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Have you ever noticed the angle formed by the droplet of water on a leaf’s surface? What is the significance of that angle? Let’s discuss this in the article.
The name was derived because of the nature of the contact with the solid surface. The angle of contact is an important topic to further comprehend the concepts like surface tension. Additionally, we will discuss the angle of contact and its significance, formula, and examples to help you understand the topic better.
What is the angle of contact?
The angle of contact is where a fluid point of interaction meets a strong surface. It is indicated by θ. It is different at connection points of various sets of fluids and solids. Or, we can say, the angle of contact deals with the connection point between the liquid and the solid surface.
It is the angle between the tangent formed through the liquid area and the solid surface in contact with the liquid portion. For instance, the droplet of water on the surface of the leaf. We will further explain this in this article.
The angle of contact varies from degree to degree and lies somewhere between 0 degrees to 180 degrees.
Forces that affect the angle of contact
Some forces can be observed when liquid is in contact with any solid surfaces. These forces are mainly of two kinds – the force of cohesion (or cohesive force) and the force of adhesion (adhesive force).
Force of cohesion or Fc:
This force holds the same molecules together, acting at an angle of around 45 degrees.
Force of adhesion or Fa
The force of adhesion holds different molecules together, and this force acts at a right angle.
Apart from the force, some other factors too can affect the angle of contact.
On the one hand, the angle of contact reduces due to the addition of impure substances; on the other hand, it increases due to an increase in a temperature gradient.
Importance of the angle of contact
The angle of contact point decides if fluid will spread on the outer layer of a strong surface or just drops as a droplet on it. If the angle of contact is in obtuse form, it will show the liquid droplet that will form on the surface. On the other hand, if it shows acute form, the liquid will just spread on the surface.
To understand the significance in detail, we have to make some assumptions in two stages:
Stage 1: This is when a drop or droplet is formed.
- We have to consider some parameters for solid surface and water droplets concerning air and liquid.
- Solid-liquid is represented by Ssl, solid-air is represented by Ssa and liquid-air is represented by Sla.
- The angle formed by Ssl with Sla is more than 90 degrees, leading to droplets’ formation.
Stage 2: This is when the water does not form droplets; instead, it spreads on the surface.
- This is because the angle formed by liquid on a solid surface is just less than 90 degrees.
The formula for angle of contact
Before deriving the formula for angle of contact, we need to study the parameters used in the formula.
- l = It represents the liquid surface
- a = It represents the air surface
- s = It represents the solid surface
Now just assume that some free energies/areas are applied to these states, which can be represented by la, ls and as.
If the limit between the three media is marginally adjusted in the neighbourhood of the edge, as shown by the dabbed line in the figure, then, at that point, the area of contact of the air with the solid surface is expanded or increased by just a small portion or barely r as per unit breadth. On the other hand, the liquid with respect to a solid surface is reduced by r. At the same time, the liquid with respect to an air surface is also decreased by r cos as per unit breadth.
If we talk about the total or net free energy change, then the equation or formula becomes like this,
as r – ls r = la r cos
Due to equilibrium, which minimises the free energy, now the angle is calculated as,
as r – ls r = la r cos
cos = as– lsla
This equation is also known as the Young equation.
Examples of the angle of contact
The angle of contact is a very common phenomenon. You can observe it between every interactive surface, like liquid and solid.
Have you seen the water droplets fall on the surface of the leaf? It is also a good example to understand and interpret this concept.
When water droplets fall on the surface like a leaf, they create some tangent that forms an angle at that surface, the angle known as the angle of contact.
Conclusion
The liquid acts differently on different surfaces, sometimes, it spreads over the surface, or sometimes it remains in its shape. But in every situation, it forms some sort of angle. The angle of contact is one of them, already discussed in the above article.
