Changes In Particle Orientation Research Articles

Settling and collision of spheroidal particles with an offset mass centre in a quiescent fluid

Gravitational sedimentation and the collision of particles play key roles in various natural and engineering processes. In practice, particles are often non-spherical in shape with non-uniform mass distribution. In this study we investigate how the mass eccentricity influences the settling and gravitational collision of non-spherical particles in a quiescent fluid. Firstly, we theoretically analyse the effect of mass centre offset on the settling motion of a single spheroid under the low-Reynolds-number assumption. We find that the competition of fluid-inertia torque and gravitational torque determines the terminal settling mode of the spheroid. As the mass centre offset increases from zero to a critical value, the orientation of a settling spheroid undergoes a transition from the broad-side-on to narrow-side-on alignment. With an intermediate mass centre offset, the settling spheroid prefers an oblique orientation with a horizontal drift. Secondly, we investigate the gravitational collision rate of settling spheroids. With the change of particle orientation, the collision kernel exhibits a non-monotonic variation with a maximum when particles settle with an intermediate oblique orientation. Therefore, adjusting the mass centre offset to alter particle orientation can indirectly affect the collision rate of settling spheroidal particles. In summary, our findings reveal the significance of the mass eccentricity on particle dynamics in fluid flows, and suggest a potential approach for manipulating the settling motion and collision rate of non-spherical particles by adjusting their mass centre position.

Dynamics of spheroids in an unbound quadratic flow of a general second-order fluid

This work employs the second-order fluid model to investigate the effect of first and second normal stresses on the motion of spheroidal particles in unbound parabolic flows, where particles migrate toward the flow center. We specifically examine the effects of fluid Weissenberg number Wi and the ratio of normal stress coefficients α = ψ2/ψ1. Previous works have considered the motion of spheroidal particles in the co-rotational limit (α = −0.5), where the effect of fluid viscoelasticity is to modify the fluid pressure but not the shear stresses. Here, we examine all ranges of α that are found for functional complex fluids such as dilute polymer solutions, emulsions, and particulate suspensions and determine how viscoelastic shear stresses alter particle migration. We use perturbation theory and the Lorentz reciprocal theorem to derive the O(Wi) corrections to the translational and rotational velocities of a freely suspended spheroid in an unbound tube or slit flow. Our results show that for both prolate and oblate particles, the viscoelasticity characterized by α significantly affects the particle cross-stream migration, but does not qualitatively change the trends seen in the co-rotational limit (α = −0.5). For a range of α (−0.9 ≤ α ≤ 0) investigated in this work, particles possess the largest mobility when α = −0.9 and smallest mobility when α = 0. Although α does not alter particle rotation at a given shear rate, we observe significant changes in particle orientation during migration toward the flow center because changes in migration speed give rise to particles experiencing different shear histories.

Structures and phase transition of liquid crystals in a dynamic slit confinement

We report on the dynamic confinement of colloidal liquid crystals in a two-dimensional slit pore with a periodically stretching and contracting boundary using Langevin dynamics simulations. The influence of moving walls on phase behavior is analyzed, and four structures are identified. It is found that boundary vibration can induce phase transition. Structural transition characterized by the change in particle orientation is caused by varying the amplitude or frequency of the oscillating boundary. The key factor determined by the work performed on the system maintaining a steady structure is also clarified from the energy perspective. The inhomogeneous mobility of these far-from-equilibrium structures is induced by the active boundary. Our results contribute to a better understanding of the slit dynamic confinement system and suggest a new way of generating order by dissipating energy in non-equilibrium systems.

Magnetic cylindrical colloids at liquid interfaces exhibit non-volatile switching of their orientation in an external field.

We study the orientation of magnetic cylindrical particles adsorbed at a liquid interface in an external field using analytical theory and high resolution finite element simulations. Cylindrical particles are interesting since they possess multiple locally stable orientations at the liquid interface so that the orientational transitions induced by an external field will not disappear when the external field is removed, i.e., the switching effect is non-volatile. We show that, in the absence of an external field, as we reduce the aspect ratio α of the cylinders below a critical value (αc≈ 2) the particles undergo spontaneous symmetry breaking from a stable side-on state to one of two equivalent stable tilted states, similar to the spontaneous magnetisation of a ferromagnet going through the Curie point. By tuning both the aspect ratio and contact angle of the cylinders, we show that it is possible to engineer particles that have one, two, three or four locally stable orientations. We also find that the magnetic responses of cylinders with one or two stable states are similar to that of paramagnets and ferromagnets respectively, while the magnetic response of systems with three or four stable states are even more complex and have no analogs in simple magnetic systems. Magnetic cylinders at liquid interfaces therefore provide a facile method for creating switchable functional monolayers where we can use an external field to induce multiple non-volatile changes in particle orientation and self-assembled structure.

Influence of particles packing in granules on the particles orientation in compacts

Particles orientation during compaction was studied in alumina granules of different packing structures and deformation properties. These granules were classified mainly in two types: loose granules prepared with flocculated slurries and dense granules prepared with dispersed slurries. Particles orientations in the granules and in the compacts were examined quantitatively with the cross-polarized light microscopy. A large difference was noted in the packing structures of granules and compacts. Orientation of particles was detected only in the surface vicinity of the dense granules. These dense granules show only a slight change in particle orientation locally and its initial structures were mostly preserved even after compaction. As a result, the green compacts containing these granules also show very low net particles orientation. In contrast, loose granules show no orientated particles. However, a major rearrangement of the particles was noted during compaction, resulting in a high net particle orientation in the compacts. The particles orientation in the green compacts affected the anisotropy in the sintering shrinkage significantly; high anisotropy was observed in compacts of high particle orientation fabricated from the loose granules.

In situ TEM sintering of nano-sized ZrO 2 particles

The sintering behavior of faceted, nano-sized single-crystal particles of ZrO 2 has been investigated using in situ transmission electron microscope heating techniques. These direct observations have provided new information on the morphological and structural evolution of particles, necks and pores during the sintering process. The results indicate that ZrO 2 particle-pairs, which are not attached to other particles and not highly constrained by the TEM grid, reorient themselves with respect to each other during sintering at 890°C. This reorientation can be explained in terms of a minimization of the grain-boundary energy between the two coalescing particles. In addition to changes in particle orientation, these experiments show that the topologies of ZrO 2 particles and their associated necks evolve during heating. During the in situ TEM heating of these particles, the dynamic motion of atom clusters (10–100 atoms) on and off of surfaces is observed, as is the formation and dissolution of ledges and steps on previously smooth facets. The phenomenon of cluster migration is discussed and the ramifications of the results of this study for more highly constrained systems of particles (e.g. ceramic green-bodies) are also considered.

The microrheology of colloidal dispersions: II. Brownian diffusion of doublets of spheres

The rotary Brownian diffusion of isolated doublets of uniform spheres of 1- and 2-μm diam was measured in water at rest and in shear (Poiscuille) flow through narrow tubes. The translational Brownian diffusion of single spheres and doublets was also measured in suspensions at rest; under the conditions of the Poiseuille flow experiments, translational diffusion was too small to be measured. Expressions for the orientation distribution function of a set of noninteracting doublets, initially randomly oriented were derived. From these, mean values of the change in particle orientation with time were computed and compared with experimental values from a large number of measurements on single, isolated doublets. In the stationary suspensions, the experimental and theoretical mean values were in good agreement over time intervals of up to 2 sec. In Poiseuille flow, the Brownian contribution to the change in particle orientation was calculated from the measured angular displacements by subtracting the theoretically predicted displacement due to shear and the mean values found to be in agreement with theory, i.e., Brownian motion is independent of shear over short time, t ⪡ T. Over longer times this agreement is no longer expected, as found experimentally.

Pressure drop due to the motion of neutrally buoyant particles in duct flows

The additional pressure drop arising from the presence of a neutrally-buoyant, eccentrically positioned, spherical particle in a Poiseuille flow is calculated toO(a/Ro)5(a= sphere radius;Ro= tube radius). Similar calculations (of a lesser order of accuracy) are given for non-circular conduits and for ellipsoidal particles. Due to changes in particle orientation resulting from rotation, the instantaneous pressure drop for an ellipsoid of revolution varies periodically with time. This pressure diminution is averaged over one period to obtain the time-average pressure drop.

Microstructure of Unconsolidated and Consolidated Marine Sediments

ABSTRACT Sediment samples from cores taken in the Gulf of Mexico were subjected to normal laterally confined loads of 0.5, 4.0, 32.0 and 64.0 Kg per cm2. Electron micrographs of their microstructure show the progressive change in particle orientation during the laboratory consolidation of a sediment sample. A predominantly random arrangement of particles exists over the greater portion of the void ratio range. Strongly preferred orientation apparently develops at pressures in excess of a few Kg per cm2.