A Capillary Tube of Radius R Immersed in Water: Understanding Capillary Action

Capillary action is a fascinating natural phenomenon that occurs when a liquid rises or falls in a narrow tube, known as a capillary tube. This behavior is primarily governed by the interplay of cohesive and adhesive forces within the liquid and between the liquid and the tube’s surface. In this discussion, we’ll delve into the intricacies of capillary action, particularly when a capillary tube with a radius R is immersed in water.

The Capillary Tube:

A capillary tube is a small-diameter tube, often made of materials like glass, plastic, or metal. These tubes are commonly used in various scientific experiments, medical applications, and everyday life, especially in measuring small quantities of liquids. The key characteristic of a capillary tube is its tiny radius, denoted as R.

Capillary Action Basics:

Before we explore the specific scenario of a capillary tube in water, let’s establish the fundamental principles of capillary action:

Cohesive Forces: Cohesive forces are the attractive forces between molecules of the same substance. In the case of water, these forces are strong due to hydrogen bonding between water molecules. This cohesion tends to keep water molecules together.

Adhesive Forces: Adhesive forces are the attractive forces between molecules of different substances. Water has strong adhesive forces with many surfaces, including glass and plastic. This means water molecules tend to be attracted to and “wet” these surfaces.

Surface Tension: Surface tension is a property of liquids that arises from the imbalance of forces at the liquid-air interface. Water has relatively high surface tension due to its cohesive forces.

Contact Angle: The contact angle (denoted as θ) is the angle formed between the liquid’s meniscus (the curved surface in the tube) and the solid surface of the tube. The value of this angle is crucial in understanding capillary action.

Capillary Rise in Water:

When a capillary tube with a radius R is immersed in water, several factors come into play to determine the height (h) to which the water will rise within the tube. These factors include:

Tube Radius (R): According to Jurin’s law, the height to which a liquid will rise in a capillary tube is inversely proportional to the tube’s radius. In simple terms, a smaller radius leads to a greater capillary rise.

Mathematically: h ∝ 1/R

This relationship means that as the radius R decreases, the water will rise to a higher level in the tube.

Surface Tension (σ) of Water: Surface tension is a measure of the strength of the cohesive forces within the liquid. Water has relatively high surface tension, which aids in its ability to rise in narrow capillary tubes.

Gravity (g): While gravity does play a role, it’s often negligible in most capillary rise scenarios unless the tube is exceptionally tall.

The formula that describes capillary rise in a cylindrical tube is:

h = (2σcosθ) / (ρgr)


h is the capillary rise (the height to which the liquid will climb).

σ is the surface tension of the liquid (in this case, water).

θ is the contact angle between the liquid and the tube wall.

ρ is the density of the liquid (water in this case).

g is the acceleration due to gravity.

r is the radius of the capillary tube.

In summary, when you immerse a capillary tube of radius R in water, capillary action causes the water to rise in the tube. The height to which it rises can be calculated using the formula above, with smaller tube radii resulting in greater capillary rise. This phenomenon has applications in various fields, from laboratory experiments to medical diagnostics, and it showcases the intriguing interplay of forces at the microscopic level