Suspensions structured

SANS Small-angle neutron scattering [175, 176] Thermal or cold neutrons are scattered elastically or inelastically Incident-Beam Spectroscopy Surface vibrational states, pore size distribution suspension structure... 

These differences in the effect of polymers on various flocculation responses have important theoretical and practical implications and can be explained in terms of various characteristics of floes and floc-aggregates. Polymer adsorption or attachment of particles to polymer can occur in any number of configurations, and as a result the aggregation of particles also can take place in many ways, leading to different floe and suspension structures which will respond differently to different tests. 

J. W. Goodwin, Rheological properties, interparticle forces and suspension structure, in D.M. Bloor and E. Wyn-Jones (Eds.), The Structure, Dynamics and Equilibrium Properties of Colloidal Systems. NATO ASI Series C 324, Kluwer, The Netherlands, 1990, pp. 659-679. 

Figure 23. The G or elastic modulus and the electrosonic amplitude (ESA) of an oil sands fine tailings as a function of pH. The G or response of the suspension structure to nondestructive shear is determined by the strength of the particle floes. The particle interactions or floe formation is a function of the surface charges on the particles (determined by the electrosonic amplitude) and this, in turn, is sensitive to the pH of the system.

For 10-/mn particles suspended in water at room temperature, fR is of the order of 10 min. Hence, it is only in dilute systems of small particles that complete relaxation of a suspension structure can take place within the shear rate range of most rotational viscometers on the market. 

Suspension Structure and Forces Acting on the Suspended Particles... 

Some examples of suspension structures are illustrated in Figure 5. Figure 5a depicts a stable suspension with only short-range repulsive forces between the suspended fine particles. Hence, this system may settle as the particles move around each other into positions of lowest free energy, a consequence of the fact that the repulsive forces act between them. Figure 5b is a stable system for a more concentrated suspension. The... 

Figure 5. Suspension structures (a) disordered (stable) (b) ordered (stable) and (c) flocculated.

In deriving equation 32 or equation 33, it is assumed that 0max is the solid volume fraction at which the suspended particles cease moving. Thus, the forces, such as shearing, that can disturb the suspension structure and hence improve the mobility of particles will have an effect on the value of max. This is confirmed by the fact that a value of kH = 6.0 is observed at low shear limit, that is, y 0 and at high shear limit, y - oo, kH = 7.1 is found. Typical values of max have been found with the use of Quemada s equation as 0max = 0.63 0.02 in the low shear limit and high shear limit for submicrometersized sterically stabilized silica spheres in cyclohexane (72, 85, 88). 

Concentrated suspensions commonly display viscoelatic behavior. The viscoelastic properties can be measured by oscillatory tests (26). Comparing with steady shear measurements, oscillatory measurements are made under small deformations, at which the suspension structure is only slightly perturbed. Hence, oscillatory measurements are suitable for correlating rheological behavior with structural data and interparticle potentials, even for strongly flocculated systems that show irreversible changes when subjected to large deformations. 

Flocculated Systems. The viscoelastic responses of flocculated systems are strongly dependent on the suspension structure. The suspension starts to show an elastic response at a critical solid volume fraction of 0ct = 0.05 — 0.07, at which the particles form a continuous three-dimensional network (211-213). The magnitude of the elastic response for flocculated suspensions above 0ct depends on several parameters, such as the suspension structure, interparticle attraction forces and particle size, and shape and volume fraction. Buscall et al. (10) found that the volume fraction dependence of the storage modulus follows a power-law behavior. 

The compressive yield properties are strongly dependent on the suspension structure. Mills et al. (186) showed that extensive preshearing of a flocculated suspension can result in a much denser floe morphology (in fractal terms), which was reflected by a decrease in the compressive yield stress by one order of magnitude. 

Electron microscopy (see section BE 181 is very valuable in characterizing partieles (see, for instance, figure C2.6.1]. The suspension structure is, of course, not represented well because of the vaeuum eonditions in the microscope. This can be overcome using environmental SEM [24]. 

Letwimolnun et al. [2007] used two models to explain the transient and steady-state shear behavior of PP nanocomposites. The first model was a simplified version of the stmcture network model proposed by Yziquel et al. [1999] describing the nonlinear behavior of concentrated suspensions composed of interactive particles. The flow properties were assumed to be controlled by the simultaneous breakdown and buildup of suspension microstructure. In this approach, the stress was described by a modified upper-convected Jeffery s model with a modulus and viscosity that are functions of the suspension structure. The Yziquel et al. model might be written ... 

The advantages and drawbacks of the three methods to prepare the sohd/solution interface discussed above are listed in table 1. As of today, the most common method is to prepare a dry layer, even though it is simpler to use a paste. However, using a paste has a major drawback since the contact between particles and the ATR crystal is not optimal, the sensitivity is low and depends on the suspension structure (which in general is pH-dependent). On the other hand, using the results obtained with a dry layer to interpret macroscopic data obtained in well-disparsed suspensions can be tricky, since effects due to... 

Fundamentally, the rheological properties of concentrated colloidal suspensions are determined by the interplay of thermodynamic and fluid mechanical interactions. This means that there exists an intimate relationship between the particle interactions, including Brownian motion, the suspension structure (i.e. the spatial particle distribution in the liquid), and the rheological response. With particles in the colloidal size range (at least one dimension <1 pm), the range and magnitude of the interparticle forces will have a profound influence on the suspension structure and hence, the rheological behaviour (4, 7). Both the fluid mechanical interactions and the interparticle forces are... 

One feature of most colloidally stable suspensions is that the compressive properties are more or less reversible, provided that no major changes in suspension structure occur. However, in the case of flocculated suspensions, the compressive properties are irreversible. In concentrated flocculated suspensions, a continuous particle network forms which can support some stress up to a critical value. Once this critical stress, also called the compressive yield stress, Py, is exceeded, the network consolidates to a higher volume fraction with a higher critical stress. 

In concentrated suspensions, the motion of particles is cmcially affected by hydrodynamic interaction between neighbouring particles, which strongly depends on the interparticle distances, i.e. on the suspension structure (cf. Overbeck et al. 1999 Watzlawek and Nagele 1997, 1999). This structure is clearly influenced by the inteiparticle forces, in particular by the forces that occur when the EDL of two particles overlap (e.g. Russel 1978 Quemada and Berli 2002). When a suspension contains only a single particulate component, such a double layer overlap leads to repulsions and, thus, decreases the particle mobility and increases the suspension viscosity (Fig. 3.5). This effect is called secondary electroviscous effect. Its... 

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. 

The above-described interparticle interactions lead to formation of suspension structures at rest. The type of suspension structure formed depends on whether the interparticle forces are attractive or repulsive in nature. With strong repulsive interactions, solid crystaUme structures can be formed. The attractive interaction appears to be more common with paste materials. The flow behavior of the suspension is strongly affected by the nature of the suspension structure. The extreme cases are the formation of chainhke structures or formation of spherically shaped clusters of particles. The two shapes are the extreme simplifications of the real structures and are often used as structural models. The type of suspension structure developed depends on interparticle interactions, the shape and size of solid particles, solid surface characteristics, particle concentration, mixing conditions, shear history, etc. The basic flow units, called floes, are formed by random packing of primary particles. At low shear or at rest, the floes group into clusters of floes called aggregates, as shown in... 

Fig. 8.86. These aggregates may form a network which can fill the entire volume of the dispersion and control its plastic or structural properties. Another method to obtain the suspension structure is through the vehicle phase. In paste formulations, rheology additives (generally organic in nature) can form a gel type structure at low shear rates. The observed yield point and viscoelastic response of the materials are indicative of a suspension structure above a critical volume fraction of the sohd phase. ...

When a shear stress is applied, the suspension structure is broken down into smaller units. The vehicle entrapped within the floe aggregates is released. This leads to a decrease in viscosity with increasing shear rate. If the rate of structure breakdown due to shear is equal to the rate of structure buildup due to brownian motion, shear thinning or pseudoplastic flow behavior is observed. The equilibrium floe is generally spherical in shape and the size distribution is narrow. ... 

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