What is Capillary Action?

The process by which liquid flows in a straight tube or porous substance is called capillary action.  It doesn’t require the force of gravity, as it frequently works against gravity. Capillary motion, capillarity, and wicking are used to describe capillary action.

The liquid is drawn into the tube by intramolecular forces: cohesion and adhesion. The combination of the liquid’s cohesive forces, and the adhesive forces between the liquid and the tube material, causes capillary action. Therefore, a tube must have a small enough diameter to allow wicking.

If any of the following occurs, the ascent will be accelerated if one of the two occurrences occurs: if  surface tension or the cohesion-to-adhesion ratio rises, the rise will be accelerated and if the liquid density rises, the ascent of the fluid in the capillary would be slowed.

Cause for capillary action

The combination of the liquid’s cohesive forces and the adhesive forces between the liquid and the tube material causes capillary action.

Intermolecular forces include adhesion and cohesion. The liquid is drawn into the tube by these forces. Therefore, a tube with a tiny enough diameter is required to form the capillary.

The formula for capillary action

The surface tension is also responsible for the rise of a liquid column within a narrow capillary tube.

Therefore, the capillary rise formula is: 

(h)=2Tcosθ/rρg

Here, r= Capillary tube glass radius

P = Density of liquid

G = Acceleration due to gravity

Θ = degree of contact between the tube wall and liquid

T = Surface tension of the liquid

Dependency of capillarity on nature of liquid and solid

If  Θ>90o, then the liquid will fall (descend) in the capillary because the meniscus is convex and this negative (h = -ve).

If  Θ=90o, then this indicates that the meniscus is planar (h = 0) and there is no capillarity.

If  Θ<90o, then h will be positive (h = +ve) because the meniscus is concave. This signifies that the liquid level in the capillary raises (ascends).

The liquid meniscus in a capillary system can be:

(i) meniscus with a concave shape

(ii) meniscus with a convex shape

iii) meniscus with a plane shape

Concave meniscus- In this, the pressure below the meniscus (po – 2T/r) is lower than the pressure above it (Po), and the excess pressure is P=2T/r

Here, the degree of contact between the tube wall and liquid is acute.

Convex meniscus-  In this, the pressure below the meniscus (po + 2T/r) is higher than the pressure above it (Po)  and the excess pressure is P=2T/r

Here, the degree of contact between the tube wall and liquid is obtuse

Plane meniscus- In this, the pressure below the meniscus is equal to the pressure above it, and the excess pressure is P=0

Here, the degree of contact between the tube wall and liquid is 90o

Capillary action forces

Below is a breakdown of the capillary action forces:

Cohesion

• The force seen between molecules of a liquid is called cohesion. 

• Surface tension is the most well-known phenomena, very few people have realised that it is also caused by the concept of cohesion. 

• Denser substances than liquids are prevented from going down by surface tension, which allows things to float over them in the absence of additional support. 

Adhesion

• Adhesion is the attractive force between two distinct things, including a hard vessel and a liquid.

• It’s the similar force that is responsible for the water to stick to the Surface of the glass

• When adhesion is stronger than cohesion, liquids saturate the surface of a material in which they come in contact, and the fluid curves upwards into the rim of the container.

Capillary action In daily life

If you drop a paper towel in water, the water will spontaneously climb up the towel, seemingly ignoring gravity. You can detect capillary activity by the water molecules crawling up the towel and pulling on other water molecules.

Plants and trees will not be able to thrive without capillary action. Plants have roots in the earth that can carry water up into the plant from the soil. Nutrients dissolved in the water enter the roots and climb on the surface. Water is delivered to the roots through capillary action.

Capillarity’s applications

Below are some well-known and commonplace applications of capillarity: 

(a) The oil in a lamp’s wick rises due to the capillary action of the wick’s threads.

(b) The capillary action of cotton in the towel is responsible for a towel’s ability to absorb moisture from the body.

(c) Water is held in a sponge piece due to capillarity.

(d) A blotting paper absorbs ink through the capillary action of the blotting paper’s pores.

(e) Plant root hairs use capillary action to extract water from the soil.

Conclusion

Capillary tubes (narrow cylindrical tubes) have minimal diameters. The liquid level in the capillary rises (or decreases) about the surrounding liquid level when these tiny tubes are dipped in liquid. Capillary action is the name of this activity, and capillaries are the tubes involved. Adhesive forces and cohesive forces are responsible for this. The formula for capillary action is (h)=2Tcosθ/rρg. The liquid meniscus in a capillary system can be of three types: the concave meniscus, convex meniscus, and plane meniscus. There are many applications of capillarity present around us.