Wall-slip effect measurement

Over the twentieth century, the mbber industry has developed special rheometers, essentially factory floor instmments either for checking process regularity or for quality control purposes, for instance, the well-known Mooney rheometer (1931), the oscillating disk rheometer (1962), and the rotorless rheometer (1976). All those instmments basically perform simple drag flow measurements but they share a common feature During the test, the sample is maintained in a closed cavity, under pressure, a practice intuitively considered essential for avoiding any wall slip effects. Indeed it has... 

Yoshimura, A. and Prud homme, R. K. 1988b. Wall slip effects on dynamic oscillatory measurements. J. Rheol. 32 575-584. 

Numerous workers have described the role of wall-slip effects on measurements made with conventional smooth-walled geometries [Barnes, 1995]. Slip can occur in suspensions at high (ca. 60%) solids volume fiaction, and can involve fluctuating torque in a rotational viscometer under steady... 

If it is known that a particular form of relation, such as the power-law model, is applicable, it is not necessary to maintain a constant shear rate. Thus, for instance, a capillary tube viscometer can be used for determination of the values of the two parameters in the model. In this case it is usually possible to allow for the effects of wall-slip by making measurements with tubes covering a range of bores and extrapolating the results to a tube of infinite diameter. Details of the method are given by Farooqi and Richardson. 21 ... 

In order to determine whether slip occurs with a particular material, it is essential to make measurements with tubes of various diameters. In equation 3.66, the value of the integral term is a function of the wall shear stress only. Thus, in the absence of wall slip, the flow characteristic 8 u/dt is a unique function of tw. However, if slip occurs, the term 8vjd will be different for different values of d, at the same value of tu., as shown in Figure 3.11. It is clear from equation 3.66 that for a given value of the slip velocity vs, the effect of slip is greater in tubes of smaller diameter. If the effect of slip is dominant, that is the bulk of the material experiences negligible shearing, then it can be seen from equation 3.66 that on a plot of... 

Having established that wall slip occurs but is not dominant, the procedure is to estimate the value of v, and hence calculate a corrected flow rate by subtracting the slip flow from the measured flow rate. In general it is found that the slip velocity increases with tw and decreases with d, although in some cases vs is independent of dt. Consequently, it can be seen from equation 3.66 that the effect of slip decreases as d, increases, and becomes negligible at very large diameters. 

To date, most wall-slip studies have concentrated on either eliminating or quantifying slip effects in laboratory rheometry. Slip effects have been traditionally treated as flow anomalies, which make the measurement of material properties a challenging task. There is less evidence on how to utilize this flow anomaly to industry s advantage. A comprehensive understanding of the slip phenomenon as it relates to food texture and food processing is of crucial importance to the food industry, where the goal is to produce quality products at reduced cost... 

The concentric cylinder viscometers are supplied with different inner and outer cylinders such that various gap widths can be formed. For rheological measurements of emulsions and suspensions, care must be taken to ensure a gap width of at least 20 times the suspended particle size in order to avoid wall effects. Moreover, experiments should be conducted with different gap widths to ensure the absence of any wall slip that is usually encountered in emulsion viscosity measurements (J6). However, uniformity of shear rate can be achieved only when the ratio of the gap width to the inner cylinder radius is small. 

Other errors, which could influence the results obtained, are, for example, wall effects ( slipping ), the dissipation of heat, and the increase in temperature due to shear. In a tube, the viscosity of a flowing medium is less near the tube walls compared to the center. This is due to the occurrence of shear stress and wall friction and has to be minimized by the correct choice of the tube diameter. In most cases, an increase in tube diameter reduces the influence of wall slip on the flow rate measured, but for Newtonian materials of low viscosity, a large tube diameter could be the cause of turbulent flow. ° When investigating suspensions with tube viscometers, constrictions can lead to inhomogeneous particle distributions and blockage. Due to the influence of temperature on viscosity (see Section Influence Factors on the Viscosity ), heat dissipated must be removed instantaneously, and temperature increase due to shear must be prevented under all circumstances. This is mainly a constructional problem of rheometers. Technically, the problem is easier to control in tube rheometers than in rotating instruments, in particular, the concentric cylinder viscometers. ... 

A suspension is a dispersion of particles within a solvent (usually a low-molar-mass liquid). Thermodynamics (Brownian motion and collisions) favours the clumping of small particles, and this can be increased by flow. However, particles over 1 pm tend to settle under gravity, unless stability measures have been considered (matching the density of the particle to that of the medium, increasing the Brownian/gravitational force ratio, electrostatic stabilization, steric stabilization). Other complications can occur in the dynamics of suspensions, such as particle migration across streamlines, particle inertial effects and wall slip (Larson, 1999). 

Choi et al. [1] examined the apparent slip effects of water in hydrophobic and hydrophilic microchannels experimentally using precision measurements of flow rate versus pressure drop. They correlated their experimental results to that from analytical solution of flow through a channel with slip velocity at the wall. There was clear difference between the flows of water on a hydrophilic and hydrophobic surface indicating the effect of slip flow (Fig. 2). Neto et al. [3] have reported clear evidence of boundary slip for a sphere-flat geometry from force measurements using atomic force microscopy. The degree of slip is observed to be the function of both liquid viscosity and shear rate (Fig. 4). 

Figure 8 summarises the key steps to ensuring the measurements of the flow curve are both accurate and relevant (Alderman and Heywood, 2004a 2004b).The need for accuracy may sometimes introduce additional laboratory experiments, particularly if end effect and/ or wall slip errors are incurred. However, ensuring that only the viscometric data that are relevant to the application of interest are measured will minimise the overall effort. Having selected the most appropriate viscometer for the sample under test and the application of interest, the next step is to generate the flow curve. 

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