Coefficient of surface tension

The coefficient of surface tension of a fluid is a quantity that accurately characterizes the ability of the fluid to contract and which is measured by the surface tension force affecting the unit of the length of the line located on the surface of the liquid. In the event that the size of the boundary of the surface of the liquid is denoted by l, and the surface tension of the film that acts on this boundary is F, so the value of the surface tension coefficient is:

Σ = F / l

The name of the coefficient of surface tension is expressed in N / m. The higher the temperature, the smaller the value of σ for pure liquids.

A consequence of the asymmetry of the molecular interaction forces of the transition layer with the molecules that surround them is the idea of the existence of normal and tangential forces with respect to the interface of the phases. These forces have a significant effect on the molecules of the transition layer. They are the forces of molecular pressure and surface tension between phases.

Dependence of the coefficient σ on the presence of various impurities

The coefficient of surface tension is directly related to the forces of molecular interaction and can assume a wide variety of values for various liquids. In liquids that evaporate very well (alcohol, gasoline, ether), the surface tension is not as large as that of liquids that are not volatile. First, we put the mesh on the hydrometer, then we lower it into the water. Due to the dense grid, the hydrometer will be held at a certain depth. Next, you need to drop a little ether on the grid, after which the hydrometer immediately rises from the water.

The coefficient of surface tension of water is related to how many impurities are in the water. On the surface of the water is put a small chip from the match. After that, a piece of soap comes down into the water. After a certain period of time it will be possible to observe the movement of the sliver to the edge of the vessel from the soap. As a result, we can conclude: the coefficient of surface tension can be reduced with soap. If you add substances that differ in biological activity (paste, soap, washing powders), then the surface tension will be reduced. Then, if you need to get bubbles, why do people add soap?

Many of us believe that due to soap the index σ increases. In fact, it just reduces the surface tension index to about one-third to the value of pure water. It should be noted that when the soap film is stretched, the concentration of soap molecules on the surface decreases, while the surface tension increases. Consequently, under the influence of soap, weak areas of the bladder intensify and do not stretch further. In addition, thanks to soap, water does not evaporate, which means that the lifetime of the bubble increases.

Now let's put this experience: put the sugar candy in the water. This will cause the chip to move towards the candy. The conclusion is unambiguous: under the influence of sugar, the surface tension index increases.

How to determine the coefficient σ by means of capillaries?

To implement this simplest kind of experience, one must have several vessels with water and capillaries.

The capillary should be lowered into a vessel with water, and then the height of the liquid should be measured. Then another capillary is placed in soapy water, after which the height of the liquid is measured. The coefficient σ can be found from the corresponding formula:

Σ = ρgdh / 4

Next, we need to compare the coefficients of surface tension of various types of liquids.