Drops, Bubbles and Capillary Rise

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Cohesive and Adhesive Forces

  • The attractive forces which are between molecules of the same type are known as the cohesive forces.

  • The forces which are attractive between molecules of different types are known as adhesive forces.

  • The forces which are Cohesive between molecules that cause the surface of a liquid to contract that too to the smallest possible surface area. This general effect is known as the surface tension.

  • The action which is of the capillary action is the tendency of a fluid which is to be raised or suppressed in a narrow tube or the capillary tube which is due to the relative strength of adhesive and cohesive forces.


The adhesive as well as the Cohesive forces are associated with bulk or macroscopic properties and hence we can say that the terms are not applicable to the discussion of molecules and atomic properties. When a liquid that comes into contact with a surface such as which is the walls of a cylinder graduated or a tabletop both cohesive forces and adhesive will act on it. These forces which we are talking about govern the shape which the liquid takes on. 

Due to the effects which are forces which are adhesive the liquid which is on a surface that can spread out to form a thin that relatively uniform film over the surface. In short we can say that a process which is known as wetting. Alternatively we can observe here that in the presence of strong cohesive forces the liquid can divide into a number of small. And even at times roughly spherical beads that stand on the surface which are  maintaining minimal contact with the surface.


Capillary Rise

The action which is of the Capillary sometimes capillarity, capillary motion or wicking is the ability of a liquid to flow that too in a space which is narrow without the assistance of or even we can say in opposition to forces which are  external like gravity. At times in a biological cell. We can more easily define the capillary and the action which is as a phenomenon where ascension of liquids through a tube or we can say the cylinder that takes place. This we can say primarily occurs due to cohesive and adhesive forces.

The liquid which is drawn that too in the upward direction which is due to this interaction between the phenomena. 

The narrower the tube, the higher will the rise in liquid. If any of the two phenomena  that we have discussed about the surface tension and a ratio which is between cohesion to adhesion increase the rise will also increase. Although we can also say that if the density of the liquid increases which is liquid the rise of the liquid in the capillary will lessen.

The amount of water we take  is held in the capillary that also determines the force with which it will rise. The material that really surrounds the files of pores also forms a film over them. The materials which are solid that are nearest to the molecules of water have the greatest property of adhesion. The thickness of the film increases as water is added to the pore and the capillary of magnitude force gets reduced. 


Capillary Rise Equation

A density of liquid denoted by ρ and surface tension denoted by σ rises in a capillary of inner radius denoted by letter r to a height:

That is h= 2σ cosθ/ρ g r

where θ, which we have seen is the contact angle that is made by the liquid meniscus with the capillary’s surface.

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The rise in the liquid which is due to the forces of adhesion, cohesion and surface tension as well. If forces which are adhesive the liquid-capillary that is more than the force which is cohesive liquid-liquid then the liquid rises as in case of water rise in a glass capillary. In this case the contact angle is less than 90 degrees and the meniscus is concave. If the force which is adhesive is less than the force which is a cohesive force then liquid depresses as in case of mercury in a glass capillary. In this case which we have discussed, the contact angle is greater than 90 degrees and the meniscus is convex.

The formula for rise in the capillary can be derived by forces which are balancing forces on the liquid column. The weight that is of the liquid πr2hρg is balanced by the force which is upward due to surface tension that is 2πrσcosθ. This formula that we have seen can also be derived using balance pressure.

The capillary which is rised experiment is used to measure the surface tension of a liquid.


Pressure inside a Bubble

We all know the thing already that plants absorb water which is from the soil to make food photosynthesis. But have we ever wondered how this all happens? For instance if we take water to rise up it has to work against gravity and yet it does happen. This is another phenomenon which often occurs which is because of the surface tension of liquids.

If water that we took in our experiments is placed in a beaker or a narrow measuring cylinder we can easily see that the surface of the water is meniscus isn’t straight. It forms a slight amount of depression. Actually, we can observe here that due to adhesive forces which are between surface and water the outer edge is pulled upwards in case of water. An image illustrating all this effect is given below:

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FAQ (Frequently Asked Questions)

Q1. Explain how Surface Tension is related to Bubbles.

Ans: The secret which we are discussing here to make bubbles is surface tension. This whole thing is because the surface tension we can say that the forces which are holding the molecules of a liquid together of water which is too high. When we take a detergent and it is added to water it lowers the surface tension so that bubbles can be formed.

Q2. Explain why Free Liquid Drops and Bubbles are always Spherical in Shape.

Ans: The Bubbles are said to be round in shape or spherical  this is because there is an attractive force known as the surface tension that pulls molecules of water into the tightest possible groupings. And the tightest possible grouping that any collection of particles can achieve is to draw itself together into a sphere.

Q3. Explain why a Soap Bubble has two Surfaces.

Ans: The picture is a soap bubble which we often see the surface tension balances the outward force which is due to the pressure difference that is between the inside and the outside air. We have two surfaces with us in this the inner and the outer surface of the bubble. The force due to the difference in pressure must balance the force from the surface tension.

Q4. Explain what Soap makes the most Bubbles.

Ans: Considering all this what detergent makes the most bubbles? In a trial of 2014 the tide detergent which made more bubbles as compared to the Palmolive dish detergent and dish Cascade detergent did not make any of the bubbles at all. In another trial conducted in 2005 we could see that the Palmolive produced more bubbles than either Joy or Dawn dish detergents.