Wetting, capillary phenomena Cards for groups

Group work

Group Task 1:

1. Get acquainted with the article "Surface tension".

2. Answer the questions:

-        What phenomenon is called surface tension?

-        Where can one observe the phenomenon of surface tension?

-        What values ​​characterize surface tension?

3. Perform the experiment:

"Study of the shape of a liquid in natural conditions."

Equipment and materials: aluminum, copper, glass, paraffin plates; sunflower or olive oil; alcohol solution in water, wire, syringe or glass tube.

Procedure

1.      Place drops of oil and water on aluminum, copper, glass, paraffin plates.

2.      View and draw the shape of the droplets.

3.      Make a conclusion about the interaction of molecules of a liquid and a solid body.

4.      The results are listed in the table.

5.      Using a glass tube or syringe, add a little olive oil to the mixture of alcohol and water.

6.      Look at the surface of the oil drop.

7.      Pass the wire through the center of the oil ball and rotate it.

8.      Consider how the drop shape changes.

9.      Make a conclusion about the shape of the surface of the liquid.

10.   Submit a report on the work done on the model:

Surface Tension

Surface tension is the phenomenon of molecular pressure on a liquid caused by the attraction of surface layer molecules to molecules inside a liquid.

When the gas temperature decreases and its pressure increases, the velocity of the molecules decreases and the average distance between them decreases. The forces of attraction of molecules become especially significant when the average potential energy of molecules is of the order of their average kinetic energy. Being attracted to each other, the molecules are combined into particles of vapor, which form a liquid. If the gas occupies the entire volume provided to it, then the liquid can only occupy a certain part of the vessel.

Molecules on the surface of a liquid are in special conditions as compared with the molecules of its inner layers, inside the liquid the resulting attractive force acting on the molecule from the side of neighboring molecules is zero.

If the molecule moves from the surface inside the liquid, the forces of intermolecular interaction will do a positive job. On the contrary, in order to pull a certain number of molecules from the depth of a liquid to the surface (i.e., increase the surface area of ​​the liquid), it is necessary to expend positive work of external forces ΔA_внеш, proportional to change ΔS surface area:

The coefficient σ is called the surface tension coefficient. (σ > 0). Thus, the surface tension coefficient is equal to the work required to increase the surface area of ​​the liquid at a constant temperature per unit.

In SI, the surface tension coefficient is measured in joules per square meter (J / m²) or in newtons per meter (1 N / m = 1 J / m²).

Consequently, the molecules of the surface layer of the liquid have excess potential compared to the molecules inside the liquid. The potential energy Ep of a liquid surface is proportional to its area:

It is known from mechanics that the equilibrium states of a system correspond to the minimum value of its potential energy. It follows that the free surface of the liquid tends to reduce its area. For this reason, the free drop of liquid takes a spherical shape.

The fluid behaves as if the forces acting on a tangent to its surface reduce (tighten) this surface. These forces are called surface tension forces.

The surface tension force is the force directed tangentially to the surface of the liquid, perpendicular to the contour section bounding the surface, in the direction of its reduction.

F_пов=σl, where σ is the surface tension, characterizing the surface tension force acting per unit length of the surface boundary

The presence of surface tension forces makes the surface of the liquid similar to an elastic stretched film, with the only difference being that the elastic forces in the film depend on the surface area (i.e., how the film is deformed) and the surface tension forces do not depend on the surface area fluid.

Since any system spontaneously enters a state in which its potential energy is minimal, the liquid must spontaneously enter a state in which the area of ​​its free surface has the smallest value.

The degree of wetting is characterized by a wetting angle. The wetting angle (or wetting angle) is the angle formed by the tangent planes to the interfacial surfaces bounding the wetting fluid, and the top of the angle lies on the boundary line of the three phases. Measured by the method of a lying drop. In the case of powders, reliable methods giving a high degree of reproducibility have not been developed until (2008). A weighting method for determining the degree of wetting has been proposed, but it has not yet been standardized.

Measuring the degree of wetting is very important in many industries (paint and varnish, pharmaceutical, cosmetic, etc.). For example, on the windshields of cars cause special coatings that must be resistant to various types of pollution. The composition and physical properties of the coating glass and contact lenses can be made optimal by measuring the contact angle.

For example, a popular method of increasing oil production by pumping water into a reservoir is based on the fact that water fills pores and squeezes out oil.