Surface tension describes the ability of a fluid to withstand gravity. For example, water forms droplets on the table surface, as water molecules are attracted to each other, which counteracts gravity. It is due to surface tension that heavier objects, such as insects, can be held on the surface of the water. Surface tension is measured in force (N) divided by a unit of length (m), or in the amount of energy per unit area. The force with which water molecules interact (cohesive force) causes tension, resulting in droplets of water (or other liquids). Surface tension can be measured using a few simple objects that are in almost every home, and a calculator.

## Lesson 21. Laboratory work No. 05. Measurement of the surface tension of a liquid (report)

**Laboratory work No. 5**

**Subject: “MEASURING SURFACE TENSION OF A LIQUID”**

**Purpose:** determine the coefficient of surface tension of water by the method of separation of drops.

**Equipment:** a vessel with water, a syringe, a vessel for collecting drops.

- Draw a table:

We find the average surface tension by the formula:

We determine the relative error by the method of evaluating the measurement results.

Conclusion: I measured the surface tension of the liquid (water), it turned out to be equal to 0.069 N / m, which, taking into account the error of 41.76%, coincides with the table value.

**Answers to security questions.**

**1. Why does surface tension depend on the type of fluid?**

Surface tension depends on the force of attraction between the molecules. Molecules of different liquids have different interaction forces, so the surface tension is different. Also, surface tension depends on the presence of impurities in the liquid, because the stronger the concentration of impurities in the liquid, the weaker the adhesion forces between the molecules of the liquid. Consequently, the surface tension forces will act weaker.

**2. Why and how does surface tension depend on temperature?**

If the temperature increases, then the speed of the molecules increases accordingly, and the adhesion forces between the molecules decrease. i.e. surface tension forces depend on temperature. The higher the temperature of the liquid, the weaker the surface tension.

**3. Will the result of calculating the surface tension change if the experiment is carried out elsewhere on Earth?**

It will change slightly, because the formula includes the value of g - gravitational acceleration. And we know that in different parts of the Earth the acceleration of gravity is different. The actual acceleration of gravity on the Earth's surface depends on latitude, time of day, and other factors. It varies from 9.780 m / s² at the equator to 9.832 m / s² at the poles.

**4. Will the calculation result change if the diameter of the tube drops is smaller?**

Changing the diameter of the tube cannot lead to a change in the measured value. A formula is used to determine surface tension.

The figure shows that a decrease in the diameter of the tube is compensated by a decrease in the mass of the droplet, and the surface tension will naturally remain the same.

**5. Why should slow drops fall?**

When liquid flows out of the capillary tube, the size of the drop grows gradually. Before separation of the droplet, a neck is formed, the diameter d of which is slightly smaller than the diameter d1 of the capillary tube. Around the circumference of the neck of the drop, surface tension forces act upward and hold the drop. As the size of the droplet increases, the gravity force mg tends to tear it off. At the moment of droplet separation, the force of gravity is equal to the resulting surface tension force F = πdσ.

It is necessary that the drops come off the tube independently, under the action of gravity. If droplet dropping is fast with an additional pressure on the syringe plunger, then at the moment of droplet detachment, gravity will not be equal to the surface tension force and this method will give a large measurement error.

## Maximum bubble pressure method (Rebinder method)

The capillary communicates with atmospheric air; therefore, atmospheric pressure P is maintained inside the tube_{0}.

The pressure P over the test liquid is gradually reduced using a water pump. Pressure difference (P_{about}−P) seeks to blow an air bubble from the capillary into the liquid, but this is counteracted by the additional pressure created by the surface tension forces of the liquid in the resulting bubble of radius r and directed tangentially to the interface Ms.

Finally, at a pressure difference (P_{about}−P) equal to P_{huts} exceeding the difference P_{i, pl.}–P_{i, vogue,} an air bubble is blown from the capillary tube into a liquid.

Here, the radius r of the blown bubble is unknown, which is extremely difficult to measure. Therefore, resort to the use of a reference fluid, the coefficient of surface tension σ_{o} which is known and close to the surface tension coefficient σ of the investigated fluid. It is believed that the radii of the bubbles blown from the same capillary will be the same in both cases.

Dividing the first equation by the second and solving with respect to σ we obtain the formula for calculating the surface tension:

## Stalagmometric method (drop counting method)

Count the number of droplets (n), collect in the container and measure the volume (V). Find the volume of one drop v.

where r is the neck radius.

Because the radius of the neck is difficult to calculate, conduct an experiment with a reference fluid.

## Ring separation method (physics lab in the first year)

A ring or frame is placed on the surface of the test fluid. If the liquid wets the ring, then the surface tension forces F_{1} and F_{2}acting on its outer and inner surfaces with diameters D and d, are directed inside the liquid:

The total surface tension is equal to:

To tear the ring from the surface of the liquid, it is necessary to apply an upward force F, which compensates for the gravity force mg of the ring and the surface tension F_{σ}:

By measuring with a dynamometer the force of separation of the ring and knowing the mass and size of the ring, you can determine the surface tension of the liquid: