What Is Surface Tension?

Liquids such as water produce a force that reduces the surface as much as possible. This force is called "surface tension." The water droplets condensed on the leaves in the morning, and the water droplets hanging down from the faucet are all formed under the action of surface tension. In addition, the reason why water striders can stand on the water surface is also due to the effect of surface tension ^[1] .

Liquids are cohesive and adsorptive, both of which are manifestations of molecular gravity. Cohesion enables the liquid to resist tensile stress, while adsorption allows the liquid to adhere to other objects ^[2]

The required surface tension can draw an arbitrary area element on the surface of the liquid. Let each area length of this area element be l , and the tension acting on each side perpendicular to the other part of the surface is F , so the surface tension is ^[3]

At room temperature (about 20 ), the surface tension of most liquids is in the range of 20 ~ 40 dynes / cm, but there are also more than this number. For example, the surface tension of water is 72 dynes / cm; the surface tension of mercury is 470 Cause / cm. The surface tension of liquid metal is relatively large. For example, the surface tension of liquid copper at 1131 ° C is 1103 dyne / cm. Some elements that are gaseous at normal temperature have a small surface tension when they are in a liquid state at low temperature. For example, the surface tension of 4.3 liquid helium is only 0.098 dyne / cm, and the surface tension of 90.2 liquid hydrogen is 0.2 dyne / Cm, the theoretical analysis also pointed out that for the same liquid, the temperature increases and the surface tension decreases ^[3]

The direction of surface tension is tangent to the liquid surface and perpendicular to the dividing line between any two parts of the liquid surface. Surface tension is only related to the nature and temperature of the liquid. Generally, the higher the temperature, the lower the surface tension. In addition, impurities will obviously change the surface tension of the liquid. For example, the surface tension of clean water has a large surface tension, and the surface tension of soap-soaked water is relatively small, that is, the surface of the clean water has a greater tendency to shrink. ^[4]

Compare the forces of molecules A and B in the liquid. With the effective force range of the molecular force as the radius and the spherical surface centered on the molecule A (Figure 1), all the molecules that have an effect on the molecule A are within the spherical surface. Choose a longer time T (the average time between two collisions of molecules). Due to symmetry, the total force of the action of each molecule on A is equal to zero during this time. Part of the spherical surface centered on the molecule B is in the liquid, and the other part is outside the liquid surface. The molecular density of this part is much smaller than that of the liquid part. If you ignore the effect of this part of molecule on B, due to symmetry, the combined force of all molecular forces between CC ' and DD' is equal to zero; the effective force on B is the downward combined force generated by all molecules below DD 'in the spherical surface. . Since every molecule in the boundary is subject to the combined force pointing to the interior of the liquid, these molecules have a tendency to descend toward the interior of the liquid, and there is also a side attraction between the molecule and the molecule, that is, the tendency to shrink the surface as much as possible . This situation makes the surface of the fluid appear to be covered by an elastic film with a solid surface area larger than it ^[3]

Examine the surface elements with lengths ds ₁ and ds ₂ on one side. If the curvature is not equal to zero (Figure 2), the resultant force of the surface tension T has a component in the direction of the surface normal, and there should be a balanced pressure difference on both sides of the surface. The relationship between pressure difference and surface tension is given by the following Laplacian formula ^[3]

In the case where the boundary surfaces of the three media intersect at one point (for example, a drop of mercury stops on the table), the contact line is subjected to the surface tension of three different boundary surfaces (Figure 3). Because the contact line has no mass, in order to maintain balance in all directions that can move freely, the component of the resultant force of surface tension in these directions must be equal to zero, which requires the three boundary surfaces to intersect at a certain angle. If | ₁₂ | is greater than the sum of | ₂₃ | and | ₃₁ |, then equilibrium cannot occur. For example, gasoline drops on the water surface. Because the surface tension of air and water is greater than the sum of the surface tensions of the other two oil surfaces, the three media cannot be in equilibrium. The gasoline will be spread over the entire water surface until the thickness of the oil layer. Up to the molecular size. If the medium 3 is melted fat, when it is placed between air and water, it forms the shape of a thin convex lens (such as a fat ball floating on a vegetable soup) ^[3]

A pipe with a very small diameter is inserted directly into the liquid. Due to the surface tension of the contact surface of the liquid, gas, and solid, the liquid will climb or fall in the tube. Let r be the inner radius of the tube, and at the same time, consider the liquid surface in the tube to be approximately a ball cap shape (Figure 4). In the case of tube wall infiltration, the resultant force of surface tension is ^[3]

The mercury droplets on the glass plate are basically spherical, because all the molecules in the thin layer on the outer surface of the mercury droplets are in a high potential energy state. Calculations show that if the total potential energy of the molecule is extremely small, the surface must be spherical. If you try to eliminate the effects of gravity, such as placing a droplet in another liquid that has the same specific gravity and does not chemically react with the droplet, or fall freely in a vacuum, or in a weightless satellite and rocket environment, then The droplets will appear ideally spherical. Spherical soap bubbles and spherical dew on lotus leaves can also be explained in the same way ^[3]

Surface tension also sometimes plays an important role in fluid movement. For example, the ripple generated by the breeze passing over the water surface is a kind of water wave whose surface tension plays a major role ^[3] .