Drops, Bubbles And Capillary Rise

Capillary action, also known as capillary motion, capillarity or wicking, is the ability of a liquid to flow through a tiny space without the assistance of, or even in opposition to, other forces such as gravity. We can more readily characterise the capillary and the action as a phenomenon that occurs when liquids ascend through a tube or, in this case, a cylinder. We may claim that this is primarily due to cohesive and adhesive forces. It is one of the most crucial surface tension phenomena we experience in our daily lives.

What do you mean by Cohesive and Adhesive Forces?

The forces of attraction between molecules of the same kind are known as cohesive forces. For example, the forces of attraction between water molecules in a glass. On the other hand, adhesive forces are attractive forces between distinct molecules. For example, the attraction between the water molecules in a glass and the glass molecules.

The adhesive and cohesive forces are related to bulk or macroscopic properties. Hence, the phrases aren’t appropriate for discussing molecules and atomic properties. When any liquid comes into touch with any surface, such as the tabletop or walls of a graduated cylinder, it is subjected to both adhesive and cohesive forces. These forces are responsible for determining the shape of the liquid.

What is Capillary Rise?

Capillarity is the depression or rise of a liquid in a narrow passage, such as the gaps between the fibres of a towel or the gaps in a porous material. Capillary action is not restricted to the vertical plane. The water gets sucked into the fibres without depending on the orientation of the towel or cloth.

The liquid is drawn upwards due to this interaction.

The capillary holds the amount of water we take, as well as the force with which it will raise. The substance that surrounds the pores produces a layer on top of them. The solid materials closest to water molecules have the best adhesion properties. As water is introduced to the pore, the thickness of the film thickens, and the capillary magnitude force decreases.

The following relation gives the height to which the water rises.

h = 2σ / ρrg

Here,

• h denotes the height of raising liquid by capillary action
• σ denotes surface tension
• r denotes the radius of the tube
• ρ denotes the density of the liquid
• g denotes acceleration due to gravity

The force of cohesion, surface tension, and adhesion increases the level of the liquid. Suppose the adhesive forces to the liquid capillary are greater than the forces that are cohesive to the liquid, the liquid level increases in the glass capillary. In the case of a less than 900 angle, the formed meniscus is concave. The liquid decreases when the cohesive force is larger than the adhesive force. The contact angle, in this case, is greater than 900 degrees, and the meniscus is convex.

Forces that balance the liquid column can get the formula for rising in the capillary. The force that is upward owing to surface tension is 2πrσcosθ, which balances the weight of the liquid πr2hρg. The balance pressure derives from this formula. The surface tension of the liquid is measured using the capillary rise experiment.

According to the above theory, 

2πr cosθ T = πr2ρgh,

Capillary rise (h)=2Tcosθrρg

At, 0° angle of contact in the capillary tube

h=2Trρg

The Phenomena of Bubbles Rising

We are all aware that plants collect water from the soil to produce food through photosynthesis. Similarly, if we consume water, it must battle against gravity to rise, but it does so. Another common occurrence is this phenomenon, caused by the surface tension of liquids.

If we put the water we used in our tests in a beaker or a small measuring cylinder; we can see that the water’s surface isn’t perfectly straight. It causes a little depressive state. In the case of water, the outer edge is dragged upwards because of adhesive forces between the surface and the water.

Conclusion

In the notes above, we have learnt the concept of capillary rise, capillary rise method and bubbles rise method. We have observed that the falling water drop always adopts the spherical shape because of the surface tension. So, we can conclude that the tendency of a liquid to minimise the surface area is known as the surface tension. Apart from this concept, we have learnt about the type of forces, i.e. adhesive and cohesive forces.