Wall-slip yield stress

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. 

Broadly speaking, the mechanical properties can be divided into two classes bulk and interfacial . Within the bulk properties are included the shear and extensional viscosities, moduli and yield stresses (material constants that relate stress to strain or strain rate), and within interfacial rheology are included the wall-slip and friction effects. The interfacial properties are independent of bulk mechanical properties and governed by the frictional or surface forces which are thought to operate at relatively... 

Case II refers to situations where the particle-wall interactions are purely repulsive. The particles are separated from the wall by a thin layer of solvent, even in the absence of any motion. Slip is thus possible for very slow flows, indicating that the sticking yield stress is vanishingly small. The residual film thickness for weak flows corresponds to a balance between the osmotic forces and the short-range repulsive forces, independently of any elastohydrodynamic contribution. This is clearly reflected in Fig. 16c, d, where we observe that the particle facet is nearly flat and symmetric. Since tire pressure in the leading and rear regions of the facet are equal and opposite, the lift force is very small. The film thickness, which is set by the balance of the short-range forces, is constant so that the stress/velocity relationship is linear. 

Interestingly, this slip behaviour of hard-sphere glasses is different in nature from that found earlier by Meeker et al. in jammed systems of emulsion droplets [60]. There, a non-linear elasto-hydrodynamic lubrication model, appropriate for deformable particles, could quantitatively account for their observations. It therefore appears that, while slip is ubiquitous for yield stress fluids flowing along smooth walls, the mechanism for its occurrence can be highly system dependent. 

Malkin (1990) reviews the rheology of filled polymers and highlights the importance of yield stresses, non-Newtonian flow, wall slip and normal stresses in filled polymer flow. Details of the effects of particle shape, concentration and adsorption on these phenomena are discussed. 

An overview of rheological measurements coupled with magnetic resonance is provided by Callaghan [67], Rheo-NMR of emulsified systems has been studied, the systems including formulations with yield stress exhibiting wall slip [68], Comparisons are provided between conventional rheological techniques and Rheo-NMR characterization. 

For concentrated suspensions, especially those near the random packing limit, the wall effect becomes so important that the viscosity may lose its meaning. Hence, other rheological properties, such as yield stress and wall boundary (slip) conditions, may be more meaningful for a concentrated system. 

Some modifications of the melt flow behavior of thermoplastics that can be observed depending on filler concentration are a yield-like behavior (i.e., in these cases, there is no flow until a finite value of the stress is reached), a reduction in die swell, a decrease of the shear rate value where nonlinear flow takes place, and wall slip or nearwall slip flow behavior [14, 27, 46]. Other reported effects of flllers on the rheology of molten polymers are an increase of both the shear thinning behavior and the zero-shear-rate viscosity with the filler loading and a decrease in the dependence of the filler on viscosity near the glass transition temperature [18, 47-49]. 

Along with wall-induced instability, the occurrence of slip between the sample and the viscometer walls is one of the most serious and prevalent, though often neglected, problems one encounters in assessing the rheology of dispersed systems in general, and concentrated emulsions in particular. Since concentrated emulsions have a yield stress, wall slip - if present -can be readily demonstrated by painting a... 

It is clear from the above that extreme care must be exercised in the characterization and rheological eva-luation of concentrated emulsions. Few, if any, com-mercial viscometers are designed to give reliable results for nonNew-tonian fluids. Not only are modifications of the hardware often called for, but also the software of automated instra-ments is generally incapable of dealing with yield-stress fluids, end effects, and wall slip. For example, to correct for end effects, it will not do to use a calibration or instrument factor for any but Newtonian fluids. Unfortunately, there are no shortcuts in this field ... 

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. 

Mason et al. (136) determined the yield stresses and yield strains of a series of monodisperse emulsions, using either a cone-and-plate or double-wall Couette geometry in oscillatory mode. Wall-induced coales-cence and wall slip were claimed to be absent, but no mention is made of attempts to reduce end or edge effects. Estimated film thicknesses were used to arrive at the effective volume fractions. Their data for the yield stress could be fit to... 

Because of the no-slip boimdary condition at both solid walls, i.e. air = oR and r = / , the velocity must be maximum at some intermediate point, say at r = XR. Then, for a fluid without a yield stress, the shear stress must be zero at this position and for a viscoplastic fluid, there will be a plug moving en masse. Equation (3.76) can therefore be re-written ... 

Advantages of NMR-based rheometry are the direct detection of phenomena like wall slip or yield stress and the model fijee evaluation of the measured data. While conventional capillary rheometry is usually used to obtain data at very high shear rates the NMR-capillary-rheometer covers a shear rate range up to approximately 500 s, a range suggesting applications in food or pharmaceutical industry where shear sensitive goods are handled. 

If the thickness or temperature is decreased below the glass transition, one observes very non-linear response to an applied stress. The films are solid-like at low stresses and shear when a yield stress is exceeded. In experiments, an alternating or constant displacement is applied through a system with some intrinsic elasticity. We illustrate the types of non-steady or stick-slip motion that results in these two cases and discuss the effect of system compliance. We also describe how molecules move when the film yields. In some cases the film transforms to a liquid-like state, and in other cases yield occurs at an interface between the film and a wall. 

As the displacement of the stage increases, the force on the top wall grows. Eventually it exceeds the yield stress of the system and produces a highly non-linear response. As shown in Figure 6(c), the top wall slips rapidly each time the yield stress is exceeded. The absolute values of the yield stresses for forward and backward slips are roughly equal, but both fluctuate from cycle to cycle. 

Wall J.F., Grieser F., Zukoski C.F. Monitoring chemical reactions at the gold/solution interface using atomic force microscopy. J. Chem. Soc., Faraday Trans. 1997 93(22) 4017-4020 Walls H.J., Caines S.B., Sanchez A.M., Khan S.A. Yield stress and wall slip phenomena in colloidal silica gels. J. Rheol. 2003 47(4) 847-868... 

Consistent with this model, foams exhibit plug flow when forced through a channel or pipe. In the center of the channel the foam flows as a solid plug, with a constant velocity. All the shear flow occurs near the walls, where the yield stress has been exceeded and the foam behaves like a viscous liquid. At the wall, foams can exhibit wall slip such that bubbles adjacent to the wall have nonzero velocity. The amount of wall slip present has a significant influence on the overall flow rate obtained for a given pressure gradient. 

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