Difference Between Surface Tension and Capillary Action

Key Difference – Surface Tension vs Capillary Action
 

Surface tension and capillary action are physical properties of liquid substances. They are macroscopic properties of liquids. The key difference between surface tension and capillary action is that, surface tension is measured as the force applied on a certain length of the liquid given by the unit N/m (Newton per meter) whereas capillary action is measured as the height of liquid column that is drawn upward, against the gravity given by the unit m (meter).

CONTENTS

1. Overview and Key Difference
2. What is Surface Tension
3. What is Capillary Action
4. Relationship Between Surface Tension and Capillary Action
5. Side by Side Comparison – Surface Tension vs Capillary Action in Tabular Form
6. Summary

What is Surface Tension?

Surface tension is a phenomenon in which the surface of a liquid, where the liquid is in contact with the gas, acts like a thin elastic sheet. The term surface tension is used only when the liquid is in contact with a gas (ex: when opened to the normal atmosphere). The term “interface tension” is used for the layer between two liquid.

Attractions between different chemical species cause the liquid molecules to unite together. The liquid molecules in the surface of the liquid are attracted by the molecules in the middle of the liquid. This is a type of cohesion. But the attraction between liquid molecules and air molecules (or the adhesive forces) are negligible. Therefore, this surface layer of liquid molecules acts as an elastic membrane. The surface layer of liquid molecules is under tension because there are no enough attraction forces to balance the cohesive forces act on them, thus this condition is called surface tension.

Formula for Calculation of Surface Tension

Surface Tension (γ) = F/d

Here, F is the surface force and d is the length along which the surface force acts on. Therefore, the measurement of surface tension is given by the unit N/m (Newton per meter).It is the SI unit for the measurement of surface tension.

What is Capillary Action?

Capillary action is the ability of a liquid to flow in narrow spaces without the assistance of, or in opposition to, external forces like gravity. It can be observed as liquid drawing through a capillary tube in the upward direction.

The capillary action occurs because of the intermolecular forces between liquid molecules and the surface of the capillary tube. Therefore, it occurs due to adhesion forces. When the diameter of the tube is sufficiently small, the liquid rises through the tube due to both adhesive and cohesive forces. The cohesive forces (attraction forces between similar molecules) cause the molecules to be drawn upward.

When a capillary tube is placed in a liquid, a meniscus is formed at the edge of the tube. Then, due to adhesion forces between liquid molecules and the wall of the tube, the liquid is pulled up until the gravitational force act on that amount of liquid is enough to overcome the adhesive force. Then the liquid molecules are pulled up due to cohesion.

Capillary action is common among plants. Xylem vessels are capillary tubes that can draw water with dissolved nutrients upward. This fulfills the requirement of water and nutrient by branches and leaves of large plants.

What is the Relationship Between Surface Tension and Capillary Action?

The capillary action creates a liquid column in a capillary tube. The height of the liquid column can be determined by the equation given below.

Formula for Calculation of the Height of the Liquid Column

h = 2γcosθ / ρgr

In this,

• h is the height of the liquid column,
• γ is the surface tension of the liquid (unit is N/m),
• θ is the contact angle between the liquid and the wall of the tube,
• ρ is the density of the liquid, g is the acceleration due to gravity (unit is Kg/m³),
• r is the radius of the tube (m).

What is the Difference Between Surface Tension and Capillary Action?

Summary – Surface Tension vs Capillary Action

Surface tension and capillary action are two types of microscopic properties of liquids. The difference between surface tension and capillary action is that, surface tension is measured as the force applied on a certain length of the liquid given by the unit N/m (Newton per meter) whereas capillary action is measured as the height of liquid column that is drawn upward, against the gravity given by the unit m (meter).

Reference:

1.Jones, Andrew Zimmerman. “What Is Surface Tension? Definition and Experiments.” ThoughtCo. Available here
2.Perlman, USGS Howard. “Capillary action.” Capillary action, from USGS Water-Science School. Available here  
3.“Capillary action.” Wikipedia, Wikimedia Foundation, 28 Feb. 2018. Available here 

Image Courtesy:

1.’WassermoleküleInTröpfchen’By User:Booyabazooka – Own work, (Public Domain) via Commons Wikimedia
2.’Figure 02 02 05’By CNX OpenStax, (CC BY 4.0) via Commons Wikimedia