Understanding Angle of Contact

The angle which is formed in between the tangent and the solid surface is drawn at the liquid surface and inside liquid at its point of contact, which is known as the angle of Contact. The angle of contact is shown using the Greek letter and mathematical symbol “θ”. This angle entirely depends on the nature of the fluid or the liquid used; in addition to the contacted solid and the medium that works on the mere surface of the fluid or liquid. 

One of the most important properties of Water is the Surface Tension and this surface tension depends on two very important phenomena. To understand the what is the angle of contact in a more precise manner, let us look at the forces or the phenomenon that acts on the molecule of liquid or fluid. These are the force of adhesion and the force of cohesion. This will help us to better define an angle of contact.

Adhesion and Cohesion

The tendency or the ability of molecules and particles to make a bond with one another is widely known as Adhesion. It means joining together by making a bond. While the attraction that these molecules have towards one another- which brings them together is commonly known as Cohesion. These two take place in different ways, irrespective of the fact that they sound similar to one another. 

Another important concept that will help us define an angle of contact is the concept of capillarity and viscosity. 

Capillarity and Viscosity

Capillarity is an invisible force that works upon the gravitational force. It has the responsibility and capacity to push the liquid up from within a pipe or a tube; the pipe must be narrow for this to happen. This pushing up of the liquid in a tube or piper is the Capillary Action. Capillarity is the rate through which fluids move across a wet or mere surface or move between two surfaces. 

The action or force or ability through which a fluid is pushed up against the force of gravitational attraction in a pipe or a tube is commonly known as the Capillary action. The fluid falls again after a certain amount of time is passed. This happens because of the Surface Tension and the ratio between the cohesive and adhesive forces.

In addition, the measure which resists the motion of the liquid or the fluids is known as the Viscosity. The Viscosity types are Kinematic and Dynamic Viscosity. Dynamic Viscosity is when the resistance is put externally. And, the resistance force that is measured under the gravitational weight is known as the Kinematic Viscosity. 

Now that we have briefly summarized the phenomenon responsible for acting on liquids, capillarity, capillary actions, and Viscosity. Let us continue our discussion of what is the angle of contact and its importance. 

Angle of Contact

As mentioned earlier to define the angle of contact, is the angle subtended by the tangent and the solid surface with the liquid surface. Whenever a liquid molecule is in contact with another molecule, which is solid, then the forces which act on the liquid or fluid molecules are the force of adhesion and the force of cohesion. The force of adhesion acts at the right angle of the wall of the given tube. However, the force of cohesion acts vertically at an angle of 45 degrees. 

Let us look at some interesting facts about the angle of contact- 

• If θ is below 90 degrees, then the liquid or fluid meniscus is concave upwards 
• If θ is above 90 degrees, then the liquid or fluid meniscus is convex upwards
• If θ is equivalent to 90 degrees, then the fluid or liquid meniscus is plane
• Again, if the θ is an obtuse angle, then the solid surface will not be wetted by the liquid or fluid
• If θ is an acute angle, then the solid surface gets wetted by the liquid or the fluid
• The angle of contact between glass and pure water is always zero

The angle of contact is dependent on the temperature of the liquid molecules. In addition to this, the θ value decreases when soluble impurities are added to the liquid. 

Conclusion

To define an angle of contact, we can say that the angle which is formed in between the tangent and the solid surface is drawn at the liquid surface and inside the liquid at its point of contact, which is known as the angle of Contact. The angle of contact is dependent on the temperature of the liquid molecules. In addition to this, the θ value decreases when soluble impurities are added to the liquid and the measure which resists the motion of the liquid or the fluids are known as Viscosity. The Viscosity types are Kinematic and Dynamic Viscosity.