Viscosity or internal friction

The microscopic description of internal viscosity of gases is based on the fact that all molecules in the limit of its layer participate in the macroscopic motion with the momentum, where u is the macroscopic velocity of a molecule with the layer. However, owing to their chaotic movement, molecules jumping over one layer to another transfer the macroscopic momentum m u. Such jumping can influence the macroscopic speed of layers molecules from faster layers accelerate slower layers and vice-versa. It looks like one layer renders a friction on its neighbor. Thus, viscosity in gases is the phenomenon of the chaotic momentum transfer of the macroscopic movement from layer to layer in a flowing gas. 

For dynamic viscosity in SI units, the coefficient of viscosity of that media is that at a speed gradient equal to unity through area S in 1 m, the total molecule momentum 

Thus the unit of viscosity in SI units is N sec/m = kg/(m sec). Another widely used unit of viscosity is g/(cm.sec) (poiseuille or poise (P)) (in honor of J.L.M. Poiseuille). In tables, viscosity is usually expressed in centipoise (cP). The ratio between units is 1 kg/(m sec)=10 P. 

Besides the coefficient of the dynamic viscosity rj, the coefficient of the kinematical viscosity V is used in technology there is a relation between the two coefficients the kinematical coefficient is the ratio of the dynamic viscosity to the medium density v =t]lp. The coefficient of the kinematical viscosity is measured in stokes (St) 1 St = 1 cmVsec. In SI the unit of kinematical viscosity is mVsec (1 mVsec = 1(P Cr). Some viscosity factors are given in Table 3.3. 

A free path flight X of COj gas at normal conditions is A = 40 nm. Determine the molecule s average speed (u) and the number of impacts z a molecule undergoes in 1 sec. 

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For simple molecular liquids Batschinski had shown that the viscosity or internal friction at different temperatures could be expressed as a function of the mole volumes at these temperatures in the form... 

This distinction from the usual transport phenomena is not the only one for rarefied gases. The flow of molecules at outflow differs significantly too (it is referred to as Knudsen s flow.) Viscosity or internal friction in high vacuum is absent as there is no collision of molecules. In spite of the fact that pressure does not influence the individual molecular movement, it does influence the current. 

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, with a unit of Pa s, refers to the resistance or internal friction existing in a solution when the solution flows. The nature of the viscosity is the interaction of the molecules in the solution. Table 5.7 lists the viscosity of sulfuric acid solution as a function of temperature and sulfuric acid concentration [8]. 

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. 

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. 

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. 

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... 

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. 

In other words, it is assumed here that the particles are surrounded by a isotropic viscous (not viscoelastic) liquid, and is a friction coefficient of the particle in viscous liquid. The second term represents the elastic force due to the nearest Brownian particles along the chain, and the third term is the direct short-ranged interaction (excluded volume effects, see Section 1.5) between all the Brownian particles. The last term represents the random thermal force defined through multiple interparticle interactions. The hydrodynamic interaction and intramolecular friction forces (internal viscosity or kinetic stiffness), which arise when the macromolecular coil is deformed (see Sections 2.2 and 2.4), are omitted here. 

The addition of low-viscosity additives can impede the formation of the melt film that is essential for melting. On the other hand, high-viscosity or solid additives can cause increased energy dissipation due to internal and external friction, which accelerates the formation of the melt film but can also lead to overheating of the viscous phase. 

Many of the comments in the previous chapter about the selection of grade, additives and mixing before moulding apply equally in preparation for extrusion. It is important of course that the material should be appropriate for the purpose, uniform, dry, and free from contamination. It should be tested for flow and while many tests have been devised for this it is convenient to classify them as either for low or high rates of shear. The main terms used in such testing ( viscosity , shear rate , shear strain , etc.) are defined in words and expressed as formulae in ISO 472, and it is not necessary to repeat them here. Viscosity may be regarded as the resistance to flow or the internal friction in a polymer melt and often will be measured by means of a capillary rheometer, in which shear flow occurs with flow of this type—one of the most important with polymer melts—when shearing force is applied one layer of melt flows over another in a sense that could be described as the relationship between two variables—shear rate and shear stress.1 In the capillary rheometer the relationship between the measurements is true only if certain assumptions are made, the most important of which are ... 

Viscosity is a characteristic property of all fluids - liquids or gases. It is defined as the property by virtue of which it tends to oppose the relative motion between two adjacent layers of the fluid. It originates from the internal friction existing between the molecules of the gas or the liquid. In gases, the viscosity arises from the transfer of momentum between two adjacent layers of the gas. The coefficient of viscosity, r, is given by the equation... 

Viscosity Internal friction or resistance to flow of a liquid. The constant ratio of shearing stress to rate of shear. In liquids for which this ratio is a function of stress, the term apparent viscosity is defined as the ratio. 

The phenomenon of friction may be described as the degradation of mechanical work (work performed by moving forces) into heat. From a molecular point of view such degradation occurs through the change in the uniform motion of an initially exerted force into an increased random motion of motecules. This latter is manifested as an increase in temperature. When such dissipation is observed in fluids, the process is referred to as internal friction, or viscosity. 

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