Popular Science Monthly/Volume 43/August 1893/Why a Film of Oil Can Calm the Sea
|WHY A FILM OF OIL CAN CALM THE SEA.|
NEXT to the oil which is used in the beacons of the world to give light to save life, that which is most effective in forestalling the loss of life and the destruction of property is the quantity that is expended by mariners in forming a film around their vessels to subdue the violence of breaking waves. The extensive practice of using oil for this purpose is the outgrowth of an age of quick ocean passages which has impelled seamen to crowd on every foot of canvas and every pound of steam in the attempt to run through storm and calm alike. In any large seaport a visit to the docks where mariners tell the experiences of their voyages will afford evidence of the extent and efficacy of this practice; but, before proceeding to point out the principles involved, it will be of interest to give extracts from the log-books of a few vessels, to show the manner and effect of the use of oil. From the official notes of Captain Tregarthen, of the British steamship Marmanhense, the following extract has been made: "On March 3d, off Cape Hatteras, in a very strong northwest hurricane, finding the ship could make no headway, I hove to. The wind was blowing in hurricane force from the northwest, and the tremendous sea which was running broke on board and did great damage. The vessel was very unsteady, coming up and falling off several points, so that I could not steer her nor keep her head to the sea, although the engines were working well. I filled the water-closet bowls with oakum and poured fish oil over it, keeping men stationed by them to replenish the supply. I also filled a small canvas bag with oakum, saturated with the same kind of oil, and towed it by a line from the weather bow of the vessel so that it would drift several fathoms to windward. The vessel now rode much more easily and could be kept head to sea. Moreover, no water came on board, and the sea was without breaking crests for thirty yards to windward of her. I feel no hesitancy in stating that, with the proper use of oil, I shall be perfectly willing to encounter the hardest gale that ever blew; and intend at the first opportunity, to stop the engines, place several oil bags to windward, and let the vessel drift as she will. I feel sure that the vessel will be safe under these conditions."
Captain Bower, while on a voyage from New York to the Mediterranean last December in the steamship Ponca, encountered a strong gale with very high seas. He says: "The vessel was deeply laden with grain and became unmanageable. We were running before the seas and shipping large quantities of water, until two small bags filled with colza oil were put over on each side of the bridge. This oil was found to be too light and of little use; but after olive oil was put in the bags no more water was shipped and the decks became almost as dry as in fine weather, although the gale continued for two days. The vessel was drawing twenty-six and a half feet of water, and, if we had not used oil, I do not think she could have withstood the storm."
Captain William Peake, master of the schooner J. F. Krantz, while making a passage from Port Spain, Trinidad, to Boston, met a terrific gale off Cape Hatteras and had the following experience: "The sails were blown away, men washed from the pumps, and boats and other things above the deck wrecked by the heavy seas. I was compelled to head southward and scud under bare poles. Then I thought of oil, and determined to see what effect it would have on the sea. Two wooden, ten-gallon kegs, containing boiled linseed oil, were lashed to the quarters of the vessel. The oil was allowed to ooze out through two small holes in the heads of the kegs. The effect was all that could be desired. After the oil had spread, no water came on board, the men returned to the pumps, the vessel was pumped out, and the decks were cleaned up. During the sixteen hours in which oil was used eight gallons were expended."
An examination of thousands of reports like the preceding ones demonstrates that a small quantity, say two quarts per hour, of the thick and heavy oils, especially those of animal and vegetable origin, when allowed to drop into the sea soon spreads over its surface, forming an oily layer within the area of which the waves, instead of breaking, become huge rollers upon which the vessels rise and fall without shocks and without shipping any water.
So much for the practical effects of oil on broken water. Now let us proceed to examine the reasons why so small a quantity of oil can produce these effects. In order to understand the methods for opposing the violence of waves, it is essential that the phenomena which constitute wave motion be understood. It can be said with some degree of confidence that there is no instance in Nature of a perfectly quiescent surface of water. Air and water are both mediums of extreme mobility, and the individual molecules of both, and of all other substances, are continually in a state of motion, with different velocities, in paths different in direction and length. There is thus a continual interlacing of particles. When air covers water, some of the particles of air, in their excursions, strike the surface of the water, producing unequal pressures upon it, and giving rise to ripples which the vision is not acute enough to detect. If the original surface of the water were perfectly smooth, and if all parts of it continued equally exposed to an equal wind, waves could not be produced. But with the minute corrugations which are always present upon the smoothest water it is to be observed that it does not occur that water is all equally exposed to equal winds. The pressure of moving air upon the crests and posterior portions of the minute corrugations is greater than that on the hollows and anterior portions. There is thus a tendency to heap up the water at the places of greatest pressure, which is augmented by the rotational or vortex motion produced by the viscosity of the air. These actions produce new forms and inequalities, which, exposed to the wind, generate new modifications of its force and give rise to further deviations from the primitive condition of the fluid. Imagine an isolated example in which the water has been suddenly heaped up by a gust of wind. The action of gravity causes the particles of water in the heap to push forward the particles immediately in front of them out of their former place to another place farther on, and they repose in their new place at rest as before the original heaping up. Thus in succession volume after volume continues to carry on a process of displacement which only ends with the exhaustion of the displacing force originally impressed and communicated from one to another successive mass of water. As the particles of water crowd upon one another in the act of going out of their old places into the new, the crowd forms a temporary heap visible on the surface of the water, and as each successive mass is displacing its successor there is always one such heap, and this heap travels apparently at that point where the process of displacement is going on; and although there may be only one crowd, yet it consists of always another and another set of migrating particles. This moving crowd constitutes a true wave. The velocity of the wave is the velocity with which the heap is seen to move. Its form is the form of the heap. Its length is the distance from crest to crest, and its height is the distance from the crest to the surface of the water before the disturbance.
The motions of the individual particles of water are different from the motion of translation which the wave has. Consider a Fig. 1. particle in a mass of water about to be traversed by a wave form. The action of gravity on the heap behind it tends to press it forward, where it is confronted by a solid wall of water. Under the action of these two opposite forces the particle is driven upward and forward until the particles which have displaced it have made room for themselves; then it sinks, and finally comes to rest a little in advance of the place from which it started. The motion of migration of each individual particle is thus in a closed orbit. The propagation of the wave is the advancement of a mere form. The actual translation of water in the propagation of unbroken waves is small. The motion of each particle takes place in a vertical plane parallel to the direction of propagation of the wave. The path of orbit described by each particle is approximately elliptic, and in water of nearly uniform depth the longer axis of the elliptic orbit is horizontal and the shorter vertical. When at the top of its path, the particle moves forward as regards the direction of propagation; when at the bottom, backward, as shown by the Fig. 2. curved arrows in Fig. 1. The straight arrow denotes the direction of propagation. The particles at the surface describe the largest orbits. The extent of the motion horizontally and vertically diminishes with the depth below the surface. A particle in contact with the bottom of water of moderate depth moves backward and forward in a horizontal straight line, as at D. On the ocean, where the water is deep as compared with the length of a wave, the paths of the particles are nearly circular, and the motion is insensible at great depths.
When waves are first raised at sea their crests are smooth and rounded, as represented in Fig. 2. As the wind freshens the crests rise higher and become moro acuminate. Rankine has investigated the limiting forms which waves assume before breaking, and has concluded that in the steepest possible oscillatory waves of the irrotational kind, the crests become curved at the vertex in such a manner that a section of the crest by the plane of motion presents two branches of a curve which meet at an angle of ninety degrees.
After the prolonged action of the wind, when the crests of the waves rise to a considerable height and become sharper and Fig. 3. sharper, the passage of the air over them with high velocity tends to impart its velocity to them. Owing to the inertia of the lower masses of water, the imparting of this velocity is resisted. The paths of the particles become distorted, as shown in Fig. 3, the front of each wave gradually becomes steeper than the back, and the crests seem to advance faster than the troughs, until at length the front of the wave curls over and breaks, as shown in Fig. 4.
Large sea-waves seem to be the result of a building-up process carried on by the joint action of large and small waves. If, for any cause, there be one wave larger than those surrounding it, its size will be continually increased at the expense of the smaller ones. For these smaller waves, in passing over the tops of the larger, offer increased obstruction to the wind and cause the formation of cusps when the waves coincide. The delicate equilibrium incident to a cusped form is easily destroyed by the action of the wind, and the crests of the waves break into fragments which go to increase the volume of large waves, leaving the small ones yet smaller. Therefore, whatever influence prevents the Fig. 4. breaking of waves acts also as an agency to prevent their increase in size. No fact of observation and no method of sound reasoning has yet led to the conclusion that the spreading of oil on the surface of water agitated by waves can exercise any sensible effect in lessening the size or velocity of the waves themselves. It is in the breaking of the waves that the oil finds its field of action.
Having reviewed the structure of sea waves, the next step is to show why oil spreads over the surface of water. There is an attraction of one particle of water for another, and there is an attraction of one particle of oil for another, but there is a repulsion between a particle of water and a particle of oil. If we attempt to mix oil and water, the two liquids separate from each other of themselves, and in the act of separation sufficient force is brought into play to set in motion considerable masses of the fluids.
Imagine an individual particle of water within a mass of water. The particles on every side of the individual particle attract it, and the attraction of opposite particles on every side tends to neutralize each other, so that the individual particle has almost perfect mobility. The surface particles, however, inasmuch as all the rest of the fluid is below them, are drawn inward toward the mass of the fluid, and a certain tension is produced. This tension Fig. 5. is potential energy, and is inherent in the surface particles in virtue of their position. If we consider an oily film to be spread over the surface of a body of water, it will appear that the particles near the surfaces which separate the oil from the water and from the air must have greater energy than those in the interior of the film. The excess of energy due to this cause will be proportional to the area of the surface of separation. When this area is increased in any way, work must be done; and when it is allowed to contract, it does work upon other bodies. Hence it acts like a stretched sheet of India rubber, and exerts a tension of the same kind.
In the above figure, which represents an exaggerated picture of a layer of oil on the surface of a body of water, let Taw represent the superficial tension of the surface separating air from water; let Tao represent the superficial tension of the surface separating air from oil; let Tow represent the superficial tension of the surface Fig. 6. separating oil from water; and let P be a point of the line forming the common intersection of the surfaces separating the air, oil, and water. For the equilibrium of these three media, the three tensions Taw, Tao, and Tow must be in equilibrium along the line of common intersection, and since these tensions have been measured and are known, the angles which their directions make with one another can be easily determined; for, by constructing a triangle, ABC, having sides proportional to these tensions, the exterior angles will be equal to the angles formed by the three surfaces of separation which meet in a line. But it is not always possible to construct a triangle with three given lines as its sides. If one of the lines is greater in length than the sum of the lengths of the other two, the triangle is impossible. For the same reason, if any one of the superficial tensions is greater than the sum of the other two, the three fluids can not be in equilibrium in contact.
If, therefore, the tension of the surface separating air from water is greater than the sum of the tensions of the surfaces separating air from oil and oil from water, then a drop of oil can not be in equilibrium on the surface of water. The edge of the drop where the air meets the oil and the water becomes thinner and thinner, till it covers a vast expanse of water.
M. Quinke has determined the superficial tensions of different liquids in contact with one another and with air, and the following is an extract from his table of results. The tension is measured in grammes per linear centimetre at twenty degrees centigrade:
|Liquid.||Specific gravity.||Tension of surface
|Tension of surface
Although olive oil is here taken as the representative of oils, it is not considered so well adapted for use at sea as some of the others. Whale oil has given the best results, but its surface tensions do not seem to have been determined. It may be presumed that they do not differ greatly from the values given for olive oil.
An inspection of the above table will show that the tension of the surface separating air from water is greater than the sum of the tensions of the surfaces separating air from oil and oil from water, which explains why a film of oil will spread over the surface of a body of water.
Through the operation of surface tensions much of the force which breakers have is lost. Let us imagine a "break" to occur after the surface of the water is covered by the oily film.
 For every square centimetre of film torn asunder there will be destroyed ·05856 centigrammetre of potential energy, being the sum of ·03760 and ·02096, the potential or surface energies, in centigram-metres per square centimetre, of the surfaces separating air from oil and oil from water; and there will be generated for every square centimetre of free surface of water formed, ·08235 centigrammetre of potential energy. The mere fact of breaking the film of oil causes an expenditure of energy, because it lays bare a surface having a tension greater than the sum of the tensions of the surfaces separating air from oil and oil from water. But there is a further loss of energy in these circumstances. Suppose after a "break" has occurred, a layer of water glides over a layer of oil. The superficial energy in the surface separating the oil from the air, amounting to ·03760 centigrammetres per square centimetre, is replaced by ·10331 centigrammetre per square centimetre, being the sum of ·08235 and ·02096, the superficial energies per square centimetre of the surfaces separating air from water and water from oil respectively. Therefore, when water breaks over an oily film, there is required for the formation of each square centimetre of a layer of water on the oily film, ·10331 minus ·03760, or ·06571 centigrammetre of work.
The film of oil also acts as a shield to prevent the derangement of the wave mechanism and to prevent the growth of waves and the formation of sharp crests. It has been pointed out that, when waves are propagated across any body of liquid, the individual particles of the liquid, having their centrifugal and centripetal forces in equilibrium, describe closed orbits. At the highest points of these orbits, or in the crests of the waves, the particles are moving in the direction of propagation of the waves.
When the wind is blowing over the waves with a velocity greater than the velocity of propagation, and in the same direction with it, the moving air tends to impart to the particles of water a velocity additional to the normal velocity of revolution in their orbits, causing the distortion of the orbits and the disintegration of the crests of the waves. The force which the moving air exerts to draw the water along with it is due to the viscosity of air.
When wind blows over water, all the air does not pass over the surface of the water. On account of the high degree of adhesion between air and water, a thin stratum of air remains in contact with the water, and it is the action of the internal friction or viscosity of air tending to draw this stratum along which causes the tractive effect of wind on water.
When a film of oil is spread over the surface, this tractive force is not brought to bear on the surface of the water as long as the film remains unbroken, but acts upon the surface of the film, whose particles, being entirely separate from the particles of water, do not share their motion. The surface of the water is thus shielded from the action of the wind in the same manner as if a skin of India rubber were spread over it, and the only action of the wind in such a case is to move the film over the surface of the water.
It is calculated that a wind moving at the rate of twenty-five miles per hour or one hundred and twelve centimetres per second, relatively to the surface of the water, exercises a tractive effect of about two thousandths of a gramme upon each square centimetre of surface; and, when we consider that this force is brought to bear upon a system of particles moving in their orbits, in the direction in which the wind blows, with a speed of about eighty centimetres per second, it will be apparent that the interposition of a film of oil between the air and water must have a powerful effect in preventing breaking crests.
Observation has shown that, in the generation of oscillatory waves, ripples or capillary waves are first formed, and that it is to the union of conterminal ripples and to their more abundant formation with the increased force of the wind that the growth of waves is due. The existence of a certain definite tension, equal to ·08235 gramme per lineal centimetre, at the common surface of air and water has been pointed out. The water surface under this tension is in perfect equilibrium.
When wind blows over the surface of a body of water, the tangential force which the air, in virtue of its viscosity, exerts on the surface of the water, is of different degrees of intensity at different places, owing to the minute corrugations which are always present on the surface of a body of water, and to the eddying motion of the air. At the places where the tangential force is greatest, the surface film of water is drawn along and the portions of the surface immediately in front of them, destroying their surface tension or energy of position, and, by laying bare new surface in places from which they are moved, generating a like amount of surface tension. Through this action heaps or ripples are formed, and surface tension is being constantly generated and destroyed. The formation of ripples takes place on waves already in existence in the same manner as upon a surface of water originally at rest, and by continually uniting with the larger waves they impart those dangerous qualities to the wave which result from high and acuminate crests.
When a film of oil is spread over the surface of the water, this heaping-up action, which in the case of the water film results in the formation of ripples, can not take place. In the figure, let Fig. 7. A represent the crest of a wave covered by the film of oil B C, and let P be a point of greatest action of the tangential force of the wind, which is supposed to move in the direction of the arrow. The tendency of this action is to drive the film into a heap immediately in front of P. By this action a greater tension is generated in the film at b and a lesser tension at a. The greater tension at b tends to draw the portion at b' ahead, and the lesser tension at a allows the tension at a' to draw the portion at a ahead; so that, instead of a tendency toward heaping up, there is a tendency to move the entire surface film along at a uniform rate. The formation of ripples is therefore stopped, and the growth of waves and the formation of breaking crests, as far as they result from this cause, are prevented.
- Above it has been assumed that the superficial tension per unit of length has the same numerical value as the superficial energy per unit of area, which can be proved as follows:
Let the equation to the curve B C A be y = f(x). Take any ordinate, as C D, whose length is y, and let the whole tension exerted across the line be represented by Φ, then the superficial tension is measured by the tension across a unit length of y, or, since Φ is the tension across the whole ordinate y, if T, which is constant, is the superficial tension per unit of length, Φ = Ty = T.f (x). Suppose that the variable ordinate y is originally in contact with the axis O B, and that the surface included between the curve and the two axes is produced by drawing the ordinate y away from the axis O B toward the right by the action of the force Φ. If we consider O B and D C, which is equal to y, to be two rods wet with oil and placed between the curve and the axis of X, and then drawn asunder, the oily film B C A D O will be formed. Let E represent the superficial energy per unit of area. Then the work done in forming the film will be = E∫f(x)dx. But if Φ is the variable force required to draw the ordinate y from the axis O B, the same work may be written = ∫ Φ dx. Therefore, work = ∫ Φ dx = E∫f(x)dx(1). Substituting the value Φ = Tf(x) in (1), we have T∫f(x)dx = E∫f(x)dx, or T = E, or that the numerical value of the superficial tension per unit of length is equal to the superficial energy per unit of area.