FRICTION AND VISCOSITY OF FLUIDS.
Frictional Resistance.—The resistance with which bodies oppose the movement of one surface on another is termed friction. It depends on the nature, and the roughness of the surfaces in contact; at the commencement of the sliding, it is greater than when the motion is continued.
Friction is in effect an equivalent force exerted in a direction opposite to that in which the sliding occurs. Its whole amount is the product of two factors: the first of these, which sums up the effect of the nature and condition of the surfaces, is called the coefficient of friction; the second, which is the sum of all pressures, as weight strain, and the adhesion due to magnetism (when employed), which act to urge the two bodies together, i.e., perpendicularly to the surface of contact, is called the normal pressure. But this law holds only where, with dry surfaces, the pressure is not enough to indent or abrade either; or, with wet surfaces, not enough to force out the unguent. In either of these cases, the friction increases more rapidly than the ratio of normal pressure.
No surfaces can be made absolutely hard or smooth; when one surface is made to slide over another, the slight roughness of the one interlock with those of the other, so that the surfaces must be separated or the points abraded to allow of the motion; but if one surface roll upon another, the prominent points are successively raised, without the need of complete lifting of the body or wearing off those points. Hence, there are two kinds of friction, the sliding and the rolling. The former of these in amount greatly exceeds the latter; it is a leading element in the stability of structures and fabrics of all lands, and the most important resistance and source of waste in all machinery, and is therefore a chief object of regard in the arts of construction and the science of engineering.
Sliding friction increases with the roughness of the surfaces in contact; hence, it is in practice diminished as these surfaces become worn, also by polishing, and by the use of lubricants, which smooth the rubbing surfaces by filling their depressions. It increases, almost universally, in exact proportion with the entire pressure, owing to weight or other causes, with which the two surfaces are held together; but at very great pressures, somewhat less rapidly. Consequently, in all ordinary cases, so long as the entire weight or pressure remains the same, the friction is, in general, entirely independent of the extent of the surfaces in contact.
The exceptions are, some increase when the rubbing surfaces under the same total pressure are very greatly extended, or when either surface is comparatively soft; and considerable lessening of friction when the bodies are very small, as in the runners of skates upon ice. For ordinary rates of motion, the total friction within a given space or distance is in like manner entirely independent of the velocity with which one surface is caused to move over the other; but in very slow motions it is increased, and in very rapid motions perceptibly diminished.
Friction is also increased in proportion to the tendency of the surfaces to adhere; hence, it is usually found greater between bodies of the same kind (steel on steel proving almost an exception) than between those of different kinds; it is usually greater when the surfaces have been long in contact, and at the beginning of motion, and always so, unless corrected by lubricants, between metallic surfaces so highly polished that air may be excluded from between them.
The frictional resistance retarding the flow of water is subject to three laws:
1. It is proportioned to the amount of surface in contact.
2. It is independent of the pressure.
3. It is proportional to the square of the velocity.
It should be remembered that the laws relating to friction, between solid bodies operate quite differently from what they do when applied to liquids; hence, the large mass of data relating to the general subject of friction must be disregarded in the consideration of hydromechanics and allied subjects.
For all fluids, whether liquids or gaseous, the resistance is independent of the pressure between the masses in contact. This is in accordance with the second law as stated.
The friction for all fluids (liquid or gaseous) is in proportion to the area of the rubbing surface; this follows from the first law and as the sectional area of a circle is the least, pipes from their circular form present the smallest resistance to the flow of water.
From the third law, in practice we desire the making of pipes as large as possible; experiment having proved that low speeds are preferable to moderate and still more so, as compared to high speeds in proportioning the piping of hydraulic apparatus.
Friction of fluids is also independent of the nature of the solid against which the stream may flow, but dependent to some extent upon their degree of roughness.
Friction for all fluids is also proportional to the density of the fluid, and related in some way to its adhesiveness.
Water flowing through a pipe tends to drag the pipe along with it on account of friction; in all actual fluids there is viscosity or internal friction, but if the relative motion is only slow enough it makes little difference whether the fluid is viscous or not.
Ordinary fluids will change in shape under the action of a force, however small, if enough time is given for the change to take place, and the rate of change of shape is a measure of its viscosity.
The laws of friction, both for solids and liquids, have been established from experiments endlessly varied. In investigating these principles we first proceed on the supposition that the forces in question act without any impediments. Great simplicity is attained by first bringing the subject to this ideal standard of perfection, and afterwards making suitable allowances for all these causes which operate in any given case to prevent the perfect application of the law.
Several tables and other data relating to the friction of water will be found in the other sections of this work, reference to which is made in the Index.
The term viscosity has been described in the Glossary at the beginning of this work; a perfect fluid is incapable of resisting—except by its weight or inertia—a change of shape. Such a substance does not actually exist for all fluids have viscosity or internal friction.
This is defined as a resistance to a change of shape depending on the rate at which the change is effected, but, as the fluids which engineers have to deal with are water and vapors and gases, it simplifies nearly all the calculations to assume that they have no internal viscosity or friction.
A slow continuous change of the shape of solids or semi-solids under the action of gravity, or external force, is also by the extension of the term called viscosity, as the viscosity of ice, as observed in the slow movement of those rivers of ice, the glaciers.
The viscosity of liquids arises from the mutual attractions of the molecules and is diminished by the effect of the wandering molecules (C. D.). The viscosity of gases increases while that of liquids diminishes as the temperature is raised.
The viscosity of fluids presents a certain analogy with the malleability of solids.
Vis Viva is equivalent to active or living force; temperature is the Vis Viva of the smallest particles of a body; in bodies of the same temperature the atoms have the same vis viva, the smaller mass of the lighter atoms being compensated by their greater velocity.
This term, which is often met with in scientific treatises, was invented by Leibnitz. It is well known that water in rapid circulation will absorb more heat than when stagnant or moving slowly; this is caused by Vis Viva of the atoms.