Double Couette

Three common designs for eliminating end effects in concentric cylinder rheometers (a) conicylinder, (b) recessed bottom, and (c) double Couette. 

Rotating cone viscometers are among the most commonly used rheometry devices. These instruments essentially consist of a steel cone which rotates in a chamber filled with the fluid generating a Couette flow regime. Based on the same fundamental concept various types of single and double cone devices are developed. The schematic diagram of a double cone viscometer is shown in... 

The premixed double emulsion is sheared in the Couette cell at different shear rates from 0 to 14200 s 1. The obtained double emulsions have diameters ranging from 7 pm to 2 pm and uniformity between 15% and 30%. Figure 15 is a mi-... 

The preparation of a ferrofluid emulsions is quite similar to that described for double emulsions. The starting material is ferrofluid oil made of small iron oxide grains (Fe203) with a typical size of 10 nm, dispersed in oil in presence of an oil-soluble surfactant. The preparation of ferrofluid oils was initially described in a US patent [39]. Once fabricated, the ferrofluid oil is in turn emulsified in a water phase containing a hydrophilic surfactant. The viscosity ratio between the dispersed and continuous phases is adjusted in order to lie in the range where monodisperse fragmentation occurs [0.01-2]. The emulsification takes place in a Couette-type cell, leading to direct emulsions with a typical diameter around 200 nm and a very narrow size distribution as can be observed in Fig. 17. 

Dynamic crystallization under precise control of shear and temperature was studied in a prototype apparatus specially developed. The whole cell, similar to a Couette viscometer, was made out of glass. The inner cylinder rotated at a controlled speed, CO, while the outer wall was fixed. A double-mantel with a circulation of water allowed precise control of the temperature. Temperature of cocoa butter in the cell was measured with a chromel-alumel thermocouple. Measures were recorded with a data acquisition system. Shear rate imposed to cocoa butter in the system could be estimated from the rotation speed of the inner cylinder, assuming that the fluid is Newtonian and incompressible. There is no normal speed, only tangential speed. The shear rate, y> in the specimen is a single function of the radiu. In the rest of this work, shear in the cell was characterized by its average value, y, calculated by integration over the cell thickness (7). 

Mason et al. (136) determined the yield stresses and yield strains of a series of monodisperse emulsions, using either a cone-and-plate or double-wall Couette geometry in oscillatory mode. Wall-induced coales-cence and wall slip were claimed to be absent, but no mention is made of attempts to reduce end or edge effects. Estimated film thicknesses were used to arrive at the effective volume fractions. Their data for the yield stress could be fit to... 

Figure 11 Schemes of the different geometries that can be used for flow or viscoelastic rheological measurements. From left to right double cylinders or Couette geometry, cone and plate geometry, and parallel plates geometry.

Two different reactions have presently been studied in the Couette flow reactor, namely the variants of the Belousov-Zhabotinsky [27-30, 32] and chlorite-iodide [29-33] reactions. The BZ reaction has revealed a rich variety of steady, periodic, quasi-periodic, frequency-locked, period-doubled and chaotic spatio-temporal patterns [27, 28], well described in terms of the diffusive coupling of oscillating reactor cells, the frequency of which changes continuously along the Couette reactor as the result of the imposed spatial gradient of constraints. This experimental observation has been successfully simulated with a schematic model of the BZ kinetics [68] and the recorded bifurcation sequences of patterns resemble those obtained when coupling two nonlinear oscillators. 

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