Getting To Know More On Excess Pressure Inside Drops And Bubbles

Whenever any matter is applied with any external force, it undergoes changes depending upon the type of the matter, and the amount of force applied. 

Pressure can be applied on any surface. The pressure exerted on the liquid matter in the earth’s atmosphere is referred to as Atmospheric Pressure. But, in still water, any object that gets located below the water surface faces or experiences more force or fluid pressure on them than other particles placed above the water surface.

Depending upon the nature of the pressure applied, the interfacial surface tension varies. Due to the interfacial surface tension that exists upon the matter, the liquid surface experiences minimum energy with a minimum area coverage. The minimum area coverage with maximum force makes the liquid spherical with a small radius.

Excess pressure inside a soap bubble

The difference in pressure between the liquid and the gas creates a pressure difference due to the surface tension at the interface of both the substances. In a soap bubble, the bubble of air bears pressure, the pressurised bubble of air exists within a thin, elastic surface of a liquid.

 So, whenever any bubble bursts, we hear an audible pop sound created due to the pressure difference between them both.

The surface tension that exists on the surface of the liquid is caused due to the attraction of particles in the external surface layer by the bulk liquid layer. The surface tension always tries to build a minimal surface with maximum pressure or force. This surface tension and difference in the pressure quotient give the shape of the bubble as a sphere. 

In any liquid surface, the force, i.e., the surface tension that exists on the concave side, is always greater than the force on the convex side of the sphere. 

The inside surface is concave in a soap bubble, and the outer surface is convex. This means that the inside pressure is higher than that of the outside pressure. And the pressure difference that exists is regulated by the surface tension.

According to the formula,

T=F/l,

Where T= surface tension. F= Surface force, l=length along which the force acts.

So, whenever the force on the surface is more, the surface tension that exists is also more, covering a less area.

The force that acts upon the bubble of the soap is represented by

Force due to surface tension that acts on the right FT=4πRT 

1. ii) Force due to outside pressure acting on the right is- FP1= P1πR2

iii) Force due to inside pressure acting on the left is- FP2= P2πR2 

bubble at equilibrium- FP2= FT + FP1

P2πR2 = 4πRT + P1πR2

(P2 − P1)πR2 = 4πRT

ΔP = 4T/R

This is the excess pressure that acts on the bubble of soap.

Excess pressure inside a liquid drop

The excess pressure that a liquid drop experiences is as similar to the excess pressure that a bubble of soap experiences. The liquid drop is also spherical, giving it a composition of both the convex and concave shapes. So the concave or the inner part of the sphere experiences a greater force than the convex part or the outer part of the sphere.

As there exists a pressure difference between the two parts of the sphere, the pressure difference created= pi-p0.

Where pi= pressure inside and p0– pressure outside.

There act two forces upon the spherical drop. One of them acts upwards and the other downwards.

Let F1 be the force acting downwards.

For the upward force, F2=PA= PπR2

For an equilibrium position, F1=F2

2 πrT= P πr2

P=2 T/r

Thus, the excess pressure that exists on the liquid drop is 2 T/r.

Difference between soap bubble and air bubble

Conclusion

The pressure on bubbles is due to the interaction of various surfaces that come in contact with each other and create tension between the two surfaces. The soap bubbles being composed of two interfaces are pulled down by the surface tension. Water bubbles having just one interface hold lesser surface tension. 

When spherical tensions between the concave and convex parts of the spherical droplet become higher, excess pressure is created. Whenever the pressure on the inner part of the sphere becomes more than that of the outer surface, a tension between the two interfaces is created, giving rise to the excess pressure. While going by the formula that regulates the pressure, i.e., the force applied on a certain area and the length to which the force is applied, the more the force is applied, the more the pressure is exerted and the lesser the area is covered.