Disperse systems compact

Non-dispersive systems are very compact, and potentially less expensive than dispersive spectrometers. For this reason they may be used for multi-channel instruments, measuring upto six elements almost simultaneously.39,40 Although... 

The concept of the free volume of disperse systems can also be correlated with the change in the structure of the composite of the type solid particles — liquid — gas during its compaction. In that case the value of the maximum packing fraction of filler (p in Eq. (80b) remains valid also for systems containing air inclusions, and instead of the value of the volume fraction of filler, characteristic for a solid particles — liquid dispersion-system solid particles — liquid — gas should be substituted. This value can be calculated as follows the ratio of concentrations Cs x g/Cs, to the first approximation can be substituted by the ratio of the densities of uncompacted and compacted composites, i.e. by parameter Kp. Then Eq. (80b) in view of Eq. (88), for uncompacted composites acquires the form ... 

Fig. 4. Laser diffraction results of disagglomeration processes after adding compact particles to a stabilized dispersed system (a) and after adding fumed silica suspension (b).

Disperse systems consist normally of two or more phases in which the continuous phases are intermixed. If, in a continuous phase (the dispersion medium), the elements of the disperse matter are embedded such that they can be individually distinguished, the system is called discretely disperse, A coherent disperse phase, which may also consist of well-defined elements adhering to or intermixed with each other, is called compact disperse. 

Many properties of disperse systems are related to the distribution of charges in the vicinity of the interface due to the adsorption of electrolytes. The adsorption of molecules is driven by the van der Waals attraction, while the driving force for the adsorption of electrolytes is the longer-range electrostatic (Coulomb) interaction. Because of this, the adsorption layers in the latter case are less compact than in the case of molecular adsorption (i.e., they are somewhat extended into the bulk of the solution), and the discontinuity surface acquires noticeable, and sometimes even macroscopic thickness. This diffuse nature of the ionized adsorption layer is responsible for such important features of disperse systems as the appearance of electrokinetic phenomena (see Chapter V) and colloid stability (Chapters VII, VIII). Another peculiar feature of the adsorption phenomena in electrolyte solutions is the competitive nature of the adsorption in addition to the solvent there are at least two types of ions (even three or four, if one considers the dissociation of the solvent) present in the system. Competition between these ions predetermines the structure of the discontinuity surface in such systems -i.e. the formation of spatial charge distribution, which is referred to as the electrical double layer (EDL). The structure and theory of the electrical double layer is described in detail in textbooks on electrochemistry. Below we will primarily focus on those features of the EDL, which are important in colloid... 

Earlier in this chapter, when we discussed mechanical properties of disperse systems that revealed viscoplasic flow, we focused on the applied shear stress and the values of G, rj, t related to it. The strength in such systems was identical to the critical shear stress. In the case of mechanical behavior of compact and primarily elasto-brittle solids it is more appropriate for one to use the uniaxial tension, thus replacing shear stress, t, with tensile stress, p the shear modulus, G, with the Young modulus, E, and using the rupture resistance, Pa, as the strength characteristic (instead of f )... 

As much as for compact tubular turbulent apparatus of cylindrical design (dd/dc=l, Ls / dd 40) in canals of divergent-convergent design under two immiscible liquid flows movement rate w distribution of disperse phase drops by sizes converges and curves are shifted to the region of homogeneous thin-disperse systems formation. 

Agglomeration (UNIT OPERATION) generic term in process engineering for all unit operations which enlarge the particle size of a dispersed system by the creation of particle — agglomerates—e.g. spray drying, granulation in dmms, and compaction of powders. 

The value of A F is positive for coarse dispersions (with respect to the initial compact phase) and drops sharply as the size of the separated particles decreases. The transition from positive to negative values of AF constitutes a necessary condition for the thermodynamically favorable formation of a dispersed system consisting of fine particles. Consequently, the AF = 0 represents a critical condition for spontaneous dispersion, known as the Rehbinder-Shchukin criterion ... 

In Section 3.1, we examined the mechanical properties of disperse systems that were capable of undergoing viscoplastic flow. In these cases, we considered the stressed state of shear with its characteristic parameters G, ti, and t. The strength of such systems could be characterized by the yield point. When we shift to describing the mechanical behavior of compact and primarily elastic-brittle solids, it is worth using the stressed state of a uniaxial extension in which we replace the shear stress, T, with the extension stress, / the shear modulus G with Young s modulus, E and the resistance to tear, P, with the yield stress, t, as the strength characteristic. 

This description holds true for disperse systems of the globular type, in which a continuous backbone is formed due to the cohesion of the individual particles in the course of transformation of a free disperse system into a connected disperse system. In such systems, the backbone formed is the main carrier of the strength. At the same time, there are also other types of systems, for example, those with a cellular structure (solidified foams or emulsions). Such structures are typical in polymeric systems and may form in the course of new phase formations by condensation in mixtures of polymers. An individual approach also needs to be employed in the description of the mechanical properties of structures with anisometric particles, due to the specifics of the cohesive forces in such systems. In addition to porous structures, we also consider various compact microheterogeneous structures, such as mineral rocks, modern composite materials, and natural materials such as bone and wood. 

A change in the properties of the suspension, in particular the turbidity of the water flow [27], and also the rheology of cohesive dispersed systems, are due to forces of autohesion. However, other still insufficiently understood factors also affect these processes. Hence, Kurgaev s attempt to associate autohesion with the rate of compaction of the residues of certain suspensions cannot be considered successful or his calculations of the forces of autohesion reliable [29]. 

The fuel for the Peach Bottom reactor consisted of a uranium-thorium dicarbide kernel, overcoated with pyrolytic carbon and silicon carbide which were dispersed in carbon compacts (see Section 5), and encased in graphite sleeves [37]. There were 804 fuel elements oriented vertically in the reactor core. Helium coolant flowed upward through the tricusp-shaped coolant channels between the fuel elements. A small helium purge stream was diverted through the top of each element and flowed downward through the element to purge any fission products leaking from the fuel compacts to the helium purification system. The Peach... 

Stability may be inherent or induced. In the latter case, the original system is in a condition of metastable or neutral eouilibrium. External influences which induce instability in a dispersion on standing are changes in temperature, volume, concentration, chemical composition, and sediment volume. Applied external influences consist of shear, introduction of a third component, and compaction of the sediment. Interfacial energy between solid and liquid must be minimized, if a dispersion is to be truly stable. Two complementary stabilizing techniques are ionic and steric protection of the dispersed phase. The most fruitful approach to the prediction of physical stability is by electrical methods. Sediment volumes bear a close relation to repulsion of particles for each other. 

The presence of the additive results in the formation of a homogeneous structure of the plugging rock, with an improved uniformity of the phase composition of the system and a more compact distribution of the dispersed particles. An increased strength of the cement rock is also obtained. 

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