Suspensions anisotropic particles

It has been shown experimentally [13] that the viscosity of anisotropic particle suspensions increases proportionally to the square of the ratio between the large and small axes for ellipsoids of revolution when the particles are prolate and increase directly proportional to the... 

Quite specific effects in the flow of dispersions of long fibers are connected with particles orientation in the flow. Indeed, the state of fibers during the flow changes greatly as compared the initial state, so that the material in a steady-state flow is an anisotropic medium. Therefore the viscosity of such a suspension may become independent of a fiber s length [30], The most strong effects caused by a deformation of anisotropic particles should be expected in transient flows, in particular if the particles themselves are flexible and deformed in the flow. 

This is obvious for the simplest case of nondeformable anisotropic particles. Even if such particles do not change the form, i.e. they are rigid, a new in principle effect in comparison to spherical particles, is their turn upon the flow of dispersion. For suspensions of anisodiametrical particles we can introduce a new characteristic time parameter Dr-1, equal to an inverse value of the coefficient of rotational diffusion and, correspondingly, a dimensionless parameter C = yDr 1. The value of Dr is expressed via the ratio of semiaxes of ellipsoid to the viscosity of a dispersion medium. 

In colloidal suspensions of anisotropic particles, the static structure factor plays a prominent role in particle size analysis. We have used transient electric birefringence (TEB) and electron microscopy, in addition to laser light scattering, to correlate our analysis of particle size distributions of bentonite suspensions. The complementary nature of TEB and photon correlation spectroscopy (PCS) in particle size analysis will be discussed. 

Figure 1. Schematic diagrams of TEB and LLS instrumentation. P, pinholes L, lenses B, polarizers C, cell Q, quarter wave plate PMT, photomultiplier tube HVG, high voltage generator MP, microprocessor TR, transient recorder CL, correlator CT, counter 6, scattering angle. For the TEB setup polarizers B-, B2 have polarization axis oriented at tt/4 with respect to the x-axis, as shown in (a). After the light beam passed through the cell with electric field in the x-direction containing a suspension of anisotropic particles and the quarter waveplate with its fast axis oriented at tt/4 with respect to the x-axis, the transmitted light beam is polarized in the direction of 71/4 + 6/2, as shown in (b). Analyzer B has polarization axis oriented at 3t/4 + a as shown in (c).

We have seen that it can be difficult to reach the critical concentration required to observe an isotropic-anisotropic transition because concentrated suspensions of colloids are not always stable. However, orientation of flexible polymers as well as of anisotropic particles in suspension can be induced by flow, a phenomenon that has long been observed, reported, and studied. This phenomenon is especially strong when a pretransitional effect exists, which can be easily observed by the naked eye on a sample that is shaken between crossed polarizers (see for example the section on clays). In these systems, birefringence is induced via mechanical forces, like the shear stresses in a laminar flow ( Maxwell-dy-namo-optic effect ). 

Coagulation structures form when disperse system undergoes aggregation if there is a sufficient amount of dispersed phase, armoring of the entire volume of disperse system takes place. The amount of dispersed phase that is needed to solidify liquid dispersion medium may sometimes be very small, especially if the system contains very fine and anisotropic particles this may be only 6% by weight in the case of bentonite clay lamella suspension, and even less for a suspension of thread-like particles of V205. 

If the particles are not spherical, even in the very dilute limit where the translational Brownian motion would still be unimportant, rotational Brownian motion would come into play. This is a consequence of the fact that the rotational motion imparts to the particles a random orientation distribution, whereas in shear-dominated flows nonspherical particles tend toward preferred orientations. Since the excess energy dissipation by an individual anisotropic particle depends on its orientation with respect to the flow field, the suspension viscosity must be affected by the relative importance of rotational Brownian forces to viscous forces, although it should still vary linearly with particle volume fraction. 

The list could be made longer, taking the idea of electrokinetics in a wide sense (response of the colloidal system to an external field that affects differently to particles and liquid). Thus, we could include electroviscous effects (the presence of the EDL alters the viscosity of a suspension in the Newtonian range) suspension conductivity (the effect of the solid-liquid interface on the direct current (DC) conductivity of the suspension) particle electroorientation (the torque exerted by an external field on anisotropic particles will provoke their orientation this affects the refractive index of the suspension, and its variation, if it is alternating, is related to the double-layer characteristics). 

Many empirical and theoretical modifications have been made to Einstein s equations. A useful extension to dilute suspensions of anisotropic particles, such as clays, is given by the Simha Equation, which is approximately... 

The viscosity of the solvent is constant but the viscosity contribution due to a suspension of large anisotropic particles may depend on the rate of shear. This is in particular true of DNA and the extrapolation of [ ] to zero rate of shear can be obtained by a variety of means [76E1]. 

Shear Thinning Flow. Dispersions showing a decrease in viscosity with shear rate (or shear stress) are described as shear thinning or pseudoplastic. Shear thinning behavior is generally produced by the reversible breakdown of suspension structures or alignment of anisotropic particles due to shear. 

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. 

There are no equivalent studies of concentrated anisotropic suspensions. This is because of the tendency of anisotropic particles to form oriented ordered structures such as micro-bundles of fibers. Batchelor [45] has modeled the flow of concentrated suspensions of large parallel fibers. Very large elongational viscosities are predicted. This was extended to suspensions in non-Newtonian fluids by Goddard [46]. 

As noted earlier, in the 1920s Taylor [38] experimentally verified Jeffery s [37] analysis for the motion of ellipsoids. There were subsequent studies by Taylor in the 1930s [39,40]. In the 1950s Mason and his coworkers [41 to 44] made extensive efforts to visualize anisotropic particle motions in dilute suspensions during flow of rigid rod- and disk-shaped particles. They observed a distribution of orbits. 

Suspensions of large particles, ca 10 pm and more, can be modeled by hydrodynamics and the theory of elasticity. With anisotropic particles such as fibers, striking anisotropic characteristics develop. 

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... 

The effect of surfactant on the dispersibility of Au nanoparticles depends on the sort of ionic type. When AOT was used instead of DOAC, the color of sols changed blue indicating the process of coagulation and the suspension was not stable. Within a few hours, the colloids fully precipitated. Figure 9-4.33 shows a TEM picture of this system sampled just after ultrasonic treatment to promote dispersion of sols. The particles in this picture are anisotropic and irregular in shape, and the size is larger than for the case of DOAC, as seen in the histogram. However, it is... 

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