Internal friction viscosity

In the plasticators output zone, both screw and barrel surfaces are usually covered with the melt, and external forces between the melt and the screw channel walls have no influence except when processing extremely high viscosity plastics such as rigid PVC and UHMWPE. The flow of the melt in the output section is affected by the coefficient of internal friction (viscosity) particularly when the die offers a high resistance to the flow of the melt (Chapter 3). Figure 5.2 shows the extruder s components where the following identifications are listed ... 

Next we consider the mechanism of internal friction (viscosity) of flowing liquid or gas. It consists in the following the neighboring layers which move with different velocities exchange impulses these impulses are transferred by particles which diffuse through the boundary between layers. Besides, the intermolecular bonds of structural elements (normally - i.e., perpendicularly - orientated to the sliding surface) deform and tear up this is the second factor of viscosity inherent to liquid only. 

Viscosity is that property of a fluid that opposes the relative motion of adjacent portions of the fluid and can consequently be regarded as a type of internal friction. Viscosity can be defined as the force required to move a layer of fluid of unit area with a velocity 1 cm/s greater than the velocity of another layer 1 cm away (see Fig. B.l). Since force is proportional to the velocity difference between the layers and inversely proportional to the distance between the layers, then... 

Unlike gases, liquid viscosity decreases as temperature increases, as the molecules move further apart and decrease their internal friction. Like gases, oil viscosity increases as the pressure increases, at least above the bubble point. Below the bubble point, when the solution gas is liberated, oil viscosity increases because the lighter oil components of the oil (which lower the viscosity of oil) are the ones which transfer to the gas phase. 

The deforming forces which induce flow in fluids are not recovered when these forces are removed. These forces impart kinetic energy to the fluid, an energy which is dissipated within the fluid. This is the origin of the idea that viscosity represents an internal friction which resists flow. This friction originates from the way molecules of the sample interact during flow. 

Viscosity. The viscosity of an oil is its stiffness or internal friction, as illustrated in Figure 7. With a surface of area moving at velocity IVat a distance AX from an equal parallel area moving at velocity V - - AV, force F is required to maintain the velocity difference according to the equation 7 ... 

Viscosity (See Sec. 5 for further information.) In flowing liquids the existence of internal friction or the internal resistance to relative motion of the fluid particles must be considered. This resistance is caUed viscosity. The viscosity of liquids usuaUv decreases with rising temperature. Viscous liquids tend to increase tlie power required by a pump, to reduce pump efficiency, head, and capacity, and to increase Friction in pipe lines. 

The viscosity of a fluid arises from the internal friction of the fluid, and it manifests itself externally as the resistance of the fluid to flow. With respect to viscosity there are two broad classes of fluids Newtonian and non-Newtonian. Newtonian fluids have a constant viscosity regardless of strain rate. Low-molecular-weight pure liquids are examples of Newtonian fluids. Non-Newtonian fluids do not have a constant viscosity and will either thicken or thin when strain is applied. Polymers, colloidal suspensions, and emulsions are examples of non-Newtonian fluids [1]. To date, researchers have treated ionic liquids as Newtonian fluids, and no data indicating that there are non-Newtonian ionic liquids have so far been published. However, no research effort has yet been specifically directed towards investigation of potential non-Newtonian behavior in these systems. 

Lubrication, in the generally accepted sense of the word, means keeping moving surfaces completely separated by means of a layer of some liquid. When this is satisfactorily achieved, the frictional resistance no longer depends on the solid surfaces but solely on the internal friction of the liquid, which, in turn, is directly related to its viscosity. The more viscous the fluid, the greater the resistance, but this is never comparable with that existing between non-lubricated surfaces. 

When reviewing the subject of plastic melt flow, the subject of viscosity is involved. Basically viscosity is the property of the resistance of flow exhibited within a body of material. Ordinary viscosity is the internal friction or resistance of a plastic to flow. It is the constant ratio of shearing stress to the rate of shear. Shearing is the motion of a fluid, layer by layer, like a deck of cards. When plastics flow through straight tubes or channels they are sheared and the viscosity expresses their resistance. 

VG - ami ton. viable - capable of living, viscosity - the resistance of a liquid to flow, resulting from the combined effects of internal friction and friction between the liquid and its surroundings, viscous - resisting flow. 

The viscosity coefficients at dislocation cores can be measured either from direct observations of dislocation motion, or from ultrasonic measurements of internal friction. Some directly measured viscosities for pure metals are given in Table 4.1. Viscosities can also be measured indirectly from internal friction studies. There is consistency between the two types of measurement, and they are all quite small, being 1-10% of the viscosities of liquid metals at their melting points. It may be concluded that hardnesses (flow stresses) of pure... 

Intrinsic resistance to dislocation motion can be measured in either of two ways direct measurements of individual dislocation velocities (Vreeland and Jassby, 1973) or by measurements of internal friction (Granato, 1968). In both cases, for pure simple metals there is little or no static barrier to motion. As a result of viscosity there is dynamic resistance, but the viscous drag coefficient is very small (10" to 10" Poise). This is only 0.1 to 1 percent of the viscosity of water (at STP) and about 1 percent of the viscosity of liquid metals at their... 

It is conceivable that the twisting motion experiences internal friction, by which is meant the occurrence of bumps or barriers in the potential surface along which the DNA deforms. This would cause y to exhibit a temperature (T) dependence differing from that due to the viscosity of water. Experimental results 4"1 give no indication of such anomalous T dependence, as shown subsequently. 

This will be found almost exclusively in the rough vacuum range. The character of this type of flow is determined by the interaction of the molecules. Consequently Internal friction, the viscosity of the flowing substance. Is a major factor. If vortex motion appears In the streaming process, one speaks of turbulent flow. If various layers of the flowing medium slide one over the other, then the term laminar flow or layer flux may be applied. 

Re is the product of the pipe diameter, flow velocity, density and reciprocal value of the viscosity (internal friction) of the gas which is flowing. Flow is turbulent where Re > 2200, laminar where Re < 2200. 

The fact that the velocity of a fluid changes from layer to layer is evidence of a kind of friction between these layers. The layers are mathematical constructs, but the velocity gradient is real and a characteristic of the fluid. The property of a fluid that describes the internal friction or resistance to flow is the viscosity of the material. Chapter 4 is devoted to a discussion of the measurement and interpretation of viscosity. For now, it is enough for us to recall that this property is quantified by the coefficient of viscosity 77 of a material. The coefficient of viscosity has dimensions of mass length-1 time-1, kg m-ls-1 in SI units. In actual practice, the cgs unit of viscosity, the poise (P), is widely used. Note that pure water at 20°C has a viscosity of about 0.01 P = 10-3kgm-ls-1... 

Internal viscosity (Section 4) provides another possible source of shear-rate dependence. For sufficiently rapid disturbances, a spring-bead model with internal viscosity acts like a rigid body for sufficiently slow disturbances it is flexible and indefinitely extensible. The analytical difficulties for coupled, non-linear spring-bead systems are equally severe in linear spring-bead systems with internal viscosity. Even the elastic dumbbell with internal viscosity has only been solved exactly in the limit of small e (559), where e is the ratio of internal friction coefficient to molecular (external) friction coefficient Co n. For this case, the viscosity decreases with shear rate. 

Both the many-bead and dumbbell models with internal friction predict limiting viscosities at high frequencies, tfa, and high shear rates t]m. The theories predict that tfm and tjx are related, such that... 

In eq. (5.27) quantities JeR and [ ] are equal to those obtained by Zimm. According to Cerf, the internal friction factor ( ) can be determined by systematically varying solvent viscosity rj0, e.g. by changing the temperature of measurement or the composition of a mixed solvent. In this way, a straight line should be obtained for a plot of tan a vs. rj0. The positive intercept of this line with the ordinate axis should be equal to the second term on the right-hand side of eq. (5.27). An indispensible condition for this procedure is that the intrinsic viscosity [rj] is independent of the solvents used. Otherwise one should plot the product rf rj0 on the abscissa. 

At very low solvent viscosity, when f is small compared with the internal friction factors, the chain molecules must behave like "frozen molecules. In this case, the initial slope of the extinction angle curve becomes again a linear function of the solvent viscosity. The slope of this straight line, however, is considerably higher than (J eRj2)(M ri jRT). Its intercept with the ordinate axis is equal to zero. This behaviour is schematically shown in Fig. 5.10 (3). 

As the results of Schwander and Cerf (188) and of Leray (180,181), which were obtained on samples of DNA from calf thymus, have been reproduced already in several review articles (1,3), the present discussion can be kept rather short. When the viscosity of the solvent (1.0 molar aquous solution of sodium chloride) is increased by replacing part of the water by glycerol, the behaviour of the initial slope of the extinction angle curve follows the qualitative pattern, as given by Fig. 5.10. At low solvent viscosities the molecules seem to behave like frozen molecules, at high solvent viscosities they seem to become flexible coils exhibiting internal friction. These results have been considered to prove the correctness of Cerf s theoretical model. 

For these measurements, temperature has been varied between 55 and 110° C. In this temperature range, the solvent viscosity changes by a factor three 4.7 to 1.5 cps). It is very improbable that a noticeable internal friction factor would change just by the same factor. Moreover, as has already been pointed out at the end of Section 5.2.2, the curves obtained by plotting cot2 c vs reduced shear stress fjN are practically coinciding for dilute solutions of cellulose tricarbanilate fractions with M S 500,000 and for anionic polystyrenes. So one can conclude that the internal friction of the thermodynamically stiff molecules of cellulose tricarbanilate must be rather low. 

As the temperature of water increases, its internal molecular friction (viscosity) decreases and both chemical and physical reactions, such as the formation of chemical inhibitor films, flocculation, and degassing, occur more rapidly. 

With the viscosity of a liquid we mean the resistance to flow of that particular liquid. This resistance is caused by internal friction and other interactions between the particles. Among other things, viscosity is dependent on temperature, the solid volume fraction and the properties of the particles. The viscosity of normal liquids, solutions and lyophobic colloids which are not too concentrated and contain symmetrical particles is measured by allowing a certain volume to flow through a capillary and measuring the time required by the liquid to flow through it. In figure 5.10 you can see the instrument which is used for this measurement the Ostwald viscometer. 

Here fi is termed the eddy viscosity, which describes the internal friction developed as the laminar flow passes around irregularities and becomes turbulent. Laminar and turbulent flow are distinguished using the Reynolds number, N R ... 

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