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# Surface Tension

## What is Surface Tension

We can define surface tension as a force acting along the surface of a liquid, causing the liquid to behave like a stretched elastic skin. We can also define it as the force per unit length acting on the surface at right angles to one side of a line drawn on the surface. Hence, its the property of a liquid in which the surface acts as though its covered with elastic skin.

Additionally, we can say that it is as a result of the cohesive forces between the molecules of the liquid. Consequently, because of this effect, they will refuse to separate from each other. In this article, we will explore its molecular and mathematical explanation. We will also look into its examples, formula, effects, physical units, and various applications.

## Surface Tension Examples

Many of the everyday observations, show that the surface of a liquid behaves as it were a stretched elastic skin. For example, close a tap of a water but not very tight. Now observe the drops of the water from the tap. You will see the water forming slowly as it comes out from the tap. Therefore, as the water is coming out from the tap. It will appear to be making a bubble (in the form of balloon or elastic skin or bag). Hence, the elastic bag supports the weight of the water until the spherical drop of water falls on the floor.

This force is due to the attraction between the same molecules (cohesive force). Therefore, it makes the surface of the liquid to behave like a stretched elastic sheet. Thanks to this type of a force, insects can comfortably float (walk) on a water. When insects walk on a water. The surface they currently occupy will behave like an elastic skin. You will see the stretch on the surface.

## Surface Tension Formula

The formula for calculating surface tension is in force per unit length. Hence we can write the formula as

Surface Tension (γ) = Force (F) / Length (L)

γ = F / L

We need to also know that the S.I unit of the surface tension is in Newton per meter (Nm-1). Another unit for this force is dynes per centimeter (dyn/cm). We also have erg/cm2, and Joules per meter square (J/m2)

We can equally write surface tension equation as

γ = (1/2) (F / L)

## Explanation

Surface tension is a physical phenomenon that results from the cohesive forces between the molecules of a liquid at the interface between the liquid and another medium, such as air or another liquid. It is a fundamental physical phenomenon that plays a crucial role in many aspects of our lives. These roles ranges from the behavior of fluids to the way our bodies function.

Therefore, these cohesive forces make the surface of the liquid to contract and form a thin, elastic layer. The layer is known as the surface film or surface skin. We should also know that this layer behaves as if it were a stretched membrane. Hence, when you place an object on it, it is capable of supporting that object. It can also resist deformation from external forces.

The magnitude of the surface tension of a liquid depends on various factors. These factors depends on

1. The nature of the liquid,
2. Temperature, and
3. The presence of impurities or dissolved substances.

The study of surface tension has applications in many fields, including physics, chemistry, materials science, and engineering. For example, it is a fundamental factor in determining the behavior of fluids in:

1. Capillary tubes,
2. The formation of drops and bubbles
3. Wetting of surfaces, and
4. Stability of emulsions and foams.

## Surface Tension Measurement

We use tensiometer to measure the surface tension of a liquid. We have different types of tensiometers which are:

1. Drop volume tensiometer
2. Force tensiometer
3. Spinning drop tensiometer
4. Bubble pressure tensiometer

These brings us to three methods for measuring surface tension:

1. Wilhelmy plate method
2. Du nouy ring method
3. Optical method

## How Does Surface Tension Work?

Surface tension is due to intermolecular forces, which are the attractive forces between molecules. These forces are stronger between like molecules than between unlike molecules. Thus, they create a cohesive force at the surface of a liquid.

At the surface of a liquid, the molecules are more strongly attracted to each other than they are to the air above the surface. Therefore, it makes the surface of the liquid to behave like a stretched elastic sheet. Consequently, this behavior is the reason why it resists penetration by objects and why it can form droplets.

## Molecular Explanation of Surface Tension

The cohesive forces between the molecules of a liquid are responsible for a stretched elastic skin of the liquid. These forces are due to the presence of intermolecular forces. The intermolecular forces responsible are van der Waals forces and hydrogen bonds. They cause the molecules to come together. This attraction creates a net inward force on the surface molecules. Subsequently, it pulls them towards the bulk of the liquid. Hence, it makes the surface to act as if it is a stretched elastic membrane.

The strength of the cohesive forces between the molecules of the liquid determines the magnitude of the stretched elastic skin of a liquid, with stronger forces resulting in higher surface tension.

## Effects

Surface tension has several effects on the behavior of liquids. For instance, it causes liquids to form droplets. These are spherical in shape due to the minimization of the surface area. Therefore, the spherical shape of droplets is due to the balance between the stretched elastic skin of the liquid and the hydrostatic pressure.

It also affects the wetting of a solid surface by a liquid. When the cohesive forces between the liquid molecules are stronger than the adhesive forces between the liquid and the solid. The liquid forms a droplet on the surface of the solid. On the other hand, if the adhesive forces are stronger. The liquid spreads over the surface, forming a thin film.

## Surfactants

We use surfactants (molecules) to lower the surface tension of a liquid. They are amphiphilic molecules, which means they have both hydrophobic and hydrophilic regions. The hydrophobic region is attracted to non-polar molecules, while the hydrophilic region is attracted to polar molecules.

When you add it to a liquid. Surfactant molecules align themselves at the surface, with their hydrophobic regions in the liquid and their hydrophilic regions in the air. This alignment reduces the cohesive forces between the liquid molecules.

## Surface Curvature and Pressure

We can describe the curvature of the surface by the radius of curvature. It is the radius of the circle that best fits the surface at a given point. Laplace equation describes the relationship between the surface tension, curvature, and pressure.

The surface curvature of a liquid interface refers to the curvature of the liquid’s surface at a given point. A liquid interface can have different curvatures at different points, and the curvature can change depending on the surrounding environment.

Surface pressure, on the other hand, refers to the force per unit length that acts on a liquid surface. This pressure is caused by the stretched elastic skin of the liquid and is directed perpendicular to the facet. It is directly proportional to the surface tension and inversely proportional to the radius of curvature of the surface.

Mathematically, the relationship between surface tension, surface pressure, and surface curvature can be expressed using the Laplace-Young equation:

ΔP = γ(1/R1 + 1/R2)

where

ΔP is the pressure difference across the interface

R1 and R2 are the principal radii of curvature at the interface.

The Laplace-Young equation states that the pressure difference across a curved interface is proportional to the surface tension and the sum of the inverse radii of curvature.

This equation explains why small bubbles have higher internal pressure than large bubbles. This is because the radius of curvature of a small bubble is smaller than that of a large bubble. Hence, the pressure difference across the interface causes the bubble to expand or contract until the internal and external pressures are equal.

## Force Due to Surface Tension

When we consider a small segment of the liquid surface with length L. The cohesive forces between the molecules of the liquid cause a net inward force on this segment. This force is directed tangentially along the surface and is perpendicular to the length of the segment.

The formula for force due to surface tension is F = γL,

This coefficient (γ) represents the amount of energy required to increase the surface area of the liquid by one unit.

If the surface area of the liquid changes by a small amount dA. Then the work done is given by

dW = FdA.

This work represents the energy required to increase the surface area of the liquid by dA, and is equal to γdA.

This force plays an important role in many natural and technological processes. We have roles like the formation of droplets, the behavior of bubbles, and the adhesion of materials. Understanding its mathematical expression helps in designing and optimizing these processes.

## Wider Applications

The study of a stretched elastic skin of a liquid has many practical applications in daily activities. One of these applications is capillary action (Capillarity). This is because it plays a key role in the transport of water in plants. It is also important in the absorption of liquids in paper towels and sponges.

Another example we can capitalize on is the formation and behavior of bubbles. The spherical shape of bubbles helps the surface of the bubble to contract and minimize its surface area.

Wetting and spreading are other important applications we need to consider. When a liquid comes into contact with a solid surface, it will either wet or not wet the surface. This is depending on the relative strengths of the adhesive and cohesive forces involved.

## The Importance of Surface Tension

This type of force plays a crucial role in many aspects of our lives. It is what allows plants to transport water and nutrients through their stems and leaves. It enables insects to walk on a water. When we look into medicine, we apply it to create microfluidic devices for drug delivery and to study the behavior of cells and proteins.

It is also responsible for the behavior of surfactants. As we earlier explained, they are molecules that reduce the surface tension of a liquid. Surfactants are found in many household products. These products can be detergents and soaps. We use them to enable more penetration in surfaces more easily.

## Its Effects in Our Daily Lives

Surface tension plays a role in many everyday activities, from washing dishes to blowing bubbles. Soap and other cleaning agents can penetrate surfaces more easily and helps to remove dirt and grease.

Additionally, it allows droplets to form on surfaces. We can see these droplets in the dew that forms on grass in the morning. It is what allows inkjet printers to create sharp, precise images. It also enables the formation of bubbles in carbonated beverages.

## Applications in Nature

Surface tension is essential to the functioning of many natural systems. In plants, it allows water and nutrients to be transported from the roots to the leaves through the stem. A quite obvious example, is the one we mentioned earlier, where Insects are able to walk on a water. Some species even use it to trap prey.

When we look into the behavior of marine organisms. We can see that the ocean allows the formation of waves. Therefore, it helps certain species to float on the surface of the water.

## Applications in Science and Technology

It is an important factor in many scientific and technological applications. In materials science, we use it to study the behavior of materials at the nanoscale. In energy harvesting, we use it to maximize the efficiency of solar cells by optimizing the surface tension of the materials we use.

We also use it in the development of microfluidic devices. Which are used in a variety of applications, from drug delivery to lab-on-a-chip technology. Deeper research can help to create precise channels for the transport of fluids and particles at the micrometer scale.

## Future Research

Research into the properties of this topic is ongoing, and there are many potential applications for this knowledge. One area of research is the development of new surfactants and other materials that can be used in a variety of industrial and biological processes.

Another area of research is in microfluidics. This is the study of fluids at the microscale. Hence, it can be used to control the behavior of fluids in microfluidic devices. This is because they have applications in fields such as medical diagnostics and drug delivery.

Q: What is the difference between surface tension and viscosity?
A: Surface tension is the cohesive force that exists between molecules at the surface of a liquid, while viscosity is a measure of a liquid’s resistance to flow.

Q: How is surface tension measured?
A: It is measured in units of force per unit length. The unit is Newtons per meter or dynes per centimeter.

Q: What are some examples of how surface tension is used in everyday life?
A: We use it for washing dishes, blowing bubbles, and even in the formation of dew on grass.

Q: How does surface tension impact the behavior of insects on water?
A: It allow insects to walk on its surface by providing a supportive layer of cohesive molecules.

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## Source

American Physical Society

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