Viscosity

“ But water also offers such resistance to flow, or viscosity, therefore the difference between iron or copper wire and liquid water as regards their fluidity is only a difference of degree, and not of kind; the demarcation between solids and liquids is not a broad, clearly-defined line, but a band of blending shade, the depths of tint representing varying degrees of viscosity.
Multitudes of examples may be cited illustrating the viscosity of bodies that we usually regard as types of solidity,370 such, for example, as the rocks forming the earth’s crust. ”

“ 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. ”

“ The processes concerned would seem to be of a rather intricate character, and largely dependent upon fluid viscosity. It may be noted, indeed, that most of the still obscure phenomena of hydro-dynamics require for their elucidation a better comprehension of the laws of viscous motion. The subject is one which offers peculiar difficulties. In some problems in which I have lately been interested, a circulating motion presents itself of the kind which the mathematician excludes from the first when he is treating of fluids destitute altogether of viscosity. ”

“ All fluids are viscous to a greater or lesser degree; the viscosity of water is small, that of air still less, whilst lubricating oil and treacle are highly viscous substances. Now just as in the study of ordinary mechanics it is found expedient initially to neglect the effects of friction, so, in connection with the present subject, we can afford to ignore the fluid friction to which the viscosity of the fluid gives rise, and in the first instance deal with resistance as a function of density alone. ”

“ I then bring the large magnet near, and you see the result.
It moves almost as freely as in the air itself. Hence we have a most convincing proof that the surface-viscosity of this solution is very much greater than the viscosity of the interior of liquid; and that the resistance offered to motion by the surface is many times larger than that experienced by moving bodies in the interior.
Another experiment will illustrate this enormous surface-viscosity of the mixture of saponine and water in a still more striking way. ”

“ Plasticity and Viscosity.—Every solid substance is found to be plastic more or less, as exemplified by punching, shearing and cutting; but the plastic solid is distinguished from the viscous fluid in that a plastic solid requires a certain magnitude of stress to be exceeded to make it flow, whereas the viscous liquid will yield to the slightest stress, but requires a certain length of time for the effect to be appreciable. ”

“ By the first of these conditions all the terms having ρ as a factor may be neglected, and the equations thus become the equations of equilibrium of the fluid; as such, they are applicable to fluid whether incompressible or elastic, and however the pressure may affect the viscosity. ”

“ The viscosity of a liquid diminishes very rapidly with rise of temperature, without any marked change in the conductivity, whereas the viscosity of a gas increases with rise of temperature, and is always nearly proportional to the conductivity. ”

“ At first sight a solid body appears to have a finite rigidity and an infinite viscosity; a liquid, on the contrary, presents a certain viscosity, but no rigidity. But if we examine the matter more closely, beginning either with the solids or with the liquids, we see this distinction vanish.
Tresca showed long ago that internal friction is not infinite in a solid; certain bodies can, so to speak, at once flow and be moulded. M.W. Spring has given many examples of such phenomena. On the other hand, viscosity in liquids is never non-existent ”

“ When a sphere, either rough or smooth, first strikes the liquid, there is an impulsive pressure between the two, and the column of liquid lying vertically below the elementary area of first contact is compressed. For very rapid displacements the liquid on account of its viscosity behaves like a solid. ”

“ However nowhere in either story is the statement made that the material of the heaviside layer was denser than air. The statement was that it was more viscous. Viscosity is not necessarily a function of density. A heavy oil such as you use in the winter to lubricate your automobile has a much higher viscosity than water, yet it will float on water, i. e. it is less dense. There is nothing in the story that would prevent the heaviside layer from having a coefficient of refraction identical with that of air. ”

“ It is these properties 271 of adhesion and viscosity that cause the 'skin friction' that impedes a ship in its progress through the water, or an aeroplane in going through the air. All fluids have these qualities—and you must keep in mind that air is a fluid, all gases are fluid, steam is fluid. Every known means of transmitting or developing mechanical power is through a fluid medium. ”

“ We know that a body submerged in a fluid experiences, when in motion, considerable resistance, but this is because our fluids are viscous; in an ideal fluid, perfectly free from viscosity, the body would stir up behind it a liquid hill, a sort of wake ”

“ It would seem, therefore, that strong solutions, say of 50 per cent. strength, would be more alike in viscosity than solutions of 5 per cent. strength of the same gums. In other words, the viscosity of a gum solution should be taken as nearly as possible to the strength it is used at, to obtain an exact quantitative idea of its gumming value.
The observation of this fact was one of the circumstances which decided us to use 5 per cent. solutions for the determination of Ghatti gum viscosities, the ratio between the 5 per cent. and 10 per cent. ”

“ When dealing with real liquids not entirely free from viscosity, such resistance as does exist is due, as already mentioned, to skin friction and eddy formation. In the next place, leaving the consideration of the movement of wholly submerged bodies through liquids whether perfect or imperfect, we shall proceed to discuss the important question of the resistance offered by water to the motion through it of a floating object, such as a ship or swan. We have in this case to take into consideration the wave-making properties of the floating solid. ”

“ The viscosity of a substance is measured by the tangential force on the unit area of either of two horizontal planes at a unit distance apart one of which is fixed while the other moves with the unit of velocity, the space between being filled with the viscous distance. ”

“ The more a liquid is expanded by a given change of temperature, the greater will be the difference of specific gravity between the part which is heated and the rest of the mass, and the more rapid, therefore, will be the circulation from the change. Any tenacity or viscosity in the liquid will impede its motion, and when water is thickened with flour, or other farinaceous substances, it parts with its acquired heat very slowly. ”

“ 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. ”

“ This sudden change in the law of resistance to the flow of fluid between solid boundaries, depending as it does on a complete change in the manner of the flow—from direct parallel flow to sinuous eddying motion—serves to determine analytically the circumstances as to the velocity and the thickness of the film under which any fluid having a particular coefficient of viscosity can act the part of a lubricant. ”

“ Viscosity Viscosity is resistance to flow. Seawater is slightly more viscous than freshwater. Its viscosity increases with greater salinity, but the effect is not nearly as marked as that occurring with decreasing temperature. The rate is not uniform, becoming greater as the temperature decreases. Because of the effect of temperature upon viscosity, an incompressible object might sink at a faster rate in warm surface water than in colder water below. However, for most objects, this effect may be more than offset by the compressibility of the object. ”

“ Experiment apart, there is a primâ facie case for the existence of a coefficient skin-friction of some considerable magnitude in the fact that the similar coefficient, as determined by Froude and others, is, in the case of water, a matter of one per cent, or thereabouts, and in the fact that the kinematic viscosity of air is fourteen times as great as that of water. ”

“ Such slipping had shown itself at high exhaustions in the experiments of A. A. Kundt and E. G. Warburg in 1875 on the viscosity of gases; its effects would be corrected for, in general, by a slight effective addition to the thickness of the gaseous layer. ”

“ It is a surprising fact that gases and vapours are possessed of this property of viscosity to a greater degree than are liquids such as water. Owing to these properties, if a solid body is moved through a fluid, more or less of the fluid is dragged along, or if a solid is put in a fluid that is moving it is carried along with the current. Also you are familiar with the great rush of air that follows a swiftly moving train. That simply means that the train tends to carry the air along with it, as the air tries to adhere to the surface of the cars, and the particles of air try to stick together. ”

“ Very few people would think of including pure water, far less spirits of wine, in a list of sticky, or viscous liquids; and yet it is quite easy to show by experiment that even these fluids possess some degree of stickiness, or viscosity. An illustration may be afforded as follows: We provide several very large glass tubes, nearly filled respectively with quicksilver, water, alcohol, glycerine, and oil. A small space is left in each tube containing a little air, and the tubes are closed by corks. ”

“ It might, without departing from the form of the expression, be possible to establish an empirical relationship, and it is in any ease of interest to endeavour to ascertain the probable magnitude of the constant for the particular case when viscosity becomes vanishingly small. ”

“ While incapable of possessing different elasticity in different directions, and thus denuded of the rich geometrical complexity of solids, they retain the variety of density, colour degrees of transparency, great diversity in surface tension, viscosity, coefficients of expansion, compressibility, and many other properties which we observe in solids, but not for the most part in gases. Though our knowledge of the physical properties of liquids is much wanting in generality at present, there is ground to hope that by degrees laws connecting and explaining the variations may be traced out. ”

“ The paraffin oil used in these experiments had, at a temperature of 12°·5 centigrade, a specific gravity ·840 and a surface-tension about ·39 of that of water. Since only a small increase of height was required with this liquid to make a smooth sphere give [pg 102] the same splash as a rough one, this liquid was found much more convenient than water in investigating the transition.
When water is made more viscid by the gradual addition of glycerine, [H] the surface-tension and the specific gravity are but little altered though the viscosity is steadily and sensibly increased. ”

“ A 10 per cent. is invariably more than twice as viscous as a 5 per cent. solution. The following curve was obtained from one of the Ghattis. Similar results were shown by other gums.
Variation of Viscosity, with Dilution. Ghatti No. ”

“ This implies that the tension of the free surface of the solid is greater than that of the surface of contact of the solid with the liquid . Now consider the tension of the free surface of the liquid . All round its edge there is a tension acting at an angle with the vertical. The circumference of the edge is , so that the resultant of this tension is a force acting vertically upwards on the liquid. Hence the liquid will rise in the tube till the weight of the vertical column between the free surface and the level of the liquid in the vessel balances the resultant of the surface-tension. ”

“ Fluid. A form of matter, in which its particles readily flow, or slide, over each other. Airs, or gases, are called elastic fluids, because they are readily reduced to a smaller bulk by pressure. Liquids, are denominated non-elastic fluids, because they suffer but little diminution of bulk, by any mechanical force.
Focus. That point in which converging rays unite.
Force. That power which acts upon a body, either tending to create, or to stop motion.
Fountain. A jet, or stream of water, forced upwards by the weight of other water, by the elasticity of air, or some other mechanical pressure. ”

“ We will now consider the stability of a fluid enclosed in a rigid envelop which is non-conducting to heat and impermeable to all the components of the fluid. The fluid is supposed initially homogeneous in the sense in which we have before used the word, i.e., uniform in every respect throughout its whole extent. Let be the ultimate components of the fluid; we may then consider every body which can be formed out of the fluid to be composed of , and that in only one way. Let denote the quantities of these substances in any such body, and let , denote its energy, entropy, and volume. ”