Cap, cap ... Here is another drop collected on the spout of the crane, swollen and fell down. A similar picture is familiar to anyone. Or a warm summer rain pours water on the yearning for moisture - and again drops. Why drops? What is the reason here? Everything is very simple: the cause of this is the surface tension of water.
This is one of the properties of water, or, more generally, of all liquids. As you know, gas fills the entire volume into which it enters, but the liquid cannot do this. Molecules inside the volume of water are surrounded by the same molecules on all sides. But those located on the surface, at the boundary of liquid and gas, are not affected from all sides, but only from those molecules that are located inside the volume, there is no effect on them from the gas side.
In this case, a force will act on the surface of the liquid, directed along it perpendicularly to that part of the surface on which it acts. As a result of the action of this force, surface tension of water occurs. Its external manifestation will be the formation of a similarity of an invisible, elastic film at the interface. Due to the effect of surface tension, a drop of water will take the form of a sphere as a body having the smallest area for a given volume.
Now we can determine that surface tension is the work of changing the surface of a liquid. On the other hand, it can be defined as the energy needed to break a unit of surface. Surface tension is possible at the interface between liquid and gas. It is determined by the force acting between the molecules, and therefore, responsible for volatility (volatility). The lower the surface tension, the more volatile the liquid will be.
You can determine what is the surface tension. The formula for calculating it includes surface area and surface tension coefficient. As mentioned earlier, the coefficient does not depend on the shape and size of the surface, but is determined by the strength of the intermolecular interaction, i.e. type of fluid. For different liquids, its value will be different.
The surface tension of water can be changed. This is achieved by heating, the addition of biologically active substances - such as soap, powder, paste. Its value depends on the degree of purity of the water. The purer the water, the greater the surface tension, and its value is second only to mercury.
A curious effect is observed when a liquid comes into contact with both a solid substance and a gas. If we put a drop of water on the surface of paraffin, then it will take the form of a ball. This is due to the fact that the forces acting between paraffin and a drop are less than the interaction of water molecules with each other , as a result of which a ball appears. When the forces acting between the surface and the droplet are greater than the forces of intermolecular interaction, water will evenly spread across the surface. This phenomenon is called wetting.
The wettability effect to some extent can characterize the degree of surface cleanliness. On a clean surface, the drop spreads evenly, and if the surface is contaminated or covered with a substance that is not wetted by water, the latter is collected in balls.
As an example of the use of surface tension in industry, casting of spherical parts, for example shotguns, can be given. Drops of molten metal simply freeze on the fly, taking a spherical shape.
The surface tension of water, like any other liquid, is one of its important parameters. It defines some characteristics of a liquid - such as volatility (volatility) and wettability. Its value depends only on the parameters of intermolecular interaction.