Surface tension is a physical property of liquids that refers to the force that acts on the surface of the liquid and tends to reduce the surface area to the minimum possible. This property is the result of the cohesive forces between the molecules of the fluid.
This property allows some objects to float on the surface of a liquid, such as aquatic insects that can walk on water.
Surface tension also plays an important role in phenomena such as capillarity, that is, the ability of a liquid to rise against gravity through a narrow tube.
As an example, the surface tension of water is one of the highest among common liquids. The typical value of the surface tension of water at 20 degrees Celsius is approximately 72 millinewtons per meter (mN/m) or 72 dyn/cm.
How Does It Work?
In a liquid, the molecules inside are surrounded and attracted in all directions by other adjacent molecules. However, at the surface of the liquid, the molecules are only surrounded and attracted to the inside and to the sides, creating a net inward force. This net force is responsible for surface tension.
Due to surface tension, liquid droplets tend to have a spherical shape, as this shape minimizes the surface area exposed to the environment.
Measurement and Units
Surface tension can be measured using a number of methods, such as the use of a tensiometer, which measures the force required to stretch or break a liquid film.
The unit of measure for surface tension is the newton per meter (N/m) or the equivalent of the joule per square meter (J/m²).
Surface Tension Formula
The mathematical formula for calculating the surface tension of a liquid is based on the relationship between the force acting on the surface of the liquid and the perimeter of the surface. The formula is the following:
T represents the surface tension of the liquid.
F is the force acting on the surface of the liquid.
L is the perimeter of the liquid surface.
Surface tension is expressed in units of force per unit length, such as newtons per meter (N/m) or dyn/cm.
It is important to note that this formula is a basic and simplified representation of surface tension. In more complex situations or with irregular surface shapes, more complex equations may be required to accurately calculate surface tension.
Examples in Everyday Life
Here are some examples that illustrate surface tension in different situations of our daily life and in natural phenomena:
Water Drops: When a drop of water forms, it tends to take on a spherical shape due to surface tension. The water molecules on the surface of the drop are attracted to each other, creating a surface film that minimizes the area exposed to air.
Insects on water: Some insects, such as cobblers or water bugs, can walk on the surface of the water without sinking. This is due to the surface tension of water. The legs of these insects barely break the surface of the water due to the force of surface tension, which allows them to float and move on it.
Capillarity in plants: This property also plays an important role in the ascent of water through the capillaries of plants. The water molecules stick to the vessel walls and are attracted to one another, creating a cohesive force that allows water to rise against gravity through the plant tissues.
Soap bubbles: When a soap bubble is blown, it assumes a spherical shape due to surface tension. The soap film on the surface of the bubble is held taut by the cohesive forces between the soap molecules.
Tear formation: The surface tension of the tear fluid in our eyes is responsible for tears forming in drops and not spreading over the entire surface of the eye. This property holds the tears together until they are large enough and heavy enough to fall.
Here's a simple experiment to demonstrate the surface tension of water using common materials:
A transparent container (it can be a glass or a cup)
a metal clip
Absorbent paper (for example, a piece of kitchen paper)
Steps to follow:
Fill the container with water to the brim.
Place the metal clip carefully on the surface of the water. Notice how the paper clip floats due to the surface tension of the water.
Take a piece of absorbent paper and place it on the surface of the water, touching the clip.
Watch what happens: the absorbent paper will instantly get wet while the clip is still floating. This occurs because water is drawn into the fibers of the absorbent paper, breaking the surface tension and allowing the water to soak into it.
This experiment demonstrates how the surface tension of water allows light objects, such as the paper clip, to float on its surface, and how the surface tension breaks when water comes into contact with a material that can absorb it, such as absorbent paper.