Capillarity

Capillarity – this phenomenon has been known to physicists for hundreds of years, but only nature could make use of this phenomenon. In a way, we talked about, but also to… crystallization of water-soluble substances. Crystallization from solutions using the capillary phenomenon occurs in areas rich in groundwater, and hot at the same time, which favors the evaporation of water from the ground, like around the Dead Sea or the famous Death Valley in California (USA). Just like when emeralds crystallize, water flowing beneath the surface of the earth dissolves some minerals. Later, when he finds himself in a region of the desert that is under the strong influence of the sun, rises with capillary channels up, in the upper layers it evaporates, and substances previously dissolved in water crystallize on the surface of the earth. In place of the evaporated water, another flows through capillaries, etc., etc.. This is how the desert areas covered with a layer of salt over a large area were created, and in sand deserts "grow" "desert roses."” – lumps of tangled plaster crystals, forming characteristic forms similar to a rose flower.

For almost seventy years, man has also learned to use the phenomenon of capillary hair to grow crystals. The first, who came up with this idea, was a Pole, professor at the Warsaw University of Technology, Jan Czochralski, who devoted his entire scientific life to researching the phenomenon of crystallization and obtaining single crystals. W 1916 He developed a method of producing single crystals, which today is known as the method of extracting crystals from liquids. This time, however, it is not water or any other solvent, only molten solid, which single crystal we want to get. It may be liquid metal, melted table salt or melted fine particles, e.g.. colorless corundum. Czochralski dealt mainly with single crystals of metals, after all, he was a metal expert and developed his method to obtain tin single crystals, lead and bismuth. To this end, he immersed a glass capillary in the molten metal, metal flowed upwards under the action of capillary forces, and solidifying under appropriate thermal conditions, it formed a thin monocrystalline rod.

However, this is only the beginning of the single crystal cultivation process. When a small amount of liquid flows "up."” into the capillary, capillary rises to that height, that its outlet just touches the surface of the liquid. The metal inside the capillary solidifies from top to bottom, because higher the temperature is lower than just above the liquid level. After some time, the solidified liquid portion increases, reaches the capillary outlet, and because it is colder than a liquid, causes its crystallization in the immediate vicinity. At this point, the crystallization process should end, because the crucible – vessel with liquid – is constantly heated, and the liquid in the crucible remains constant, constant temperature, only slightly above the freezing point.

The method used by Czochralski to extract metal crystals from a liquid.

To grow large crystals, Czochralski used one more idea. When the liquid in the capillary has solidified all the way to the bottom of the capillary, it was beginning to lift it above the liquid level. Together with the capillary and the solidified inside it the crystallization seed rose to the top. The same physical law is at work here, as in the phenomenon of capillary hair and during the formation of a concave meniscus – the forces of adhesion of the liquid to the solidified single crystal are greater than the cohesive forces of the liquid. The liquid raised above the mirror level contacts the air with its side surfaces, it is cooled and solidifies, so a single crystal grows. And again we can raise the capillary to a certain height, the next layer of single crystal will clot and again, and again. The condition for this crystallization method to be successful is that the capillary is raised a little, such, that the difference between the adhesive and cohesive forces is less than the attractive force, that is, the force of gravity of the fluid lifted upwards. In fact, when crystallizing by pulling the crystals from the liquid, we do not lift the capillary with a single crystal in steps, but continuously at very low speed 13 do 40 thousandths of a millimeter per second, that is, from 5 do 15 cm in an hour. Very slowly, but we are sure about it, that the cohesion between the liquid and the nascent single crystal will not be broken, that we will grow a big one, regular metal crystal, semiconductor or salt weighing up to several kilograms. The same method is used to grow single crystals of noble varieties of minerals, and among them – grenades.