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# Angle of Contact- Definition, Shape of Meniscus, Practice Problems, FAQs

If we put a drop of water and mercury on the glass surface. Then both drops will form different shapes on the glass surface. Now if we change the material of the surface then the shape will also change. From this we can observe that this shape depends on the materials that are in contact with each other. This shape is decided by the angle between liquid drop and contact surface. In this article we will be studying the angle of contact and the shape of the meniscus in detail.

• Angle of Contact
• Shape of Meniscus
• Practice Problems
• FAQs

## Angle of Contact

When liquid is in contact with a surface, various surface tensions come into play along various interfaces. These are:

Here angle of contact is important in the context that it decides the shape of liquid surface near its plane of contact with another medium. The angle between the tangent planes at the solid surface and the liquid surface at contact is known as the contact angle. The direction of the tangent plane should be taken as follows:

1. Tangent plane to the solid surface should be towards the liquid surface.

2. Tangent plane to the liquid surface should be away from the nearest solid surface.

Wetting is the study of how a liquid deposits and spreads out on a solid (or liquid) substrate. The same liquid can behave differently on different surfaces and the spreading of the liquid is determined by the contact angle. Lower the contact angle, higher is the wettability.

The nature of the solid and liquid in contact determines the angle of contact. At the point of contact, the surface forces between the three media must be in equilibrium. See the figure given next.

Case (a): angle of contact is obtuse

Applying the condition of equilibrium for the three surface tensions at the point of contact, we get

Since is obtuse, cos is negative

The greater value of surface tension for solid-liquid interface indicates that liquid molecules are weakly attracted to those solids and are attracted strongly to themselves. Hence, the liquid does not wet the solid surface.

For example: Mercury on any surface, and water on a greasy surface. Mercury separates into fine droplets when spread over our hand because of this.

Case (b): angle of contact is acute:

Again applying condition of equilibrium at the point of contact, we get

Since is acute, cos is positive

In such a case liquid molecules are strongly attracted towards the solid. Then liquid wets the solid. For example, water makes a glass tube wet when poured in it.

## Shape of Meniscus

Consider a liquid in a container. The upper surface of the liquid takes a curvature due to the interaction between the liquid and the surface of the container. The upper surface of the liquid is known as the meniscus.

Forces governing shapes of the meniscus:

There are two forces (adhesive force and cohesive force) responsible for the different shapes of the meniscus.

The force of attraction between the molecules of different substances is known as adhesive force

• Cohesive force:

The force of attraction between the molecules of same substances is known as cohesive force()

These two forces depend on the nature of the liquid and the solid used. The resultant of and is defined as and the meniscus is such that the resultant force will be perpendicular to the upper surface. Based on the direction of the resultant force, the meniscus can be concave, flat or convex.

Yellow arrows in the figure represent the directions of the tangent plane to solid and liquid surfaces.

Convex Meniscus:

The contact angle, θ, for a convex meniscus is an obtuse angle. It means that θ > 90°.

Example: Mercury (when exposed to air, θ ≅ 138° with glass)

Flat Meniscus:

The contact angle, θ, for a flat meniscus is 90°.

Example: For pure water in contact with pure silver, θ ≅ 90°.

Concave Meniscus:

The contact angle, θ, for a concave meniscus is an acute angle. It means that θ < 90°.

Example: For ordinary water and glass, 8° < θ < 16°.

## Practice Problems

Q. What is the shape of meniscus when a non-wetting liquid is placed in a capillary tube?

A. Non-wetting liquids have an obtuse angle of contact. So the shape of the meniscus will be convex upward.

Q. When the temperature is increased the angle of contact of a liquid?

A. When temperature is increased the cohesive force decreases so angle of contact also decreases.

Q. A drop of liquid under no external forces is always spherical in shape. Explain why?

A. A liquid tends to acquire the minimum surface area to minimise the energy by the virtue of surface tension. For a given volume, the sphere has the least surface area.

Q. Water on a clean glass surface tends to spread out while mercury on the same surface tends to form drops. Explain why?

A.

When is an obtuse angle, liquid molecules are strongly attracted to themselves and weakly attracted to those of solids; creating a liquid-solid surface takes a lot of energy, and the liquid does not moisten the solid. This is what happens with mercury on a glass surface. On the other hand, if the molecules of the liquid are strongly attracted to those of the solid, this will reduce Ssl and therefore, cosθ may increase or may decrease. In the case of water, it is at an acute angle and hence water wets the glass surface.

## FAQs

Q. What is the mercury meniscus in a capillary tube?
A. Mercury has a convex shaped meniscus.

Q. A liquid does not moisten the sides of a solid, If the angle of contact is?
A. If angle of contact is obtuse then liquid does not wet the slides of solid.

Q. What is the nature of meniscus of liquid having angle of contact?
A. Liquid having zero degree angle of contact have semi-spherical meniscus.

Q. A drop of liquid under no external forces is always spherical in shape, why?
A. A liquid tends to acquire the minimum surface area to minimise the energy by the virtue of surface tension. For a given volume, the sphere has the least surface area.

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