Rheology anisotropic particles

Many of the fillers used in industry are anisotropic in character. Depending on the shape of fillers, they are subdivided into isotropic particles, flakes, and fibers. Anisotropic particles may take on states of orientation because of flow and packing processes. Whether developed during flow or processing, particle orientation influences phenomena ranging from rheological properties to compound processability in industrial processing equipment, electrical characteristics, and mechanical performance. 

Before discussing theoretical models for the rheology of fiber suspensions and its connection to fiber orientation, there are three topics that must be discussed Brownian motion, concentration regimes, and fiber flexibility. Brownian motion refers to the random movement of any sufficiently small particle as a result of the momentum transfer from suspending medium molecules. The relative effect that Brownian motion may have on orientation of anisotropic particles in a dynamic system can be estimated using the rotary Peclet number, Pe s y Dm, where y is the shear rate and Ao is the rotary diffusivity, which defines the ratio of the thermal energy in the system to the resistance to rotation. Doi and Edwards (1988) estimated the rotary diffusivity, Ao, to be... 

Mourchid, A., Delville, A., Lambard, J., Lecolier, E. and Levitz, P. (1995) Phase Diagram of Colloidal Dispersions of Anisotropic Charged Particles Equilibrium Properties, Structure, and Rheology of Laponite Suspensions. Langmuir 11, 1942-1950... 

Composites with filler concentrations close to the percolation threshold exhibit conductivity which is sensitive to compressive deformation, since this brings the metal particles into contact, thereby forming percolation pathways. This sensitivity has been exploited especially in anisotropic composites. These are made by prealigning the metal particles with either electric or magnetic fields. This alignment is identical with that produced by external fields in electro- and magneto-rheological fluids where at a critical field continuous threads of... 

Due to the magneto-rheological effect, the resulting magnetic composite becomes anisotropic in terms of mechanical and magnetic properties. One can easily vary the direction of the particle chains by the direction of the applied magnetic field, as shown in Fig. 6. 

Mourchid A, Delville A, Lambard J, L 233 colier E and Levitz P 1995 Phase diagram of colloidal dispersions of anisotropic charged particles equilibrium properties, structure, and rheology of Laponite suspensions Langmuir AA 1942-50... 

The objective of the polarization model is to relate the material parameters, such as the dielectric properties of both the liquid and solid particles, the particle volume fraction, the electric field strength, etc., to the rheological properties of the whole suspension, in combination with other micro structure features such as fibrillatcd chains. A idealized physical model ER system—an uniform, hard dielectric sphere dispersed in a Newtonian continuous medium, is usually assumed for simplification reason, and this model is thus also called the idealized electrostatic polarization model. The hard sphere means that the particle is uncharged and there are no electrostatic and dispersion interactions between the particles and the dispersing medium before the application of an external electric field. For the idealized electrostatic polarization model, there are roughly two ways to deal with the suspensions One is to consider the Brownian motion of particle, and another is to ignore the Brownian motion and particle inertia. For both cases the anisotropic structure of such a hard sphere suspension is assumed to be represented by the pair correlation function g(r,0), derived by... 

In this chapter we consider the characteristics of binary polymer-soHd particle suspensions. Our concern is with polymer-particle interaction and particle-particle interactions, especially in their roles to influence the melt flow and enhance solid mechanical behavior. We discuss the behavior of isotropic- and anisotropic-shaped particle compounds in thermoplastics, including rheological behavior from low loadings to high loadings obtained using various instruments. 

The characteristics of incorporated fillers influence the rheological properties of TLCPs. Moreover, the fibre orientation is also influenced by the rod like molecules of LCPs (Ramazani et al. 2001). In general, the diameter of TLCP fibers is 1 pm (or less) compared to 10-15 pm length typically observed for glass fibers (Laun 1984). For particles of 10 pm or larger, the hydrodynamic interactions can cause a change in the velocity distribution in the vicinity of other fibers (Barnes 2003). Fiber-fiber collisions may also influence suspension viscosity by creating anisotropic structures in the fluid. 

Flows were also utilized to direct the assembly of submicrometer-diameter particles in ID structures. For dilute dispersions, their rheological characteristics such as viscosity and elasticity determined the type of the resulting stmcture. When particles are dispersed in a polymer solution, melt, or concentrated surfactant solution, flow can induce anisotropic viscoelastic stresses which govern ID particle self-assembly. The assembly of particles occurs at high shear rates such that the Weissenberg number (the ratio of the first normal stress difference over the shear stress) of the suspending medium exceeds a critical value. In addition to the shear rate and shear strain, particle concentration, polydispersity, and particle interaction potentials play a major role in the formation of ID stmctures. One example includes shear-induced... 

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