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In today’s class, we will be talking about capillarity. Enjoy the class!
The tendency of a fluid to be raised or suppressed in a narrow tube, or capillary tube, is called capillary action.
If a capillary tube is placed vertically into a liquid, capillary action will raise or suppress the liquid inside the tube depending on the combination of substances. The actual effect depends on the relative strength of the cohesive and adhesive forces and, thus, the contact angle θ given in the table. If θ is less than 90º, then the fluid will be raised; if θ is greater than 90º, it will be suppressed.
Mercury, for example, has very large surface tension and a large contact angle with glass. When placed in a tube, the surface of a column of mercury curves downward, somewhat like a drop. The curved surface of a fluid in a tube is called a meniscus. The tendency of surface tension is always to reduce the surface area. Surface tension thus flattens the curved liquid surface in a capillary tube. This results in a downward force in mercury and an upward force in the water.
Mercury is suppressed in a glass tube because its contact angle is greater than 90º. Surface tension exerts a downward force as it flattens the mercury, suppressing it in the tube. The dashed line shows the shape the mercury surface would have without the flattening effect of surface tension.
Water is raised in a glass tube because its contact angle is nearly 0º. Surface tension, therefore, exerts an upward force when it flattens the surface to reduce its area.
|Interface||Contact angle Θ|
|Organic liquids (most)–glass||0º|
Table 6.1 Contact Angles of Some Substances
Capillary action can move liquids horizontally over very large distances, but the height to which it can raise or suppress a liquid in a tube is limited by its weight.
If we look at the different factors in this expression, we might see how it makes good sense. The height is directly proportional to the surface tension γ, which is its direct cause.
Furthermore, the height is inversely proportional to tube radius—the smaller the radius r, the higher the fluid can be raised since a smaller tube holds less mass. The height is also inversely proportional to fluid density ρ since a larger density means a greater mass in the same volume.
Capillary action depends on the radius of a tube. The smaller the tube, the greater the height reached. The height is negligible for large-radius tubes. A denser fluid in the same tube rises to a smaller height, all other factors being the same.
Factors that affect capillary
- Intermolecular forces
- Density of liquid
- Size of the tube
Angle of contact
The angle of contact is defined as the angle between the tangent drawn to the liquid surface at a point of contact of liquid and solid insides the liquid. It depends on the nature of solid and liquid both. For concave meniscus of liquid will be acute and for convex meniscus of liquid, it will be obtuse.
Liquid meniscus in capillarity
In capillarity liquid meniscus can be:
- concave meniscus
- convex meniscus
- plane meniscus
To understand this let us take a liquid drop (or) bubble as we know due to the property of surface tension, every liquid tries to minimize (or) contract its free surface area. Similarly, a liquid drop (or) bubble also tries to compress (contract) its surface and so it compresses the matter enclosed.
This, in turn, increases the internal pressure of the liquid drop (or) bubble, which prevents further contraction and equilibrium is achieved. In the equilibrium state, the pressure inside the bubble (or) drop is greater than outside the bubble (or) liquid drop and this difference in pressure between inside and outside the liquid drop (or) bubble is called excess pressure.
In the case of a liquid drop:
For a liquid drop excess pressure is provided by the hydrostatic pressure of the liquid.
In the case of a bubble:
For the bubble, the excess pressure is provided by the gauge pressure of the gas confined in the bubble.
Applications of capillarity
- The oil in the wick of a lamp rises due to the capillary action of threads in the wick.
- The action of a towel in soaking up moisture from the body is due to the capillary action of cotton in the towel.
- Water is retained in a piece of sponge on account of capillarity.
- A blotting paper soaks ink by the capillary action of the pores in the blotting paper.
- The root-hairs of plants draw water from the soil through capillary action.
Forces in capillary action
The force that exists between the molecules of specific liquids is termed as cohesion. Raindrops, before they fall to the earth, are also kept together by the same force. Surface tension is a phenomenon that most of us are aware of but not many of us know that it is also due to the concept of cohesion. Surface tension allows objects that are denser than the liquids to float on top of them without any support and doesn’t let them sink.
Another concept that can be understood with this phenomenon is one of adhesion. Adhesion is the force of attraction between two dissimilar substances such as a solid container and a liquid. This is the same force that allows water to stick on the glass.
If the phenomenon of adhesion is more than that of cohesion the liquids wet the surface of the solid it is contacted with and one can also notice the liquid curving upwards towards the rim of the container. Liquids like mercury have more cohesion force than adhesion force and thus can be termed as non-wetting liquids. Such liquids curve inwards when near the rim of the container.
In our next class, we will be talking about Elasticity. We hope you enjoyed the class.
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