Viscometer capillary flow type

In practical applications, flow of the material through an orifice is perhaps the most frequently encountered rheological phenomenon. It is then natural to be used for the viscosity measurement of suspensions (53-55). However, the flow through an orifice is not precise in terms of shear measurement because the shear rate is not well defined under such circumstances. To meet this objection, the orifice is in most cases extended to a tube. This leads to the capillary flow type of viscometers, the simplest, and for Newtonian fluids, the most accurate type, comprising the familiar Ostwald und Ubbelohde viscometers. The fully developed axial velocity in the laminar regime is given by... 

Viscometers n. Instruments for measuring viscosity including mechanical probe and torque types as the Brookfield viscometer, capillary tube types as the Cannon-Fenske or Ostwald-Fenske, and flow through orifice types as the Ford cup. 

Shear Viscosity. There are three main types of viscometers the rotational type, the flow-through-constriction type, and the flow-around-obstruction type. The concentric cylinder and the cone-and-plate are the two primary classes of rotational viscometers. The capillary tube is an example of the flow-through-constriction viscometers. Falling-ball or falling-needle viscometers are examples of the flow-around-obstruction type. 

It should, however, be noted that there exist some hints that bicontinuous microemulsions behave elastically. This has been assumed to be due to the differences observed in measuring viscosities once in a Couette flow and in the other case by a capillary viscometer. Here it was observed that the values obtained with the capillary viscometer are markedly higher. It has been suggested that in capillary flow a component of elongational flow is observed and that in this type of flow elastic components can be observed much earlier than in shear flow [106,107]. 

The hyaluronidase-induced fall in the viscosity of hyaluronic acid is usually determined in a capillary flow viscometer of the Ostwald type and is expressed in various terms. For the sake of clarity, it may be recalled that the relative viscosity, is the ratio of the flow time of the solution to the flow time of the solvent, although it is occasionally referred to the flow time of water. The specific viscosity, i peo.v is 1, also called the viscosity increment. The reduced viscosity is rei./c, where c is the concentration in grams per 100 milliliters. The extrapolation of the reduced viscosity for c = 0 is called the intrinsic viscosity [ ] => lim Vapeejc. The... 

The Cannon-Fenske viscometer (Fig. 24b) is excellent for general use. A long capillary and small upper reservoir result in a small kinetic energy correction the large diameter of the lower reservoir minimises head errors. Because the upper and lower bulbs He on the same vertical axis, variations in the head are minimal even if the viscometer is used in positions that are not perfecdy vertical. A reverse-flow Cannon-Fen ske viscometer is used for opaque hquids. In this type of viscometer the Hquid flows upward past the timing marks, rather than downward as in the normal direct-flow instmment. Thus the position of the meniscus is not obscured by the film of Hquid on the glass wall. 

In shear studies, the most commonly used type of device for the generation of well-defined flow fields is the rotational viscometer. The use of these devices for the rheological characterization of liquids is well established [137]. Compared with the capillary and jet devices (Sects. 5.1 and 5.2), rotational viscometers allow the investigation of the effects of continuous rather than intermittent shearing. 

A Cannon-Fenske viscometer is a capillary type of viscometer. It utilizes the flow through a capillary tube as a means of measuring viscosity. 

Two main types of viscometers are suitable for the determination of the viscosity of a polymer melt The rotation viscometer (Couette viscometer, cone-plate viscometer) and the capillary viscometer or capillary extrusiometer. The latter are especially suitable for laboratory use since they are relatively easy to handle and are also applicable in the case of high shear rates. With the capillary extrusiometer the measure of fluidity is not expressed in terms of the melt viscosity q but as the amount of material extruded in a given time (10 min). The amount of ex-trudate per unit of time is called the melt index or melt flow index i (MFI). It is also necessary to specify the temperature and the shearing stress or load. Thus MFI/2 (190 °C)=9.2 g/10 min means that at 190 °C and 2 kg load, 9.2 g of poly-... 

Earlier experiments involved the collection of SEC effluent aliquots to measure solution viscosity in batches with the very time consuming Ubbelohde drop-time type viscometers. A continuous capillary type viscometer was first proposed for SEC by Ouano. Basically, as shown in Figure 1, a single capillary tube with a differential pressure transducer was used to monitor the viscosity of SEC effluent at the exit of the SEC column. As liquid continuously flows through the capillary (but not through the pressure transducer), the detected pressure drop (AP) across the capillary provides the measure for the fluid viscosity (h) according to the Poiseuille s viscosity law ... 

Fig. 17.1 Two types of viscometers Ubbelohde (left) and Cannon-Fenske (right). The Ubbelodhe viscometer has the following components (1) fill tube, (2) capillary outlet, (3) pressure relief tube, (4) solution bulb, (5) suspended volume bulb, (6) lower flow bulb, (7) upper flow bulb, (8) upper timing mark, and (9) lower timing mark. The Cannon-Fenske Viscometer has the following components (1) fill tube, (2) capillary outlet tube, (3) solution bulb, (4) lower flow bulb, (5) upper flow bulb, (6) upper timing mark, and (7) lower timing mark.

Since fluid shear rates vary enormously across the radius of a capillary tube, this type of instrument is perhaps not well suited to the quantitative study of thixotropy. For this purpose, rotational instruments with a very small clearance between the cup and bob are usually excellent. They enable the determination of hysteresis loops on a shear-stress-shear-rate diagram, the shapes of which may be taken as quantitative measures of the degree of thixotropy (G3). Since the applicability of such loops to equipment design has not yet been shown, and since even their theoretical value is disputed by other rheologists (L4), they are not discussed here. These factors tend to indicate that the experimental study of flow of thixotropic materials in pipes might constitute the most direct approach to this problem, since theoretical work on thixotropy appears to be reasonably far from application. Preliminary estimates of the experimental approach may be taken from the one paper available on flow of thixotropic fluids in pipes (A4). In addition, a recent contribution by Schultz-Grunow (S6) has presented an empirical procedure for correlation of unsteady state flow phenomena in rotational viscometers which can perhaps be extended to this problem in pipe lines. 

The technique involving an evacuated capillary viscometer is limited to the measurement of polymer solutions having viscosities less than ca. 103 poise a point which has been made previously by Hadjichristidis and Roovers 132). This is a consequence of the viscometer type (Ubbelohde) which has been used in these determinations. Despite this practical limitation, it has been stated 78) that there is no inherent limit for viscosity measurements, even in conventional types of viscometers provided that the tubes are sufficiently wide. However, the limitations of operating with an evacuated viscometer, and the flow behavior of high viscosity (> 103 poise) polymer solutions, clearly reveals that the foregoing claim is unrelated to reality. 

In addition to high-speed photography, which was used to obtain the results discussed above, other methods have been used to visualize flow, for example, the tracer method.279"281 A typical experimental device consists of a capillary viscometer connected to a mold, which is a simple model of plunger-type injection molding unit.279 A special device was used to introduce tracers of different colors in regular succession. After sample solidification, it is possible to examine the position of the various colors on the surface (Fig. 4.47 a) or through the volume by cutting sections (Fig. 4.47 b). It can be seen that the tracers are positioned symmetrically near the surfaces and that they have a V-type shape, with the tip oriented toward the stream. 

Since pressure driven viscometers employ non-homogeneous flows, they can only measure steady shear functions such as viscosity, 77(7). However, they are widely used because they are relatively inexpensive to build and simple to operate. Despite their simplicity, long capillary viscometers give the most accurate viscosity data available. Another major advantage is that the capillary rheometer has no free surfaces in the test region, unlike other types of rheometers such as the cone and plate rheometers, which we will discuss in the next section. When the strain rate dependent viscosity of polymer melts is measured, capillary rheometers may provide the only satisfactory method of obtaining such data at shear rates... 

Determine the viscosity in capillary tubes by measuring the amount of time it takes for a given volume of liquid to flow through a calibrated capillary tube. Calibrate the capillary tube by using liquids of known viscosity. The calibration may be supplied with the viscometer tube when purchased along with specific instructions for its use. Many types of capillary viscometer tubes are available, and exact procedures will vary... 

A variety of laboratory instruments have been used to measure the viscosity of polymer melts and solutions. The most common types are the coaxial cylinder, cone-and-plate, and capillary viscometers. Figure 11 -28 shows a typical flow curve for a thermoplastic melt of a moderate molecular weight polymer, along with representative shear rate ranges for cone-and-plate and capillary rheometers. The last viscometer type, which bears a superficial resemblance to the orifice in an extruder or injection molder, is the most widely used and will be the only type considered in this nonspecialized text. 

---