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CHILDREN'S SCIENTIFIC LABORATORY
Big spoon of nature. Children's Science Lab
Directory / Children's Science Lab In February 1970, not far from the island of Martinique in the Caribbean, three American scientists - G. Stommel, L. Howard and D. Nergard - with enviable persistence tried to drive a kilometer-long plastic gut under water, like the one that gardeners use to water flowers and trees. The flexible gut got tangled and broke, causing a lot of trouble for scientists, but they still achieved their goal: in the end, the gut "hung" vertically - from the surface of the water to a depth of 1000 meters. And then the scientists saw what they wanted to see: they experimentally tested the theoretical propositions expressed 14 years earlier by G. Stommel, A. Arong and D. Blengard in the work "Oceanographic Riddle", and made sure that these provisions are true. The authors of this theoretical work, having studied the distribution of water density depending on its salinity and temperature in different areas of the World Ocean at different depths, came to the conclusion that if, for example, in the Sargasso Sea near Bermuda, a copper pipe, say, 1000 meters long, is lowered vertically and an inner diameter of 2 centimeters so that the end does not stick out too high above the water, then it will be possible to observe an amazing phenomenon, which the authors called the "eternal salt fountain". To start this fountain, it is enough to connect the upper end of the pipe to the pump, turn it on and keep it on for exactly as long as it takes to raise a portion of less salty water from a thousand meters deep. After that, the pump can be disconnected, and the water from the pipe will fountain on its own. The fact is that the pump draws cold water, less salty than water in higher layers, into the pipe from a thousand-meter mark. Rising up, the water heats up somewhat, receiving heat through the pipe walls from the somewhat warmer water of the upper layers. The copper walls of the pipe provide heat exchange, but not salt exchange, so that the water in the pipe becomes warmer as it moves up, remaining slightly salty, and therefore relatively less dense. Thus, a column of water contained in a pipe is lighter than an equivalent column of water outside the pipe. The difference in weight gives rise to a difference in pressure, which ultimately causes the less salty water to rise up the pipe. If the end of the pipe does not protrude too high above the surface, then there will be enough excess pressure to actuate the "perpetual fountain", and less salty water will continuously pour out of the protruding end of the pipe. This process will continue until the water in the Sargasso Sea is well mixed, that is, almost indefinitely.
Having received a salt fountain 60 centimeters high, scientists suddenly began to doubt: what if not the difference in density, but the waves on the surface make the water rise up? The waves move a flexible, elastic hose attached to the float, and perhaps turn it into a kind of pump, which just feeds the "eternal fountain" with energy. The repetition of the experiment with a rigid hose made it possible to eliminate doubts: the salt fountain worked in this case too. Let's try to get a salt fountain and we. You don't need a kilometer hose for this, and instead of Bermuda, we just have to go to the kitchen. And the tropical ocean, where the water is warmer and saltier at the surface and colder and less salty at depth, we will model using a wide pan. We will also need a plastic cup, say, from under the Volna cheese, in the bottom of which a hole should be pierced with a pin.
To begin with, pour cold tap water into the pan-ocean so that the depth of this bottom layer is 3-4 centimeters. We put a plastic cup with a hole upside down in the water. Now, very carefully, so as to avoid mixing as much as possible, we will pour warm water into the pan until cold water appears from the hole in the glass. And finally, let's simulate the surface layer of the tropical ocean - for this (again with extreme caution) we will pour a thin layer of hot salty water on top of the layer of warm water. Ocean is ready. If you now drop paint or ink over the hole in the cup, you can see that a small fountain of water beats out of the hole, simulating an oceanic salt fountain. The water flowing out of the cup has about the same temperature as the water outside it at the same depth, but it is less salty, and therefore lighter. This causes the water to flow out of the cup. The fountain will operate until the salt and heat are distributed evenly throughout the volume of our "ocean". salt fingers Due to the fact that in saline solutions heat spreads much faster than salt - about - once in a hundred - in the ocean, under certain conditions, there can be a kind of natural copper pipe, or rather, many small tubes - invisible channels through which movement occurs water in water. If a layer of warm salty water is placed above a layer of cold, not very salty water, then miniature salt fountains, called "salt fingers", are formed on the interface - streams of less salty water beat up, separated from each other by falling strings of more salty water.
It was not possible to observe salt fingers directly in the ocean, but in the kitchen, please! To do this, it is only necessary to pour tinted salty hot water into a glass of cold tap water. Pour, of course, should be very careful, so that the interface between cold and hot water is fairly clear. To get a clear interface between the layers of water in the "ocean", D. Walker advises pouring hot water from a small height onto a piece of floating plank; K. Stong recommends using a paper circle lowered on a string to the very surface of cold water in a jar.
In a few minutes, after the model is ready, salt fingers will grow on the interface, from 1 to 5 centimeters long and about a millimeter thick. This phenomenon lasts for quite a long time - from several minutes to several hours. The emergence and development of salt fingers can be explained by wave excitation, which deforms the initially calm interface. Drops of cold water move up into hot water and vice versa. Due to the difference in the rate of heat propagation and salt diffusion, the droplets that are on top of the dividing line basically only heat up, the salt concentration in them hardly changes, they become lighter and continue to rise; droplets that find themselves below the dividing line give off heat, become colder, become heavier and sink.
Due to large heat losses through the walls of the vessel, the experiment with fingers in a salty-warm environment is not always immediately successful. The English physicist S. Turner proposed for the experiment a more rational salt-sugar system formed by two solutions. The first solution is salty-sweet: two and a half teaspoons of salt and one teaspoon of granulated sugar per glass of tap water. The second solution is sweet-salty: two teaspoons of sugar and one teaspoon of salt in a glass of tap water. First, a salty-sweet solution is poured into a glass jar - it forms the bottom layer of the entire system. Then, very carefully, keeping the interface, the sweet-salty solution is poured into the same jar; it must be tinted (ink "Rainbow" - blue or red). Salt fingers will appear within an hour and last for several hours. The growth rate of the fingers in this experiment depends on the rate of salt diffusion, and their very appearance is due to the fact that salt diffuses faster than sugar. The upper layer (sweet-salty) has a lower density than the lower one, and the boundary between the layers, it would seem, should be stable. But a random initial instability sends a small amount of the sugar solution down, and the salt penetrates the resulting bulge faster than the sugar diffuses into the surrounding brine. The bulge with the addition of salt becomes denser than its surroundings and rushes down, forming a finger. In the same way, a small bulge of salt water from the lower, denser layer, penetrating upward into the sweet-salty solution, loses its salt faster than it acquires sugar, becomes lighter than its surroundings, and rushes upward in the form of a growing finger. Salt Oscillator And finally, another amazing experience based on the difference in densities of salt and fresh water. For the experiment, you will need a glass jar from canned vegetables or a thin tea glass, an aluminum cartridge from under validol or photographic film. You can also use a plastic cup from under some medicine. Pierce the bottom of the glass with a needle, preferably heated, so that the edges of the hole are smooth. It is easy to punch a hole in an aluminum cartridge with the same needle.
Pour cold water into the jar almost to the rim. Prepare salt water (one to one and a half teaspoons of salt per glass of water), tint it with rainbow ink (blue or red). Fix the cup in a cardboard holder by cutting a hole in it according to the diameter of the cup. Then lower it into the jar and, while pouring the salt solution, make sure that the water level in the glass becomes slightly higher than in the jar. Now watch what happens. The denser, heavier salt water begins to flow through the hole in the glass into the fresh water. It can be assumed that it will flow evenly until the level of brine in the glass decreases so much that the pressure of the outflowing brine equals the pressure of fresh water in the jar at the level of the hole. It all seems to be happening. The tinted stream becomes thinner and disappears. All? No, after a while the jet appears again and disappears again. This goes on for quite some time.
What happens in the glass at the time when the jet has stopped is easy to guess, remembering the first experience: there is a fountain of fresh water - from the bottom of the glass, more precisely, from the hole, fresh, that is, lighter, water rises up through the thickness of the brine. If fresh water were tinted, we would be able to observe this fountain. Thus, a certain oscillatory system was obtained, which was called the "Martin salt oscillator" after the scientist who first discovered this effect in 1970. The oscillation period of the oscillator depends mainly on the size of the hole and the temperature of the fresh water. The operation of the oscillator is based on the same mechanisms as in previous experiments.
A. The system is in equilibrium. Below the hole in the glass is fresh cold water; above the hole is a denser liquid, brine. B, C. The emergence of Rayleigh-Taylor instability, "swing" and the beginning of the upward flow of fresh water. A salt oscillator, writes D. Walker, is an example of a system that begins to oscillate after self-excitation due to Rayleigh-Taylor instability (instability at the interface of a layer of a denser liquid lying above a less dense one when the interface is in hydrostatic equilibrium), followed by a rapid excitation (buildup) at the interface between two liquids. In other words, in our experiment, despite the equalization of pressures in the hole, a layer of a denser liquid lying above a layer of a less dense liquid is unstable and is subject to some weak, random perturbations. Such perturbations generate a slight bulge at the interface between two fluids. Due to the density difference, some of the less dense liquid is on top of the old interface and some of the denser liquid is pushed down. This instability quickly increases, the salt oscillator begins to operate. Fresh water, penetrating upward, accelerates its flow through the hole, because it is lighter than salt water at the same level on the other side of the hole. A fountain of fresh water begins to beat, and there comes a moment when this jet stops the outflow of salt water. Pumping water into the cup gradually leads to an increase in the height of the liquid in it and, consequently, to an increase in pressure at the level of the hole. The loss of water from the jar slightly reduces the level of water in it, since the jar is wider than the cup. Finally, there comes a moment when the pressure of salt water in the hole becomes great enough to reduce, and then completely stop the fresh water fountain. The cycle is over. There is now too much water in the cup and the jet reappears. Gradually, the flow decreases until the pressure at the orifice is equalized again. Then some random perturbation again causes a bulge on the interface - a fountain of fresh water appears. Thus, the flow alternates: either up or down - this is the salt oscillator. The flow rate depends on the diameter of the hole in the cup and on the viscosity of the liquid. As in the previous experiment, you can try other liquids, it is only important that they differ in density and do not mix, as, for example, alcohol and water mix. D. Walker reports that he tried to work with water, slightly tinted blue, and a solution of molasses, tinted red, and observed, according to him, an almost fabulous spectacle. For the device of the oscillator, S. Martin used a medical syringe. The oscillation period was in this case equal to 4 seconds, and the period of operation of the oscillator was 20 cycles. Our oscillator with an aluminum cartridge from validol, lowered into a tea glass, worked with a 10-second cycle for an hour.
A large oscillator, made up of a five-liter jar and a polyethylene bottle of Iskra-2 bleach, in a saline solution slightly sweetened with sugar and heavily tinted with blue ink, gave a long stream with a 20-second cycle. In addition to the vortex "umbrella" at the end of the string, which appears at the beginning of each cycle, vortex rings can also be observed here. They move down, overtaking, penetrating each other, and blur at the very bottom of the can. Some of the rings were photographed.
We talked about three experiments based on differences in the density of salt and fresh water. In nature, the vertical mixing of ocean waters, caused by the difference in densities, is of great importance for the life of the entire ocean. Thanks to him, the solar heat, absorbed by a thin layer of water, spreads into the depths. (Reference from TSB: a layer only 1 centimeter thick absorbs 94% of the solar energy incident on the surface of normal sea water and brine formed a warm water zone of 44,2 ° C. to a salinity of 123 grams per kilogram. Interest in these depressions is also caused by the fact that in the bottom sediments there was found an increased content of zinc, copper, lead, silver and gold - in the 10-meter upper layer of sediment they accumulated (according to preliminary estimates) worth 2,5 billion dollars. Soviet scientists also took part in the study of these depressions on the ships Akademik Sergey Vavilov and Vityaz. Scientists suggest that the age of the brine in the depressions is about 10000 years. Another example of such an anomaly is Lake Vanda in Antarctica. Directly under the ice, the water in it is fresh, and its temperature is 0 ° C, and at a depth of 220 meters the water temperature is already 25 ° C, and the salinity is about 150 grams per kilogram. How did salt depressions form? How accurately can you determine the age of the brine contained in them? Scientists find it difficult to answer these questions. To do this, one must learn how to calculate the rate of convective mixing of hot and dense brines with less salty cold water located on top. It is necessary to thoroughly study the mechanism of action of the "big spoon" in the ocean. References:
Author: V.Lagovskiy
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