For several centuries fluid dynamics studies have relied upon the assumption that when a liquid flows over a solid surface, the liquid molecules adjacent to the solid are stationary relative to the solid. This no-slip boundary condition (BC) has been applied successfully to model many macroscopic experiments, but has no microscopic justification. In recent years there has been an increased interest in determining the appropriate BCs for the flow of Newtonian liquids in confined geometries, partly due to exciting developments in the fields of microfluidic and microelectromechanical devices and partly because new and more sophisticated measurement techniques are now available. An increasing number of research groups now dedicate great attention to the study of the flow of liquids at solid interfaces, and as a result a large number of experimental, computational and theoretical studies have appeared in the literature. We provide here a review of experimental studies regarding the phenomenon of slip of Newtonian liquids at solid interfaces. We dedicate particular attention to the effects that factors such as surface roughness, wettability and the presence of gaseous layers might have on the measured interfacial slip. We also discuss how future studies might improve our understanding of hydrodynamic BCs and enable us to actively control liquid slip.

https://iopscience.iop.org/article/10.1088/0034-4885/68/12/R05/pdf

Boundary slip in Newtonian liquids: a review of experimental studies

Viscoelastic instabilities are of practical importance, and are the subject of growing interest. Reviewed here are the fresh developments as well as earlier work in this area, organized into the following categories: instabilities in Taylor-Couette flows, instabilities in cone-and-plate and plate-and-plate flows, instabilities in parallel shear flows, extrudate distortions and fracture, instabilities in shear flows with interfaces, instabilities in extensional flows, instabilities in multidimensional flows, and thermohydrodynamic instabilities. Emphasized in the review are comparisons between theory and experiment and suggested directions for future work.

Instabilities in viscoelastic flows

The no-slip boundary condition at a solid-liquid interface is at the center of our understanding of fluid mechanics. However, this condition is an assumption that cannot be derived from first principles and could, in theory, be violated. In this chapter, we present a review of recent experimental, numerical and theoretical investigations on the subject. The physical picture that emerges is that of a complex behavior at a liquid/solid interface, involving an interplay of many physico-chemical parameters, including wetting, shear rate, pressure, surface charge, surface roughness, impurities and dissolved gas.

/pdf/microfluidics-the-no-slip-boundary-condition-4yg4du3ofj.pdf

Microfluidics: The no-slip boundary condition

▪ Abstract Polymer melts exhibit extrusion instabilities at sufficiently high levels of stress, and they appear to exhibit wall slip. I explore the evidence for slip, the possible mechanisms of slip, and the relation between slip and extrusion instabilities.

Extrusion Instabilities and Wall Slip

Viscous flow is familiar and useful, yet the underlying physics is surprisingly subtle and complex. Recent experiments and simulations show that the textbook assumption of 'no slip at the boundary' can fail greatly when walls are sufficiently smooth. The reasons for this seem to involve materials chemistry interactions that can be controlled — especially wettability and the presence of trace impurities, even of dissolved gases. To discover what boundary condition is appropriate for solving continuum equations requires investigation of microscopic particulars. Here, we draw attention to unresolved topics of investigation and to the potential to capitalize on 'slip at the wall' for purposes of materials engineering.

/pdf/slippery-questions-about-complex-fluids-flowing-past-solids-23mqzajf4h.pdf

Slippery questions about complex fluids flowing past solids.

Non-Newtonian fluid mechanics is often distinguished from its Newtonian counterpart by the additional requirement that a constitutive equation be specified as part of the problem statement. For some modeling problems important to polymer processing, a wall boundary condition in which provision is made for slip is also a necessary ingredient. This requires that attention be directed to microscopic processes influencing flow behavior on a macroscopic scale. In this paper theoretical and experimental consequences of slip phenomena are reviewed. It is maintained that future progress will depend heavily on productive interplay between fluid mechanics and materials science.

The behavior of complex fluids at solid boundaries

Pseudo-spectral analysis of the stability of pressure-driven flow of a giesekus fluid between parallel planes

Carefully prepared aqueous solutions of drag‐reducing polymers have been characterized by means of steady viscometric and molecular weight measurements. Solutions of anionic polyacrylamide (Cyanatrol‐750), 0.5–50 ppm, and nonionic poly (ethylene oxide) (Polyox WSR‐303), 100–1000 ppm, were investigated. Steady shear viscosity measurements, with a Rheometrics Stress Rheometer and a Contraves LS30 rheometer, reveal substantial shear thinning for Cyanatrol solutions above 2 ppm and Polyox solutions above 500 ppm, for the shear‐rate range investigated. Zero‐shear viscosity was accurately determined for all solutions. Intrinsic viscosity,hydrodynamic radius of gyration, and overlap concentration were also estimated. The Carreau and Carreau–Yasuda models successfully fit all shear viscosity data, and allow estimates of elasticity. Reduced variables were used for generalizing the shear viscosity data. Aging tests show that both polymers are stable. The molecular weight and polydispersity of Cyanatrol were measured using the technique of band sedimentation/low angle laser light scattering. The drag‐reducing capability of these polymers has been confirmed by pipe flow experiments.

Steady viscometric properties and characterization of dilute drag‐reducing polymer solutions

The effect of viscoelasticity of dynamics on single nonspherical bubbles has been determined for donditions comparable to those present during cavitation. It is shown that, in contrast to dynamics of spherical bubbles under cavitation conditions, fluid rheology has a noticeable influence on the bubble dynamics. Earlier non-Newtonian studies have been restricted to bubbles of fixed volume oscillating around a spherical shape, but in the present work an approach has been developed which allows one to study nonsphericities associated with bubble growth or collapse. An important feature of these problems is the coupling provided by fluid rheology between bubble dynamics and deformation history of the surrounding fluid.

Dynamics of nonspherical bubbles surrounded by viscoelastic fluid

Measurements were made of the steady secondary flow which is caused by a cylinder executing small-amplitude oscillations perpendicular to the cylinder generators. Test fluids were a water-glycerine mixture and two non-Newtonian fluids: aqueous solutions of Separan AP-30 and PolyHall 295 polyacrylamide polymers. The fluids were contained in a cylinder whose center coincides with the rest position of the oscillating cylinder. Experimental results are consistent with a theoretical analysis for a three-constant Oldroyd fluid model in its simplified convected-Maxwell form. Both experiment and theory show that at an oscillation frequency of 40 Hz the secondary flow of the dilute polymer solutions is, essentially, in a sense opposite from that of a Newtonian fluid.

Secondary flow in the neighborhood of a cylinder oscillating in a viscoelastic fluid

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