Viscosity slip characteristics

For practical purposes, casting slips are often characterized by so-called apparent viscosity. This characteristic is defined for any point on the rheological curve (Fig. 155) as a ratio of shear stress to the deformation rate at the given point. The inadequacy of this characteristic is demonstrated by Fig. 155 a dilatant and a pseudoplastic mix have the same apparent viscosity at a stress corresponding to the intersection of their rheological curves, despite their quite diverse rheological behaviours. However, determination of apparent viscosity may be useful, for example in routine qualitative inspection. ... 

Plain slideways are preferred in the majority of applications. Only a thin film of lubricant is present, so its properties - especially its viscosity, adhesion and extreme-pressure characteristics - are of vital importance. If lubrication breaks down intermittently, a condition is created known as stick-slip , which affects surface finish, causes vibration and chatter and makes close limits difficult to hold. Special adhesive additives are incorporated into the lubricant to provide good bonding of the oil film to the sliding surfaces, which helps to overcome the problems of table and slideway lubrication. On long traverses, oil may be fed through grooves in the underside of the slideway. 

The fact that the appearance of a wall slip at sufficiently high shear rates is a property inwardly inherent in filled polymers or an external manifestation of these properties may be discussed, but obviously, the role of this effect during the flow of compositions with a disperse filler is great. The wall slip, beginning in the region of high shear rates, was marked many times as the effect that must be taken into account in the analysis of rheological properties of filled polymer melts [24, 25], and the appearance of a slip is initiated in the entry (transitional) zone of the channel [26]. It is quite possible that in reality not a true wall slip takes place, but the formation of a low-viscosity wall layer depleted of a filler. This is most characteristic for the systems with low-viscosity binders. From the point of view of hydrodynamics, an exact mechanism of motion of a material near the wall is immaterial, since in any case it appears as a wall slip. 

Indeed, the shear stress at the solid surface is txz=T (S 8z)z=q (where T (, is the melt viscosity and (8USz)z=0 the shear rate at the interface). If there is a finite slip velocity Vs at the interface, the shear stress at the solid surface can also be evaluated as txz=P Fs, where 3 is the friction coefficient between the fluid molecules in contact with the surface and the solid surface [139]. Introducing the extrapolation length b of the velocity profile to zero (b=Vs/(8vy8z)z=0, see Fig. 18), one obtains (3=r bA). Thus, any determination of b will yield (3, the friction coefficient between the surface and the fluid. This friction coefficient is a crucial characteristics of the interface it is obviously directly related to the molecular interactions between the fluid and the solid surface, and it connects these interactions at the molecular level to the rheological properties of the system. 

Plant food dispersions such as tomato concentrates and concentrated orange juice are important items of commerce. The viscosity function and the yield stress are two important rheological properties that have received considerable attention. Corrections for slip, due to the formation of a thin layer of fluid next to solid surfaces, in a concentric cylinder viscometer depended on the magnitudes of applied torque and on the shear-thinning characteristics of the dispersion. Mixer viscometers were used for obtaining shear rate-shear stress and yield stress data, but the latter were higher in magnitude than those obtained by extrapolation of flow data. 

This formula for the electroosmotic velocity past a plane charged surface is known as the Helmholtz-Smoluchowski equation. Note that within this picture, where the double layer thickness is very small compared with the characteristic length, say alX t> 100, the fluid moves as in plug flow. Thus the velocity slips at the wall that is, it goes from U to zero discontinuously. For a finite-thickness diffuse layer the actual velocity profile has a behavior similar to that shown in Fig. 6.5.1, where the velocity drops continuously across the layer to zero at the wall. The constant electroosmotic velocity therefore represents the velocity at the edge of the diffuse layer. A typical zeta potential is about 0.1 V. Thus for = 10 V m" with viscosity that of water, the electroosmotic velocity U 10 " ms, a very small value. 

The interphase is a separate phase with its own characteristics and two interfacial tension coefficients, Vj -f V2 = Vjj, with Vj2 being the experimental quantity. The lattice theories predict that in binary blends (1) there is a reciprocity between Vi2 and the interphase thickness, VjjA/ = constant (2) the surface energy is proportional to (3) polymer chain ends concentrate at the interface and (4) any low molecular weight component migrates to the interface. In consequence, the inter-phase is characterized by low entanglement density and viscosity, often evidenced by the interlayer slip [3]. 

Critical Entanglement Chain Length Viscoelasticity in polymers ultimately relates back to a few basic molecular characteristics involving the rates of chain molecular motion and chain entanglement. The increasing ability of chains to slip past one another as the temperature is increased governs the temperature dependence of the melt viscosity. One embodiment... 

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