Elasticity and viscosity
Contents
Introduction
"The fundamental property of a fluid is that it cannot be in equilibrium in a state of stress such that the mutual action between two adjacent parts is oblique to the common surface. This property is the basis of Hydrostatics and is verified by the complete agreement of the deductions of that science with experiment. Very slight observation is enough, however, to convince us that oblique stresses may exist in fluids in motion. Let us suppose for instance that a vessel in the form of a circular cylinder, containing water (or other liquid), is made to rotate about its axis, which is vertical. If the motion of the vessel be uniform, the liquid is soon found to be rotating with the vessel as one solid body. If the vessel be now brought to rest, the motion of the fluid continues for some time, but gradually subsides, and at length ceases altogether; and it is found that during this process the portions of fluid what are further from the axis lag behind those which are nearer, and have their motion more rapidly checked. These phenomena point to the existence of mutual actions between contiguous elements which are partly tangential to the common surface. For if the mutual action were everywhere wholly normal, it is obvious that the moment of momentum, about the axis of the vessel, of any portion of fluid bounded by a surface of revolution about this axis, would be constant. We infer, moreover, that these tangential stresses are not called into play so long as the fluid moves as a solid body, but only whilst a change of shape of some portion of the mass is going on, and that their tendency is to oppose this change of shape."
A treatise on the mathematical theory of the motion of fluids; Horace Lamb; Cambridge Press: Cambridge; 1879; pp 2-3.
"Long-range elasticity has been observed only in substances and materials known to contain flexible molecular chains. The chains may be crosslinked into a network structure, but this is not essential. Stretching produces molecular configurations in which the orientation about the single bonds of the structure is less random than in the unstretched specimen. This produces a decrease in the entropy of the system. Elastic recovery is a result of the tendency for the system to increase its entropy. Energy changes may also take place in the stretching and recovery processes, but they are not necessary for the phenomenon. It can readily be shown by thermodynamic reasoning that, if energy changes are negligible, the elastic modulus should be directly proportional to absolute temperature. Certain rubber samples have, in fact, been found to conform to this relationship."
Physical chemistry of high polymers; Maurice L. Huggins; John Wiley & Sons: New York; 1958; p 102.
"If portions of a mass of liquid are caused to move relatively to one another, the motion gradually subsides unless sustained by external forces; conversely, if a portion of a mass of liquid is kept moving, the motion gradually communicates itself to the rest of the liquid. These effects, which are matters of immediate observation, were ascribed by Newton to a 'defectus lubricitatis,' i.e., a 'lack of slipperiness' between the particles of the liquid, which may be fairly interpreted to mean a property resembling friction between solid surfaces, and Newton, in fact, uses the the term 'attritus,' i.e. friction, several times in the course of his deduction. These terms "internal friction" and "viscosity" have been used indifferently to describe this property of liquids; it is not clear when the latter work acquired a strict technical meaning, since etymologically (from viscum, the mistletoe) it would ween to have been applied originally to liquids possessing the property to an abnormal degree."
The viscosity of liquids; Emil Hatschek; G.Bell and Sons: London; 1928; p. 1.
What is the meaning of elasticity?
Elasticity is a material property characterizing the compressibility of a fluid - how easy a unit of the fluid volume can be changed when changing the pressure working upon it.
Strain in tension: <math>\varepsilon =\frac{\Delta l}{l}\,\!</math>
Strain in shear: <math>\gamma =\frac{\Delta x}{h}\,\!</math> Stress: <math>\sigma =\frac{F}{A}\,\!</math> | |
<math>\begin{align}
& \sigma =G\gamma \text{ for linear-shear elasticity} \\ & \sigma =E\varepsilon \text{ for linear-tensile elasticity} \\ \end{align}\,\!</math> |
& G\text{ or }E=\frac{force}{area}\text{ } \\ & G\text{ or }E=\frac{energy}{volume} \\ & G\text{ or }E=\text{bond energy}\centerdot \text{bond density} \\
\end{align}\,\!</math>
When we stretch a solid, just a little; or when we move liquid, for a very short time, we expect the stress (force per area) to be proportional to the strain: | <math>\sigma =G_{0}\gamma \,\!</math> |
The stored energy per unit volume is the work done by the stress: | <math>w=\int\limits_{0}^{\gamma }{\sigma d\gamma }=G_{0}\int\limits_{0}^{\gamma }{\gamma d\gamma }=\frac{1}{2}G_{0}\gamma ^{2}\,\!</math> |
<math>G_{0}</math> has units of energy/volume. To be “soft” matter the energy per molecule is about kT so that the energy per volume is: | <math>G_{0}\approx \frac{kT}{a^{3}}\,\!</math> |
See Witten, p. 29
Viscosity and molecular properties
Newtonian liquids | <math>\sigma =\eta \dot{\gamma }\,\!</math> | Viscosity is an empirical constant. |
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The work per time is force x velocity. | <math>\begin{align}
& \dot{W}=Force\cdot velocity \\ & \text{ }=\left( \sigma \Delta x\Delta y \right)\left( \dot{\gamma }\Delta z \right) \\ & \dot{w}=\sigma \dot{\gamma }=\eta \dot{\gamma }\cdot \dot{\gamma } \\ & \dot{w}=\eta \dot{\gamma }^{2} \\ \end{align}\,\!</math> |
First calculation of volume work rate. |
Therefore viscosity is energy per volume times time.
Think of flow as repeated steps: Stretch then relax. | “Affine” transition – as if in rubber. | |
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Stress per step: | <math>\sigma =G_{0}\gamma \,\!</math> | |
The energy stored per unit volume is | <math>w=\int\limits_{0}^{\gamma }{\sigma d\gamma }=G_{0}\int\limits_{0}^{\gamma }{\gamma d\gamma }=\frac{1}{2}G_{0}\gamma ^{2} \,\!</math> | |
The work rate is the work divided by a relaxation time: | <math>\dot{w}=\frac{G_{0}}{2\tau }\gamma ^{2} \,\!</math> | The second calculation of volume work rate. |
See Witten pp 28-29.
These two derivations give us two models for the volume work rate.
What is the meaning of viscosity?
The definition of "Viscosity" is the resistance to flow of a liquid (normally).
A high viscosity means that the liquid will not flow easily - imagine treacle.
A low viscosity means that the liquid flows very easily - imagine water or gasoline.
Work (rate) done by viscous flow. | <math>\begin{align}
& \dot{w}=\eta \dot{\gamma }^{2} \\ & \text{or }\dot{w}=\frac{\eta }{\tau ^{2}}\gamma ^{2} \\ \end{align}\,\!</math> |
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Work (rate) done by stretching and relaxation. | <math>\dot{w}=\frac{G_{0}}{2\tau }\gamma ^{2}\,\!</math> |
Combining the two gives: | <math>\eta =\frac{G_{0}\tau }{2}\,\!</math> |
Viscosity is an energy/volume x a characteristic time.