Many defects in paint films are related to surface tension. Surface tension arises due to the different energies of the molecules inside and on the surface of a liquid. Surface molecules have higher energy than molecules inside the liquid. Because any molecule in the liquid is surrounded by the same kind of molecules, the gravitational force of the surrounding molecules is spherically symmetrical, and the forces in all directions cancel each other out, and the resultant force is zero. So the molecules in the liquid can move freely without consuming work.
But the molecules at the surface are different. The gravitational force of internal molecules on it is greater than that of external gas molecules, and the resultant force is not equal to zero. Molecules on the surface are pulled inward. Therefore, in daily life, we can often see that liquid tends to be spherical, such as water droplets and soap bubbles falling on lotus leaves, which are formed to reduce the surface area. Since the molecules on the surface are pulled by the inside, to transfer the liquid molecules from the interior of the liquid to the surface, work needs to be done against the force of gravity pointing towards the interior of the liquid.
On Earth, spheres are often broken due to gravity. But by the same token, surface tension will drive a liquid from a matte surface to a smooth one, since a smoother surface has less surface area ( giving the paint its levelling properties ).
As shown in the figure, if the liquid is made into a film and placed on a metal frame, in order to maintain the surface balance, an appropriate force of width L is required on the liquid.
In the tangent direction, there must be a force equal to f and opposite in direction (this is surface tension), whose value is
In the formula, y is the coefficient of surface tension, also known as surface tension, N/m; multiplied by 2 because the liquid film has two sides.
Surface tension can also be seen as surface free energy, since liquid surface contraction can also be understood in terms of energy. If a weight is marked in the figure, mg=f, and the infinitesimal mass is reduced, the liquid film will rise, and the weight m will be raised at the same time, and the surface liquid film can do work when it shrinks, and the maximum work is
where a is the shrinkage surface area. therefore
The unit of the surface tension coefficient can be the unit of force or the unit of work, such as the unit of y N/m=N/(m2/m)=J/m2. Generally, the surface tension of the liquid is below 0.1N/m, and the surface tension decreases with the increase of temperature; the addition of surfactant can greatly reduce the surface tension, which is one of the reasons why the emulsion can be stable.
