Couette-type device

Midler and Finn (1966) also studied disruption at relatively high shear rates in a Couette-type device. At shear rates below 1200sec only slight damage was observed. As shown in Fig. 12, data for two runs at the same viscosity of 69c.p. and shear rate of 1970sec", but at different gap widths, fall on the same line. Such consistency supports 2 ... 

When the concept of the mean shear rate predicted by Metzner (1961) was applied, an agitator speed of 700 rpm used in Midler s experiment should have an effea equivalent to only 120 see . However, comparing this with the results in the Couette type device, the equivalent shear rate in those turbulent conditions should be 10-100 times those predicted by Metzner. Therefore, Midler and Finn suggested that maximum rather than average shear stresses must be considered in the presence of high local velocity gradients. 

The shear mode of operation is the term generally given to the simple shearing of the fluid, as in a Couette rotational or parallel plate type of viscometer but with an electric field applied between the moving and the stationary electrodes of gap size h (Fig. 6.63). With zero voltage V = 0) applied, most ER fluids exhibit near-Newtonian properties. When an electric field E = V/h) is applied to the fluid, there is an increased resistance to its movement which must be overcome before motion can take place (see Fig. 6.64 which is an idealised representation). Conventional constant temperature O and speed lv Couette laboratory techniques can normally only encompass shear rates (7 = cuR/h) up to several hundred s although cooled purpose made industrial clutch-type devices of similar geometry may reach 6000 s. ... 

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 Couette rheometer. Another rheometer commonly used in industry is the concentric cylinder or Couette flow rheometer schematically depicted in Fig. 2.48. The torque, T, and rotational speed, 0, can easily be measured. The torque is related to the shear stress that acts on the inner cylinder wall and the rate of deformation in that region is related to the rotational speed. The type of flow present in a Couette device is analyzed in detail in Chapter 5. 

In a rotational viscometer the solution is filled in the annulus between two concentric cylinders of which either the external (Couette-Hatschek type) or the internal (Searle type) cylinder rotates and the other, which is connected to a torsion-measuring device, is kept in position. Let R, and Ro be the radii of the inner and outer cylinders, h the height of the cylinder which is immersed in the solution or its equivalent height if end effects are present, a> the angular velocity of the rotating cylinder, and T the torque (or moment of force) required to keep the velocity constant against the viscous resistance of the solution. It can be shown that the shearing stress (see, for example, Reiner, 1960) ... 

The flow-property or rheological constants of non-Newtonian fluids can be measured using pipe flow as discussed in Section 3.5E. Another, more important method for measuring flow properties is by use of a rotating concentric-cylinder viscometer. This was first described by Couette in 1890. In this device a concentric rotating cylinder (spindle) spins at a constant rotational speed inside another cylinder. Generally, there is a very small gap between the walls. This annulus is filled with the fluid. The torque needed to maintain this constant rotation rate of the inner spindle is measured by a torsion wire from which the spindle is suspended. A typical commercial instrument of this type is the Brookfield viscometer. Some types rotate the outer cylinder. 

There are two types of fluid flows that can be important in microfluidic devices. One type of flow is called Couette flow, which is the steady viscous flow between parallel plates, one of which is moving relative to the other, as shown in Figure 6.1. The velocity of the fluid varies linearly from zero at the stationary plate up to the velocity V at the moving plate. Another type of fluid flow is Poiseuille flow. This is a pressure-driven flow between stationary parallel plates, as shown in Figure 6.3. It has a parabolic variation of the pressure with the maximum flow velocity in the middle of the plates and zero flow velocity at the walls. 

Devices used for flow-induced crystallization (FIC) experiments are aU types of rheometers rotational plate/plate, cone/plate, Couette, sliding plates, capillary rheometers, including the multipass rheometer (MPR) [24-26], and shear devices in-house built [27], Linkam shear cell [28,29], fiber pull-out [30-34], FS, and complex flows and contraction/expansion and cross-slot [35-38]. 

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