Sedimentation speed

This brief section provides a historical and practical overview of useful empiricisms employed in suspension theories, including a few useful formulas. Early investigators were mainly concerned with the measurement and correlation of two fundamentally important, but apparently unrelated, quantities (i) The effective viscosity fi of sheared suspensions of neutrally buoyant particles and (ii) the sedimentation speed us of suspensions of non-neutrally buoyant particles. Upon appropriate normalization, both were regarded as being functions only of the volumetric solids concentration ... 

To obtain reliable experimental data and to correctly interpret them, we used such physicochemical and analytical techniques as dilatometry, viscometiy, UV and IR spectroscopy, electroiuc paramagnetic resonance, Raman light scattering spectroscopy, electron microscopy, and gas-liquid chromatography. To analyze the properties of polymeric dispersions, the turbidity spectrum method was used, and the efficiency of flocculants was estimated gravimetrically and by the sedimentation speed of special water-suspended imitators (e.g. copper oxide). 

As an example, consider small spheres of density 2 x 10 kg/m dispersed in water (so that Ap = 10 kg/m ), at room temperature (T = 300 K), in a container of height 10 cm H = 0.1 m). The ratio between the particle concentration near the surface and that near the bottom of the container is of order zero for particles of diameter 100 nm (eventual sedimentation), whereas it is 0.88 for particles of diameter 10 nm (almost no sedimentation occurs). This clearly illustrates the effect of particle size on phase separation, but it gives no idea of the rate at which it happens. The sedimentation speed can easily be estimated by formulating the fact that it is the limiting speed at which the force of gravity is balanced by frictional forces with the suspension fluid (Stokes law) ... 

The correlation between the terminal velocity of a sphere F) and the sedimentation speed is correlated to the void fraction e (Cheremisinoff, 1984) by the following equation ... 

Under the transmission electron microscope, the interior of the bacterial cell appears granular. This is due to the ribosomes, which are essential players in protein synthesis. They ensure, along with the t-RNA, the translation of the genetic code. The ribosomes consist of two parts characterized by their sedimentation speed, expressed in Svedberg values (S). These two sub-units are different in size 30 S and 50 S in procaryotes. The assembled ribosome has a sedimentation constant of 70 S. The 30 S sub-unit contains a 16 S ribosomal RNA molecule (1542 nucleotides) and 21 different protein molecules its molecular mass is 900 KDa. The larger 50 S sub-unit contains two ribosomal... 

Fig. 11.15. Effect of pectolytic enzymes on the sedimentation speed of white must lees (Canal-Llaub res, 1989)...

The quantity of lees formed during juice extraction and the speed of sedimentation speed depend on variety, grape disease status, maturity and especially winemaking methods (crushing, draining, pressing, etc.) (Section 13.3). 

Obviously there is no linear correlation between solid content, sedimentation speed, and viscosity From the results, it can be assumed that slowly increasing the solid content results in an increased viscosity and parallel increased sedimentation speed because of, for example, the binding of particles to flocks. If the solid content... 

Fig. 11.20 Change of viscosity and sedimentation speed at increased solid content...

Furthermore, mixtures of the described ptimaiy particle batches were generated at various relations For Mix 1 (Table 11.10) the particle charges were mixed in a weight ratio of 1.7. The mix of coarse and fine particles results in average values regarding viscosity, dynamic mobility, and sedimentation speed. So a direct dependence of these characteristics from particle size and therewith available surface area can be assumed. 

The piimaiy particles within a suspension containing 40 wt% of AI2O3 fine particles and a polyvinyl alcohol binder Mowiol 4-88 were dispersed using an electrosteric sodium polyacrylate dispersant with a chain length of 8000 g/mol NaPA 8000. The dispersant addition leads to a slight reduction of the viscosity of the suspension. The pH value is slightly increased at constant suspension density. As already described in literature [26], the additirm of the dispersant results in an increased dynamic mobility of the primary particles. The sedimentation speed is reduced. 

The variation of additives showed no defined impact on the determined suspension densities or pH values. Opposite to that, the varied additive types showed an effect on the resulting suspension sedimentation speed, viscosity, and dynamic mobility of the primary particles (Table 11.17). 

The addition of 3 wt% Tylose H15YG4 leads to a very low sedimentation speed, correspondingly a very low viscosity is expected. The measured viscosity of 56.3 mPa s was the second highest value of all investigated suspensions — a contradiction. The highest viscosity was determined for Duramax BIOOO even... 

For the investigation of the changed suspensitm properties because of varied additive amount, suspensions of comparable solid content (40 wt%), primary particle size, and additive type polyvinyl alcohol Mowiol 4-88 were compared (Table 11.19). The stepwise increase of the additive amount leads to a slightly decreasing pH value. The viscosity increases while the sedimentation speed decreases. This is confirmed by the determined decreasing dynamic mobility values. An explanation can be found in the increasing amount of polymer molecules... 

From the studied variations of suspension formulation it became clear that for suspensions containing a liquid medium (water), a disperse phase (particles), and eventually a dispersing agent the known correlations between dynamic mobility, sedimentation speed, and viscosity are valid. An increased viscosity is a result of reduced dynamic mobility. Parallel the sedimentation speed is increased. This leads to the formation of granules with higher sheU thickness, the amotmt of homogeneous granules increases. 

Particles in suspension in a liqnid medium are subjected to three kinds of forces (a) gravitation forces the particles to faU down, (b) viscosity of the liquid decreases the speed of their displacement and (c) Archimedes force is opposed to gravitation forces in this case. By applying fundamental relation of dynamics, the expression of the steady state sedimentation speed... 

The results show the advantage of using smaller IF-MS2 particles instead of the corresponding 2H structure. The sedimentation speed is much higher in the case of the lamellar structure (which is a few tenths of a micrometre in size). Thus, calculations predict much better stability of IF suspensions, the distance due to sedimentation being lower than that of the Brownian motion. However, flocculation of particles could explain the poor stability of the suspensions because of the absence of any polar molecules. Calculations show that an aggregate of particles with a diameter of 650 nm will cover about the same distance by sedimentation and by Brownian motion. 

The rotor of an ultracentrifuge has a sample cell with a transparent top and bottom so that a beam of hght can pass through the cell each time it passes the location of the beam. The value of r, the distance of the molecules from the axis of rotation, can be measured by observing the position dependence of the index of refraction, which depends on composition. After the rotor has been spinning a short time, the sedimentation speed Used = dr/dt will attain a steady value so that the frictional force provides the centripetal force given by Eq. (10.4-8). 

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