Surface tension refers to the ability of a liquid to resist the force of gravity. For example, water forms droplets on the table because the molecules along the surface cluster together to balance gravity. This tension is what allows an object with greater density (for example an insect) to float on the surface of the water. Surface tension is measured as a force (N) exerted over a length (m) or as an amount of energy measured over an area. The forces that the molecules of a liquid exert on each other, called cohesion, trigger the phenomenon of surface tension and are responsible for the shape of the drops of the fluid itself. You can measure the voltage with a few household items and a calculator.
Steps
Method 1 of 3: with an arm scale
Step 1. Define the equation to solve to find the surface tension
In this experiment it is determined by the formula F = 2sd, where F is the force expressed in newtons (N), s is the surface tension in N / m and d is the length of the needle used in the experiment. By modifying the arrangement of the factors to find the voltage, we obtain that s = F / 2d.
- The force is calculated at the end of the experiment.
- Measure the length of the needle in meters using a ruler before starting the test.
Step 2. Build a balance with equal arms
For this experiment you need such a structure and a needle that floats on the surface of the water. The scale must be carefully constructed to obtain accurate results. You can use many different materials; just make sure the horizontal bar is made of something sturdy, such as wood, plastic, or rather dense cardboard.
- Draw a mark in the center of the material you use to make the two arms (plastic ruler, straw) and drill a hole right above it. The hole is the fulcrum of the scale, the element that allows the arms to rotate freely; if you have decided to use a straw, you can simply pierce it with a pin or a nail.
- Make two holes, one at each end of the arms, making sure they are equidistant from the center; pass a string through each hole to support the scales.
- Support the central nail (fulcrum) horizontally using books or a piece of rigid material that does not yield; the scale must rotate freely around the fulcrum.
Step 3. Fold a piece of aluminum to make a plate or box
It doesn't need to be perfectly round or square; it must be filled with water or other ballast, so check that it is sturdy enough.
Hang the plate or aluminum box on the scale; make small holes in it to thread the string dangling from the end of one arm
Step 4. Secure a needle or paper clip horizontally on the other end
Hang this element on the string on the opposite end of the scale, taking care that it assumes a horizontal position, as it is an important detail for the success of the experiment.
Step 5. Put some plasticine or similar material on the scale to balance the weight of the aluminum container
Before starting the experiment you must make sure that the arms are perfectly horizontal; the plate is obviously heavier than the needle and therefore the scale is lowered towards its side. Add enough plasticine to the end of the other arm to balance the tool.
Plasticine acts as a counterweight
Step 6. Put the needle or paper clip dangling into a bowl of water
During this phase you must be very careful to make sure that the needle stays on the surface of the liquid; you must prevent it from being submerged. Fill a container with water (or another fluid whose surface tension you do not know) and place it under the needle at a height that allows it to rest on the surface.
Make sure the string holding the needle stays taut once the needle is in the liquid
Step 7. Weigh a few pins or several drops of water with a postal scale
You have to add them one at a time to the aluminum plate you built earlier; to make the calculations it is important to know exactly the weight needed to lift the needle out of the water.
- Count the number of pins or water drops and weigh them.
- Find the weight of each item by dividing the total value by the number of drops or pins.
- Suppose that 30 pins weigh 15 g, it follows that 15/30 = 0, 5; each weighs 0, 5 g.
Step 8. Add them one at a time to the foil tray until the needle rises from the surface of the water
Go slowly adding one item at a time; look closely at the needle on the other arm to pinpoint the exact moment it loses contact with the water.
- Count the number of items needed to raise the needle.
- Write down the value.
- Repeat the experiment several times (5-6) to get accurate data.
- Calculate the average value of the results by adding them and dividing the number obtained by that of the experiments.
Step 9. Convert the weight of the pins (in grams) into force by multiplying it by 0.0981 N / g
To calculate the surface tension you need to know the amount of force required to lift the needle out of the liquid. Since you weighed the pins in the previous step, you can easily find this quantity using the conversion factor of 0.00981 N / g.
- Multiply the number of pins you added to the pot by the weight of each; for example, 5 elements of 0.5g each = 5 x 0.5 = 2.5g.
- Multiply the total grams by the conversion factor 0, 0981 N / g: 2, 5 x 0, 00981 = 0, 025 N.
Step 10. Insert the variables into the equation and solve it
Using the data you collected during the experiment, you can find the solution; replace the variables with the appropriate numbers and carry out the calculations respecting the order of the operations.
Still considering the previous example, suppose the needle is 0.025m long; the equation becomes: s = F / 2d = 0, 025 N / (2 x 0, 025) = 0, 05 N / m. The surface tension of the liquid is 0.05 N / m
Method 2 of 3: by Capillarity
Step 1. Understand the phenomenon of capillarity
To do this, you first need to know the forces of cohesion and adhesion. Adhesion is the force that allows a liquid to adhere to a solid surface, such as the edges of a glass; the forces of cohesion are those which attract the various molecules towards each other. The combination of these two types of forces causes a liquid to rise towards the center of a thin tube.
- The weight of the rising liquid can be used to calculate its surface tension.
- Cohesion allows water to bubble or collect in droplets on a surface. When a liquid comes into contact with the air, the molecules undergo the forces of attraction towards each other and allow the development of bubbles.
- The adhesion causes the development of the meniscus, which is seen in liquids when they adhere to the edges of the glass; it is the concave shape that you can see by aligning the eye with the surface of the fluid.
- You can see an example of capillarity by observing the water rising through a straw threaded into a glass of water.
Step 2. Define the equation to solve to find the surface tension
This corresponds to S = (ρhga / 2), where S is the surface tension, ρ is the density of the fluid you are considering, h is the height reached by the liquid inside the pipe, g is the acceleration of gravity acting on the fluid (9, 8 m / s2) and a is the radius of the capillary tube.
- When using this equation, make sure that all numbers are expressed in the correct unit of measurement: density in kg / m3, height and radius in meters, gravity in m / s2.
- If the problem does not provide the density data, you can find it in the textbook table or calculate it using the formula: density = mass / volume.
- The unit of measurement of the surface tension is the newton per meter (N / m); one newton corresponds to 1 kgm / s2. To confirm this statement you can perform the dimensional analysis. S = kg / m3 * m * m / s2 * m; two "m" cancel each other leaving only 1 kgm / s2/ m i.e. 1 N / m.
Step 3. Fill the container with the liquid whose surface tension you do not know
Take a shallow dish or bowl and pour in about 2.5 cm of the liquid in question; the dose is not important as long as you can clearly see the substance rising up the capillary tube.
If you repeat the test with different liquids, remember to wash the container thoroughly between experiments; alternatively use different dishes
Step 4. Put a thin clear tube into the liquid
This is the "capillary" you need to take the measurements you need and calculate the surface tension accordingly. It must be transparent for you to see the fluid level. It should also have a constant radius throughout its length.
- To find the radius, simply place a ruler on top of the pipe to measure the diameter and halve the value to know the radius.
- You can buy this type of pipe online or in hardware stores.
Step 5. Measure the height reached by the liquid in the tube
Place the base of the ruler on the surface of the liquid in the bowl and observe the height of the fluid level in the tube; the substance rises upwards thanks to the surface tension which is more intense than the force of gravity.
Step 6. Enter the data found in the equation and solve it
Once you have found all the necessary information, you can substitute them for the variables of the formula and carry out the calculations; remember to use the correct units of measurement so as not to make mistakes.
- Suppose you want to measure the surface tension of water. This liquid has a density of about 1 kg / m3 (approximate values are used for this example). The variable g is always equal to 9.8 m / s2; the radius of the pipe is 0, 029 m, and the water goes up into it for 0, 5 m.
- Replace the variables with the appropriate numerical information: S = (ρhga / 2) = (1 x 9, 8 x 0, 029 x 0, 5) / 2 = 0, 1421/2 = 0, 071 J / m2.
Method 3 of 3: with a coin
Step 1. Gather the materials
For this experiment, you need a dropper, a dry dime, water, a small bowl, liquid dish soap, oil, and a cloth. Most of these items are available at home or you can buy them at the supermarket; it is not essential to use soap and oil, but you must have two different liquids to compare their respective surface tensions.
- Make sure that the coin (the five cents one is fine) is perfectly dry and clean before starting; if it were wet, the experiment would not be accurate.
- This procedure does not allow to calculate the surface tension, but to compare those of the different liquids with each other.
Step 2. Drip one liquid at a time onto the coin
Place the latter on the cloth or on a surface that can get wet; fill the dropper with the first fluid and let it go down slowly, making sure it is one drop at a time. Count the number of drops it takes to fill the entire surface of the coin until the liquid begins to flow away from the edges.
Write down the number you found
Step 3. Repeat the experiment with a different liquid
Clean and dry the coin between experiments; also remember to dry the surface you placed it on. Wash the dropper after each use or use several (one for each type of fluid).
Try mixing a little dish soap with the water and drop the drops to see if anything changes in the surface tension
Step 4. Compare the number of drops of each liquid needed to fill the surface of the coin
Try repeating the test several times with the same liquid to get accurate data. Find the average value for each fluid by adding the number of drops dropped and dividing this sum by the number of experiments performed; write which is the substance that corresponds to the greatest number of drops and the one of which only a minimum quantity is enough.
- Substances with a high surface tension correspond to a greater number of drops, while those with a lower tension require less liquid.
- Dish soap reduces the surface tension of the water by allowing you to fill the face of the coin with less fluid.
