Wall-slip effect elimination

Princen [57, 64, 82] and others [84] also noted the presence of wall-slip in rheological experiments on HIPEs and foams. However, instead of attempting to eliminate this phenomenon, Princen [64] employed it to examine the flow properties of the boundary layer between the bulk emulsion and the container walls, and demonstrated the existence of a wall-slip yield stress, below that of the bulk emulsion. This was attributed to roughness of the viscometer walls. Princen and Kiss [57], and others [85], have also showed that wall-slip could be eliminated, up to a certain finite stress value, by roughening the walls of the viscometer. Alternatively [82, 86], it was demonstrated that wall-slip can be corrected for and effectively removed from calculations. Thus, viscometers with smooth walls can be used. This is preferable, as the degree of roughness required to completely eradicate wall-slip is difficult to determine. 

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... 

Using their modified concentric-cylinder viscometer -equipped in this case with polished glass inner and outer cylinders to allow unimpeded wall slip, and a mercury pool to eliminate the lower end effect -Princen and Kiss (126) determined the yield stresses, tq, and effective viscosities. 

The effects of the Reynolds number on the extrusion of Newtonian fluid from square and rectangular dies has been considered. As with planar and axisymmetric jets, extrudates from three-dimensional dies swell at low Reynolds numbers but contract at high ones. Depending on its aspect ratio, limiting die swell from the rectangle varies that of the square (0.7255) and that of two-dimensional planar case (0.8333). Wall slip reduces die swell and in the cases of perfect slip, completely eliminates it. 

How do we eliminate these wall-depletion/slip effects The answer is to take the viscometer wall motion into the bulk of the liquid, which is done most easily by either roughening or profiling the wall. Sandblasting with a coarse grit should be enough, since this results in approximate 10 - 20 micron surface variations. However, most rheometry manufacturers supply grooved geometries. 

Many different techniques have been used to either minimize or eliminate this effect. One technique involves the use of a mixing mechanism in the slip reservoir. Another feeds the slip streams through a screen mesh. (This usually only breaks the flow into much smaller streams and produces streaks that are much closer together and harder to see.) The best technique is to use a feed system that does not introduce the problem in the first place. The slip is fed to the reservoir under the surface, not on the top surface. This has been accomplished by the use of a reservoir system like that shown in Figure 4.3. This reservoir also uses a "weir feed system, in which the slip must flow from the first chamber over a retaining wall into the main feed chamber of the doctor blade assembly. The flow down the retaining wall is similar to a waterfall and provides a uniform and... 

Nunez et al. (1996) indicate that the migration of viscous droplets from the wall in an emulsion exhibit the Segre-Silberberg effect. However, they pointed out that not all emulsions experience sUp and that the phenomena of slip appeared to be characteristic of the crude oil or viscous component. In some respects, slip does reduce the friction factor by lubrication from the least viscous phase. However, Aral and Kaylon (1994) found that increasing the surface roughness tended to reduce or eliminate slip. 

Since the slip of particle appears during the impact to the wall, a negative distribution function / i is introduced to eliminate the effect of sUp. The accuracy of simulation by this method is better than that by bounce-back. 

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