---
title: "Explain capillarity phenomenon"
subject: "Fluid Mechanics"
chapter: 1
key: "semester_3/fluid_mechanics:capillarity-phenomenon-explain"
source: https://www.bctengineer.com/q/semester_3/fluid_mechanics/capillarity-phenomenon-explain
site: BCT Engineering
---

# Explain capillarity phenomenon

Fluid Mechanics · Chapter 1: Fluid and its physical properties

- Asked in: 2076 Asoj, 1 marks
- Video: [Viscosity, Cohesive and Adhesive Forces, Surface Tension, and Capillary Action](https://www.youtube.com/watch?v=P_jQ1B9UwpU) — Professor Dave Explains
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/fluid_mechanics/capillarity-phenomenon-explain
- Whole chapter: https://www.bctengineer.com/md/semester_3/fluid_mechanics/chapter-1

## Question

Explain capillarity phenomenon.

## Short answer

Capillarity is the rise or fall of liquid in a narrow tube due to surface tension and contact angle. For wetting liquid (θ < 90°), rise h = 4σ cosθ/(ρ g d); for non-wetting, depression. Smaller diameter gives larger rise. Examples: soil moisture, wicking.

## Answer

**Capillarity** is the rise or fall of a liquid in a narrow tube relative to the surrounding free surface, caused by surface tension and the contact angle between the liquid and the tube wall.

- If cohesion is weaker than adhesion (water-glass, $\theta<90^\circ$), the liquid wets the wall and **rises**.
- If cohesion is stronger (mercury-glass, $\theta>90^\circ$), the liquid is **depressed**.

Balancing the vertical pull of surface tension against the weight of the column in a tube of diameter $d$:

$$
h=\frac{4\sigma\cos\theta}{\rho g d}
$$

The rise is larger for smaller tubes. Examples: rise of water in soil pores and plant stems, wick action, and errors in piezometer and manometer readings when the tube is very narrow.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is capillarity?
   A. Compression of a liquid under pressure
   B. Rise or fall of a liquid in a narrow tube relative to the surrounding free surface
   C. Resistance of a liquid to flow
   D. Evaporation of a liquid from a tube
   Answer: B. Surface tension and contact angle make the liquid rise or fall in a narrow tube. (Key point: Definition of capillarity)

2. Why is mercury depressed in a glass tube while water rises?
   A. Mercury is lighter than water
   B. Mercury has lower surface tension
   C. Mercury cohesion exceeds adhesion, contact angle > 90 degrees
   D. Mercury adhesion exceeds cohesion
   Answer: C. When cohesion is stronger than adhesion theta > 90 degrees and cos(theta) is negative, so the liquid is depressed. (Key point: Rise vs depression)

3. From h = 4 sigma cos(theta)/(rho g d), what happens to the rise as the tube becomes narrower?
   A. It decreases
   B. It stays the same
   C. It becomes zero
   D. It increases
   Answer: D. h is inversely proportional to d, so smaller tubes give larger rise. (Key point: Effect of tube diameter)