Angle of Contact

Briefly, at the point of contact when a liquid comes in contact with a solid, the angle produced between the solid surface and the tangent towards the liquid’s free surface is called the angle of contact. The surface tension forms this angle and its measure also relies on some factors. Moreover, this angle is usually checked to determine the wettability of liquid. This concept is an interrelated branch of the surface tension property of liquids. To understand other related concepts, it is necessary to clearly understand what angle of contact is . For which, a curated angle of contact study material is here below.

About angle of contact

The angle of contact can be defined as the angle formed by the solid surface and the tangent to the liquid’s free surface at the point of contact where a liquid comes into contact with a solid; the Angle is known as the angle of contact. The angle of contact is usually formed by the surface tension property of liquid. Though, before moving further, have a glance at what surface tension is.

What is surface tension?

Surface tension is a characteristic of any liquid that seeks to keep its open surface area as small as possible. The force operating on distance over a tangential invisible line upon that free liquid surface is surface tension. 

Influencing factors of angle of contact

The measure of angle of contact depends on the four factors:

• The surface tension of the liquid
• Nature of the liquid, E.g., Temperature, density.
• Nature of solid surface inside, E.g., glass, floor.
• The space or say medium above the liquid’s free surface

Properties of angle of contact

• The angle of Contact is highly dependent on the surface tension of the liquid.
• The angle of contact will be an acute angle when the liquid surface is concavely curved upwards. Whenever this angle forms, the liquid will be deemed a wet area.
• Similarly, it’ll be obtuse when the surface is convexly curved upwards. The surface area won’t be wet if this angle is in place. E.g., Mercury does not wet the glass.
• The angle of contact works as a constant in the associated pair of solid and liquid.
• The surface with the lower angle of contact possesses the properties of detergent.
• Its measure of angle highly depends on the temperature of a liquid or say, surroundings.
• When the angle of Contact is zero, the liquid wets the solid surface entirely.
• The drop in the measure of angle of contact can be seen when soluble impurities in liquid increase.

The shape of a liquid drop

The angle of the contact of solid and liquid pairs decides whether the liquid dropped on the surface will form a droplet shape or not. Usually, if the angle of contact is larger, the droplet will retain its shape. Conversely, if the angle of contact is smaller, the droplet wouldn’t be formed instead, it’ll spread on the surface. 

The angle of contact in different solid-liquid pairs

• Water and glass are at zero degrees of contact.
• Alcohol and a clean glass: Shape of surface, concave. The angle of contact = 0° (Approx.).
• Plain water and glass: Shape of surface, concave. The angle of contact is 8° (Approx.)
• Clean water and silver: Shape of surface, plane. The angle of contact = 90°.
• Mercury and glass. : Shape of surface, convex. The angle of contact = 138° (Approx.)
• Wax coated glass and plain water: angle of contact would be obtuse 

Difference between angle of contact of water and mercury

Conclusion

We came through the various aspects of the angle of contact from all above. We learned its definition formation, along with general properties and effects. In a nutshell, the angle of contact is the angle formed at the point of contact of solid and liquid and it is the angle between the liquid surface and the tangential line to the free surface above the liquid surface. Also, this angle forms due to the liquid’s surface tension property and depends on several factors.