Angle of Contact

Inside a liquid, a single molecule is surrounded by other molecules from all sides and is attracted to them. As a result, the molecule acquires negative energy. Conversely, a molecule on a free surface is only partially surrounded by other molecules, resulting in a lower negative potential energy. Therefore, the free surface of a liquid has more energy than the inner molecules, which leads to surface tension, which we have already studied. But how do we calculate the angle of contact for a liquid? Is it calculated under the water or over it? Let us find out the answers to these questions.  

Angle of contact 

The angle of contact is the angle formed by a tangent to the liquid surface at the point of contact and a solid surface within the liquid. It is represented by the symbol θ. It varies at the interfaces of different liquids and solids. The value influences whether a liquid spreads or forms droplets on the surface of a solid. The angle of contact is the angle formed when a liquid-vapour interface encounters a solid surface. It is traditionally measured through the liquid.

If 𝛉 < 90°, the liquid’s surface will have a concave upwards meniscus.

If 𝛉 > 90°, the liquid’s surface will have a convex upwards meniscus.

If 𝛉 = 90°, the liquid’s surface at the point of contact is smooth.

Characteristics of the angle of contact

• It is constant for a particular liquid-solid pair and depends on the type of solid in contact with the liquid. Other parameters stay unchanged.
• It is unaffected by the angle at which the solid meets the liquid surface.
• Surface tension influences it. The temperature and purity of the liquid fluctuate as the surface tension changes. It is determined by the medium above the liquid’s free surface.

Effect of temperature on the angle of contact

To begin with, the angle of contact between any two materials can only be between 0° and 180°. Since the angle of contact is a whole surface phenomenon rather than a bulk attribute, students should not presume that it can be affected by the amount of a substance.

The force of attraction between molecules of two different materials is known as adhesive force. For a glass of water, the adhesive force is the force between the molecules of water and the glass at the border.

Cohesive force, on the contrary, is the force that exists between molecules of the same object. This is the tendency of molecules from the same material or liquid to attract one another. The angle of contact is the angle formed by the liquid’s surface and the outline of the contact surface. The contact angle is smaller than 90° when the adhesive force is larger than the cohesive force. 

The angle of contact is greater than 90° if the adhesive force is less than the cohesive force. As the temperature rises, the cohesive forces in the material diminish as the energy and vibrations of the molecules increase. As a result, the angle of contact narrows.

To put it simply, as the temperature rises, the angle of contact reduces and as the temperature falls, the angle of contact rises.

Conclusion

We have previously examined what surface tension is and how it helps understand the behaviour of fluid around us: the angle of contact of water with the surface defines the angle between two lines of tangents as we saw above. We went through the characteristics of the angle of contact and found out that it is constant for a particular liquid-solid pair and depends on the type of solid in contact with the liquid. Other parameters stay unchanged. We use a goniometer to measure the angle of contact of a drop is placed on a plane. The goniometer is an instrument that was specially designed to calculate and measure angles of contact, but all of this is only possible for the static method. The dynamic method is identical to the static method, except that the liquid droplet that is placed on the surface is changed.