Couette flow technique

The above emulsification methods (perhaps except the Couette flow technique) have as a common feature that the final DSD is primarily determined by the interaction of turbulent eddies with interfaces. Note, however, that turbulence is hard to control and to maintain consistently throughout the whole reactor volume. From a practical point of view, it is almost impossible to predict the DSD after a scale-up based on laboratory-scale experiments. Emulsification techniques based on other principles are necessary to overcome these drawbacks. An alternative technique is the so-called membrane emulsification method where the liquid forming the disperse phase is pressed through a porous membrane. The other side of the membrane where the droplets are formed is in contact with the continuous phase. This concept is simple and it is assumed to be superior to the above techniques (35). The basic relationship of membrane emulsification (equation (8.10)) correlates the trans-membrane pressure required to start the drop-wise flow through the pores (ft) with the average pore diameter of the membrane (Dm) with being the contact angle of the mixture with the wall of the pore ... 

Critical values for Weber numbers at which droplet break-up occurs have been determined using the moving belt, roller and Couette flow techniques (Chapter 2). 

Under steady-state conditions, as in the Couette flow, the strain rate is constant over the reaction volume for a long period of time (several hours) and the system of Eq. (87) could be solved exactly with the matrix technique developed by Basedow et al. [153], Transient elongational flow, on the other hand, has two distinctive features, i.e. a short residence time (a few ps) and a non-uniform flow field, which must be incorporated into the kinetics equations. In transient elongational flow, each rate constant is a strongfunction of the strain-rate which varies with time in the Lagrangian frame moving with the center of mass of the macromolecule the local value of the strain rate for each spatial coordinate must be known before Eq. (87) can be solved. 

The results presented here illustrate the general feasibility of this technique. They relate primarily to the behavior of thermal and current noise in the glass transition (Tg) or melting (Tm) region of an amorphous (polystyrene) and a crystalline (HD-polyethylene (HDPE)) polymer rendered conductive by the addition of minor amounts of carbon black, and further they relate to the noise of aqueous solutions of certain polymers during Couette flow. Because of experimental diflBculties, noise measurements on solid polymers during deformation and flow have not yet produced useful results. 

Fig. 4.4.2 The discrete data points represent Taylor-Couette-Poiseuille flow regimes observed with MRI for r = 0.5 [41]. The curved boundaries were obtained for r = 0.77 with optical techniques [38]. The two inserts show MRI spin-tagging FLASH images of the SHV and PTV hydrodynamic modes.

MRI has been used as a non-invasive quantitative visualization technique to investigate a class of complex Taylor-Couette-Poiseuille (TCP) flows, which constitute a prototype of many mixing or fractionation processes. Here we focused on the vicinity of the Stationary Helical Vortex (SHV) regime characterized by a... 

Measurement of the flow properties of non-Newtonian fluids is typically accomplished via rotational techniques. The rotational methods fall into two basic types concentric cylinder and cone and plate rheometers. In a concentric cylinder rheometer, a bob is placed inside a cylinder so that the fluid to be studied may be placed into the gap between the cyhnders. This arrangement helps approximate a uniform shear rate throughout a sample by shearing only a thin film of sample fluid between the cyhnders. In the Searle method, the inner cylinder is rotated so that interchangeable bobs can be used (to vary the gap). The outer cylinder does not rotate making it easier to apply a temperature control jacket. In the Couette method, the outer cylinder turns. In each case, the torque on the inner bob is measured. 

Barrat and Bocquet [6] reported slip in Couette and Poiseuille flows using molecular dynamics simulation (Fig. 7). Tretheway and Meinhart [4] reported micron-resolution velocity profile in hydrophilic and hydrophobic microchannels of cross section 30 x 300 pm using the p-PIV technique (Fig. 5a). Their results showed significant fluid velocity near a hydrophobic (octadecyltrichlorosilane or... 

Many other techniques exist to determine the melt viscosity (log < 5) and are extensively used by polymer scientists sliding plate, poiseuille, rotational plates, dynamic shear, couette, cone-plate, uniaxial flow, etc. These were thoroughly reviewed in several rheology textbooks, see for instance [8] and will not be detailed here. In this chapter we will describe only the main techniques used to determine viscosities in the upper range (14 < log rj < 5) which have received much less coverage in the literature. 

Numerous methods for measuring fluid viscosity exist, for example, capillary tube flow methods (Ostwald viscometer), Zahn cup method, falling sphere methods, vibrational methods, and rotational methods. Rotational viscometers measure the torque required to turn an object immersed or in contact with a fluid this torque is related to the fluid s viscosity. A well-known example of this type of system is the Couette viscometer. However, it should be noted that as some CMP slurries may be non-Newtonian fluids, the viscosity may be a function of the rotation rate (shear rate). An example of this is the dilatant behavior (increasing viscosity unda increasing shear) of precipitated slurries that have symmetrical particles [33]. Furthermore, the CMP polisher can be thought of as a large rotational plate viscometer where shear rates can exceed 10 s and possibly affect changes to the apparoit viscosity. The reader can refer to the comprehensive review of viscosity measurement techniques in the book by Viswanath et aL [34]. 

Reactor configurations involved in continuous emulsion polymerization include stirred tank reactors, tubular reactors, pulsed packed reactors, Couett-Taylor vortex flow reactors, and a variety of combinations of these reactors. Some important operational techniques developed for continuous emulsion polymerization are the prereactor concept, start-up strategy, split feed method, and so on. The fundamental principles behind the continuous emulsion polymerizations carried out in the basic stirred tank reactor and tubular reactor, which serve as the building blocks for the reaction systems of commercial importance, are the major focus of this chapter. 

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