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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FAQs on Drops, Bubbles and Capillary Rise
1. What is capillary action and what causes it?
Capillary action, or capillarity, is the natural tendency of a liquid to flow into a narrow space, like a thin tube, without any external help and sometimes against gravity. It is caused by the interplay of two types of molecular forces: cohesive forces (attraction between similar molecules, e.g., water-water) and adhesive forces (attraction between different molecules, e.g., water-glass). When adhesive forces are stronger than cohesive forces, the liquid rises in the tube.
2. Why are small liquid drops and soap bubbles spherical?
Small liquid drops and bubbles assume a spherical shape due to a phenomenon called surface tension. A liquid's surface always tries to contract to the minimum possible surface area to achieve the lowest energy state. For a given volume, a sphere has the smallest surface area, which is why the cohesive forces within the liquid pull it into a spherical form.
3. What is the difference between cohesive and adhesive forces?
Cohesive and adhesive forces are intermolecular attractive forces. The main difference is:
- Cohesive Force: The force of attraction between molecules of the same substance. For example, the forces holding water molecules together.
- Adhesive Force: The force of attraction between molecules of different substances. For example, the force between a water droplet and a glass surface.
4. How do cohesive and adhesive forces determine capillary rise or fall?
These forces determine whether a liquid will rise or fall in a capillary tube.
- Capillary Rise: Occurs when the adhesive forces (between the liquid and the tube) are stronger than the cohesive forces (within the liquid). The liquid "wets" the surface and is pulled upwards. Water in a glass tube is a classic example.
- Capillary Fall: Occurs when the cohesive forces are stronger than the adhesive forces. The liquid molecules are pulled more strongly towards each other than to the tube, causing the liquid level to be depressed. Mercury in a glass tube demonstrates this.
5. What is the formula for the height of capillary rise, and what do its components represent?
The height (h) to which a liquid rises in a capillary tube is given by the formula: h = (2σ cosθ) / (ρgr). Each component has a specific meaning:
- σ (sigma) represents the surface tension of the liquid.
- θ (theta) is the angle of contact between the liquid and the tube surface.
- ρ (rho) is the density of the liquid.
- g is the acceleration due to gravity.
- r is the radius of the capillary tube.
6. Why does a soap bubble have two surfaces, while a liquid drop has only one?
The key difference lies in their structure. A liquid drop is a solid volume of liquid in air, so it only has one surface—the outer boundary between the liquid and the air. A soap bubble, however, is a thin film of soapy water enclosing a pocket of air. This creates two surfaces: an outer surface in contact with the surrounding air and an inner surface in contact with the air trapped inside.
7. What is the angle of contact and how does it affect the shape of a liquid's meniscus?
The angle of contact (θ) is the angle formed by the tangent to the liquid surface at the point of contact with a solid surface, measured inside the liquid. It determines the shape of the meniscus (the curved upper surface of a liquid in a tube).
- If θ is acute (< 90°), the liquid wets the surface, and the meniscus is concave (curved upwards), like water in glass.
- If θ is obtuse (> 90°), the liquid does not wet the surface, and the meniscus is convex (curved downwards), like mercury in glass.
8. What is a real-world example of capillary action?
A common real-world example of capillary action is how plants draw water from the soil. The narrow, tube-like structures called xylem in plant stems act as capillaries. Water adheres to the walls of the xylem and is pulled upwards, against gravity, to reach the leaves for photosynthesis. Another example is a paper towel absorbing a liquid spill.
9. What is meant by excess pressure inside a liquid drop and a soap bubble?
Excess pressure is the pressure difference between the inside and outside of a curved liquid surface, caused by surface tension. Due to its two surfaces, a soap bubble has double the excess pressure of a liquid drop of the same radius.
- For a liquid drop, the excess pressure is ΔP = 2σ/r.
- For a soap bubble, the excess pressure is ΔP = 4σ/r, where σ is the surface tension and r is the radius.
10. How does adding a detergent to water help in forming stable bubbles?
Pure water has very high surface tension, which causes small bubbles to collapse almost instantly. Adding a detergent (like soap) reduces the surface tension of the water. This lower surface tension makes the water film more elastic and flexible, allowing it to stretch and form a stable, thin layer that can trap air to create a lasting bubble.
