Basics of Surface Tension

We all have indulged ourselves at some point in certain science experiments at school (and even outside) that we can never forget. Making a needle float on the water’s surface is one such experiment. Watching an object of a greater density than water float on top of water makes us question the physics of liquids. At least, that’s what happened with Agnes Pockels, who discovered surface tension. Surface tension is a characteristic trait of liquids to shrink to the minimum possible surface area. Let us understand the basics of surface tension through this article. 

Basics of Surface Tension Definition

Liquids are generally known for their fluid-like properties – the ability to flow and to take up the shape of the container they are occupying. But liquids also possess a property that causes their surface to act like a stretched elastic sheet. This property is called Surface Tension. If we had to write the basics of surface tension definition, it would go like – “Surface tension is a property of liquids characterised by the surface molecules’ tendency to shrink into a lower surface area as a result of bulk force from inner molecules.” 

Formula of Surface tension: T = F/L 

Where,

F – force per unit length

L –  length 

T – surface tension of the liquid

What is Basics of Surface Tension? 

Surface tension explains an ant’s ability to walk on the surface of the water or the near-perfect spherical shape of raindrops. But to fully understand the basics of surface tension, first, we need to understand what causes surface tension in liquids. 

Cause of Surface Tension

We already know that molecules of any kind of matter – solids, liquids or gases, are held together by certain forces like Van der Waals force. These forces are attractive in nature and cause molecules to pull the neighbouring molecule towards themselves. Such forces are called intermolecular forces. Intermolecular forces can either be cohesive or adhesive. The cohesive force causes molecules to attract towards other molecules, while the adhesive force causes molecules to stick to the surface of the container. 

All the molecules in a mass of liquid are subject to equal cohesive force from all sides. But the molecules present on the surface only experience the cohesion from inside the liquid. To put it simply, the particles on the surface of a liquid experience a pulling force from the rest of the liquid, thereby causing a thin elastic-like film to appear on the surface. Thus, we can say that surface tension is caused by cohesive forces that pull liquid molecules together. 

Examples of Surface Tension

• Insects walking on water: Certain tiny insects (like water striders) can be seen walking on the surface of the water, because their weight is not enough to penetrate the surface tension of the water. 
• The needle experiment: Even though it is many times the density of water, a correctly positioned tiny needle may float on the surface. However, the needle will descend fast if the surface tension is disturbed.
• ‘Waterproof’ tents: Certain tent manufacturers brand their tents to be waterproof. These tents’ waterproof quality is a result of the surface tension of water bridging the pores of finely woven tent fabrics, making them slightly rainproof. However, if you press your finger on the tent material, the surface tension is broken, and rain will trickle through.
• The round shape of bubbles: The tendency of liquids to reduce wall tension causes the bubbles to become spherical in shape.

• The shape of droplets: The shape of liquid droplets is influenced by surface tension. The cohesive forces of the surface layer tend to draw water droplets into a spherical shape.

Applications of Surface Tension

Although nature presents various instances and examples of surface tension. Humans have found numerous applications for surface tension too. The basics of surface tension can be seen in many of our everyday applications like:

• The test for jaundice: Understanding the concept of surface tension has come as a boon in the medical field. As the surface tension of urine helps determine if a person has jaundice (surface tension of normal urine is 66 dynes/cm but the presence of bile makes it drop to 55 dynes/cm).
• Surface disinfectants: Disinfectants are typically low-surface-tension solutions. Having a low surface tension allows them to spread out and damage the cell walls of bacteria and other germs. 
• Soaps and detergents: Soaps and detergents primarily help water in cleaning clothes by reducing the surface tension of water so that water can ease its way into the pores of the cloth. 

Conclusion

Now that we understand the basics of surface tension and some of the examples and applications of surface tension, this understanding can go a long way in comprehending the nature of liquids and fluids as a whole and is of great importance in fluid mechanics (capillary action, etc.).