Apparent Slip

One characteristic of shear banded flow is the presence of fluctuations in the flow field. Such fluctuations also occur in some glassy colloidal materials at colloid volume fractions close to the glass transition. One such system is the soft gel formed by crowded monodisperse multiarm (122) star 1,4-polybutadienes in decane. Using NMR velocimetry Holmes et al. [23] found evidence for fluctuations in the flow behavior across the gap of a wide gap concentric cylindrical Couette device, in association with a degree of apparent slip at the inner wall. The timescale of these fluctuations appeared to be rapid (with respect to the measurement time per shear rate in the flow curve), in the order of tens to hundreds of milliseconds. As a result, the velocity distributions, measured at different points across the cell, exhibited bimodal behavior, as apparent in Figure 2.8.13. These workers interpreted their data... 

Laminar flow of concentrated suspensions and apparent slip at the pipe wall... 

Y. Cohen, Apparent Slip Flow of Polymer Solutions, in "Encyclopedia of Fluid Mechanics. Rheology and Non-Newtonian Flows", N.P. Cheremisinoff (ed.), Gulf Publishing, Houston, TX, vol. 7,1988, pp. 407-457. 

H. Muller-Mohnssen, Direct Determination of Apparent Slip for a Ducted Flow of Polyacrylamide Solutions, J. Rheol., 31(4) 323-336 (1987). 

H. Muller-Mohnssen, D. Weiss and A. Tippe, Concentration Dependent Changes of Apparent Slip in Polymer Solution Flow, J. Rheol., 34 223-244 (1990). 

The slip of viscoelastic polymeric materials (and flow instabilities) was reviewed in detail by Denn (6). Apparent slip at the wall was observed with highly entangled linear polymers, but not with branched polymers or linear polymers with insufficient numbers of... 

Three theories were proposed to explain wall-slip (a) adhesive failure at the wall, (b) cohesive failure within the material as a result of disentanglement of chains in the bulk and chains absorbed on the wall, and (c) the creation of a lubricating surface layer at the wall either by a stress-induced transition, or by a lubricating additive. If the polymer contains low molecular weight components or slip-additives, their diffusion to the wall will create a thin lubricating layer at the wall, generating apparent slip. 

The existence of a stress-free meniscus introduces a boundary discontinuity on both upper and lower rims. This discontinuity is bound to result in sharply enhanced stress build-up at the upper and lower contact lines. It may produce an effective slip layer on the sample/plate interfaces at the meniscus, yielding an overall torque, which is less than anticipated on the basis of Fig. 7a, since a large portion of the measured torque on the plate arises from the stress contribution at the rim r=R. This correction may increase with lowering the gap distance. Without a proper analytical treatment, it remains unknown how the magnitude of such an apparent slip depends on the rheological properties of the sample and whether the reported slip like behavior [ 19,33] is a manifestation of such a plausible edge effect. 

EFFECT OF SURFACE TREATMENT. A gap dependence of the flow curve is indirect evidence of slip. In addition, some role of the solid boundary in slip is clearly implicated in cases where one can show that changes in the wall material or surface treatments of it (such as a teflon coating) influence the magnitude of the apparent slip velocity (Ramamurthy 1986 Hatzikiriakos and Dealy 1991 Wang and Drda 1996). 

Doi and Edwards 1979). The predicted shear thinning is so severe that as the shear rate increases, the shear stress r y is pre,dicted to pass through a maximum and then decrease. with further increases in y (see Fig. 3-33). Hence, at each shear stress there are at least two values of the shear rate. This, it is predicted, should lead to material instabilities—that is, apparent slip phenomena, such as spurt, sometimes observed in flow through capillaries (see Section 3.7.5.3). 

When applied to geometries with moving boundaries, such as the cone-and-plate geometry, the Helfrich argument suggests that the flow should be concentrated in thin zones of width proportional to mesh size, and hence there should be apparent slip. 

The most direct manifestation of wall effects is that of (apparent) slip in many... 

D. C. Tretheway and C. D. Meinhart, Apparent fluid slip at hydrophobic microchannel walls, Phys. Fluids 14, L9-L12 (2002) Chang-Hwan Choi, J. A. Westin, and K. S. Breuer, Apparent slip flows in hydrophilic and hydrophobic microchannels, Phys. Fluids 15, 2897-902 (2003). 

P. G. de Gennes, Physics of Polymer Surfaces and Interfaces, edited by Isaac C. Sanchez (Butterworth-Heinemann, Burlington, MA (1992), Chap. 3, pp. 55-71 J. L. Goveas and G. H. Fredrickson (1992), Apparent slip at a polymer-polymer interface, Eur. Phys. J. B2,79-92 (1998) R. Zhao and C. W. Macosko, Slip at polymer-polymer interfaces Rheological measurements on coextruded multilayers, J. Rheol. 46, 145-67 (2002). 

Cohen, Y., and Metzner, A. B. (1986). An analysis of apparent slip flow of polymer solutions. Rheol. Acta 25,28-35. 

At the edges of the coated layer its free surface ends in ordinary static contact lines. These lateral contact lines necessarily bend round upstream and connect with the wetting line. Thus at each edge of the layer where it is being delivered to the substrate there must be a curved segment of dynamic contact line, and the apparent slip of the liquid... 

The described phase separation of extrusion bodies gives rise to the formation of a low-viscosity boundary layer at the wall. The thickness hs of this lubricating film is very small compared to the dimensions of the non-separated core material. Wall slip is taken into account using an apparent slip velocity Us at solid boundaries (Mooney 1931, Wein and Tovchigechko 1992, Yoshimura and Prud Homme 1988). Above the slip stress level where sliding is initiated, the slip velocity Us relative to the wall is assumed to be proportional to the shear stress along the wall... 

Apparent slip Boundary condition at interface Perfect slip... 

The slip flow phenomena can be explained by some possible mechanism. The fluid slip can be described as true or apparent slip. The true slip occurs at a molecular level, where liquid molecules are effectively sliding on the solid surface. The apparent slip occurs not at the solid/fluid interface but at the fluid/fluid interface where a thin layer of liquid/gas molecules is tightly bound to the solid surface. For apparent slip the velocity gradient close to the soUd is so high that the molecules beyond the layer of hquid/gas molecules appear to slide on the surface. 

For apparent slip, a thin gas-liquid layer with a modified viscosity and/or mobility is created near the solid surface. At room temperature and pressure, there is always some residual gas dissolved in a liquid. Critical level of shear might induce cavitations in a liquid, and the generated gas bubbles might adhere to the solid surface forming a thin layer at the interface onto which the liquid can slip. The other factor can be the critical shear rate at which a microscopic surface roughness or cormgation can favor the... 

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). 

Choi C-H, Westin JA, Breuer KS (2003) Apparent slip flows in hydrophilic and hydrophobic micro channels. Phys Fluids 15 2897-2902... 

Zhang J, Kwok DY (2004) Apparent slip over a solid-liquid interface with a no-slip boimdary condition. Phys Rev E 70 056701... 

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