Liquid applied shear stress

When a shear stress is applied to a suspension or liquid exhibiting laminar flow, a velocity gradient (the rate of shear) is established. When the rate of shear varies linearly with the applied shear stress, the system is termed Newtonian and the proportionality constant is termed the viscosity. Newtonian flow is usually observed in dilute... 

This work shows that high shear rates are required before viscous effects make a significant contribution to the shear stress at low rates of shear the effects are minimal. However, Princen claims that, experimentally, this does not apply. Shear stress was observed to increase at moderate rates of shear [64]. This difference was attributed to the use of the dubious model of all continuous phase liquid being present in the thin films between the cells, with Plateau borders of no, or negligible, liquid content [65]. The opposite is more realistic i.e. most of the liquid continuous phase is confined to the Plateau borders. Princen used this model to determine the viscous contribution to the overall foam or emulsion viscosity, for extensional strain up to the elastic limit. The results indicate that significant contributions to the effective viscosity were observed at moderate strain, and that the foam viscosity could be several orders of magnitude higher than the continuous phase viscosity. 

Even at a phenomenological level, the flow behavior of polymers differs significandy from that of low-viscous liquids such as water. The flow behavior of polymer melts is markedly dependent on the applied shear stress o, the shear rate y, and on time. Elastic effects and normal stresses also occur in practice, and their effects can either help or hinder the processes underway. 

As described in the previous section, solid particles can be broken up if the applied shear stresses are sufficiently large. In contrast, however, liquid droplets cannot be dispersed under certain conditions, even in shear fields with very large shear stresses. The Weber number We is often used to characterize the dispersion of droplets (another notation often used instead... 

Viscoelastic A liquid (or solid) with both viscous and elastic properties. A viscoelastic liquid will deform and flow under the influence of an applied shear stress, but when the stress is removed the liquid will slowly recover from some of the deformation. 

Fluids (liquids and gases) are a form of matter that cannot achieve equilibrium under an applied shear stress but deform continuously, or flow, as long as shear stress is applied. 

Powders can withstand stress without flowing, in contrast to most liquids. The strength or yield stress of this powder is a function of previous compaction, and is not unique, but depends on stress ap ication. Powders fail only under applied shear stress, and not isotropic load, although they do compress. For a given apphed horizontal load, failure can occur by either raising or lowering die normal stress, and two possible values of failure shear stress are obtained (active versus passive failure). 

The shear viscosity /(r) measures the rate at which a liquid can rearrange in response to an applied shear stress. As explained in Section 1, the extent to which the temperature dependence of / (r) conforms to, or deviates from, Arrhenius behavior. 

Newton defined the absolute viscosity of a liquid as the ratio between the applied shear stress and the resulting shear rate. If two plates of equal area A are considered as separated by a liquid film of thickness A as in Fig. 1.4, the shear stress is the force F applied to the top plate causing it to move relative to the bottom plate divided by the area of the plate A. The shear rate is the velocity V of the top plate divided by the separation distance D. 

All of these boundary conditions are to be applied at the interface z = h (x s, / ) In writing these conditions, we have used the usual symbol a to represent the surface tension at the gas liquid interface, and the ambient pressure in the gas above the thin film is denoted as//. Finally, t, represents the possibility of an externally applied shear stress on the interface, and will be a useful ad hoc way of including the possibility of something like a wind-induced stress at the interface, without actually calculating the velocity profiles and stress in the outer fluid. 

Viscosity rj is the ratio of the applied shear stress to the rate of flow, t , of a liquid. If a liquid contained between two parallel plates of area A and a distance d apart is subjected to a shear force F. then... 

Although one can measure deformation in a solid, one cannot normally do this in a liquid since it undergoes a continuously increasing amount of deformation when a shear stress is applied. But one can determine the deformation rate (the shear rate) caused by an applied shear stress or vice versa, and fluid behavior can be classified on this basis. 

Viscosity of Newtonian liquids ean be measured by calibrated glass capillary viscometer. Kinematic (the resistance to flow of a fluid under gravity) and dynamic (the ratio between the applied shear stress and the rate of shear of a liqitid) viscosities can be calculated from measured time of flow using the following equations ... 

Conversely, the viscosity of non-Newtonian fluids is dependent on the applied shear stress and is referred to as apparent viscosity. When a fluid exhibits plastic flow, a certain minimum shear stress must be applied, called yield stress, before the fluid starts to flow (Fig. 18.6b). At a shear stress of less than the yield stress, the viscosity is thus infinitely large and the liquid behaves like a solid. Above the yield stress, the viscosity decreases with increasing shear stress. Also in the straight part of the curve in Fig. 18.6c, the viscosity decreases with increasing shear stress. This is... 

A more rigorous approach is to make a primary distinction between a solid and a fluid, based on the phase s response to an applied shear stress, and then use additional criteria to classify a fluid as a liquid, gas, or supercritical fluid. Shear stress is a tangential force per unit area that is exerted on matter on one side of an interior plane by the matter on the other side. We can produce shear stress in a phase by applying tangential forces to parallel surfaces of the phase as shown in Fig. 2.1 on the next page. 

Rheological properties of foams (elasticity, plasticity, and viscosity) play an important role in foam production, transportation, and applications. In the absence of external stress, the bubbles in foams are symmetrical and the tensions of the formed foam films are balanced inside the foam and close to the walls of the vessel [929], At low external shear stresses, the bubbles deform and the deformations of the thin liquid films between them create elastic shear stresses. At a sufficiently large applied shear stress, the foam begins to flow. This stress is called the yield stress, Tq- Then, Equation 4.326 has to be replaced with the Bingham plastic model [930] ... 

Applying shear stress is not the only way that a liquid repellent surface can be damaged. Vertical compression or expansion can also lead to degradation of the surface properties. Furthermore, material removal can occur by vertical compression or expansion when two solid surfaces are in contact. In the following section we will describe techniques that apply normal (and not shear) force to the tested surface. 

Figure 2.7 A volume of liquid subjected to an applied shear stress.

Viscosity is defined as the property of resistance to fiow exhibited within the body of a material and expressed in terms of a relationship between applied shearing stress and resulting rate of strain in shear. In the case of ideal or Newtonian viscosity, the ratio of shear stress to the shear rate is constant. Plastics typically exhibit non-Newtonian behavior, which means that the ratio varies with the shearing stress. There are two different aspects of viscosity. Dynamic or absolute viscosity, best determined in a rotational type of viscometer with a small gap clearance, is independent of the density or specific gravity of the liquid sample and is measured in poises (P) and centipoises (cP). Kinematic viscosity, usually determined in some form of efflux viscometer equipped with a capillary bore or small orifice that drains by gravity, is strongly dependent on density or specific gravity of the liquid, and is measured in stokes (S) and centistokes (cS). The relationship between the two types of viscosity is... 

When the applied shear stress system is removed from an elastically deformed solid, the solid regains its original shape completely and the work of deformation is also recovered. No such recovery occurs in a liquid the work done in producing a given rate of shear is completely dissipated against the liquid friction forces. The resistance to flow offered by a liquid when it is subjected to a shear stress is called a viscous force and the liquid is said to possess viscosity. 

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