientations and aggregate morphologies characterized by the same fractal parameters, decreases by increasing the fractal dimension, that is, from rod-like to spherical-like aggregates. Shear-thinning further reduces the intrinsic viscosity showing a linear dependence with the flow index in the investigated range. The intrinsic viscosity can be properly scaled with respect to the number of primary particles and the flow index to obtain a single curve as a function of the fractal dimension.</jats:p

Discrete Element Method Simulations of an Innovative Magnetic Stirred Device for the Top-down Production of Ferromagnetic Nanoparticles

Nanoparticles (NPs) are relevant in several industrial applications due to their peculiar properties with respect to their bulk precursor material. Hence, there is a growing need to develop novel technical solutions to synthetize such NPs by simple, eco-friendly, and cost-effective processes. In this regard, the authors have recently proposed a strategy for the safe and sustainable production of NPs involving a mechanical refining using magnetic agitation in wet-operating stirred media, which minimizes the NPs air dispersion and improves the control over the final product specifics. However, the magnetic agitation poses heavy limits of applicability in the case of synthesis of ferromagnetic NPs. In the present contribution, an alternative device configuration developed to overcome this limitation is investigated though a numerical approach. Discrete element method (DEM) simulations are performed to model the grinding and primary particles collisions and to clarify the effect of the parameters involved in both process setup and operation. The results are reported in terms of frequency and velocity of collision and compared to those of the standard device configuration to derive useful information about the functioning and capabilities of the novel system

Nanoparticles Synthesis in Wet-Operating Stirred Media: Investigation on the Grinding Efficiency

The use of nanomaterials, thanks to their peculiar properties and versatility, is becoming central in an increasing number of scientific and engineering applications. At the same time, the growing concern towards environmental issues drives the seeking of alternative strategies for a safer and more sustainable production of nanoparticles. Here we focus on a low-energy, magnetically-driven wet milling technique for the synthesis of metal nanoparticles starting from a bulky solid. The proposed approach is simple, economical, sustainable, and provides numerous advantages, including the minimization of the nanoparticles air dispersion and a greater control over the final product. This process is investigated by experiments and discrete element method simulations to reproduce the movement of the grinding beads and study the collision dynamics. The effect of several parameters is analyzed, including the stirring bar velocity, its inclination, and the grinding bead size, to quantify the actual frequency, energy, and angle of collisions. Experiments reveal a non-monotonous effect of the stirring velocity on the abrasion efficiency, whereas numerical simulations highlight the prevalent tangential nature of collisions, which is only weakly affected by the stirring velocity. On the other hand, the stirring velocity affects the collision frequency and relative kinetic energy, suggesting the existence of an optimal parameters combination. Although a small variation of the stirring bar length does not significantly affect the collision dynamics, the use of grinding beads of different dimensions offers several tuning opportunities.</jats:p