Modeling Capillary Action Experiment

Grade Level: K-8th grade
Time Required: 15 Minutes
NDT Focus Area: Liquid Penetrant Testing (PT) 

Students will observe capillary action, which is used in PT to help liquid penetrants be drawn into very tight discontinuities on the surface that would usually be invisible to the naked eye. This introduces students to the physics behind PT and how it is used to inspect real-world materials.

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Learning Objectives

By the end of this activity, students will be able to: 

  • Explain the basic principles of capillary action

  • Describe how capillary action applies to liquid penetrant used in inspection

  • Observe and interpret the differences in how liquid travels through spaces of different sizes.

  • Relate the demonstration to real-world uses (i.e., finding discontinuities on the surface of a part).

Key Concepts 

  • Water “climbs” up the straws, climbing higher in the skinnier straws, in a demonstration of capillary action.

  • Capillary action can only be seen when two different substances interact with each other (in this case, the liquid interacting with the material of the straw).

  • The two substances “adhere,” meaning the water molecules “stick” to the inner plastic of the straw.

  • Cohesion describes the way molecules within a substance interact with each other; water molecules like to stick together.

  • In the larger straws, the force of the straw causing the water to adhere to the sides also breaks up the water’s cohesion more, causing the water to climb more slowly than in the smaller straws.

Vocabulary

Here are some key terms you will need to know to support your understanding during the experiment.

Capillary action

The ability of a liquid to flow in narrow spaces, even against gravity. 

Adhesion

The attraction between molecules of the liquid and the molecules of the solid material (like the walls of a tube).  

Cohesion

The attraction between molecules of the liquid itself. 

Discontinuity

An internal flaw or abnormality in a material.

Nondestructive

Testing that does not damage or destroy the part being inspected.

Materials Needed

Before you start, collect the following items for the experiment.

Water

Food coloring

Spoon

Glass

Notebook & pencil

Regular drinking straw

Miniature drinking straw

Large drinking straw

(e.g. boba tea straw)

Experiment Procedure

Follow these steps one at a time to carry out the experiment. Read carefully and take your time to make sure you complete each part safely and correctly.

Fill your glass with water

Add a few drops of dark food coloring.

Use your spoon to mix the food coloring and the water until the liquid is all one color. Still too light? Squeeze in as many drops of food coloring as you like!

Set your three different sized straws beside the glass.

Try peeking through each one. Which straw would you like to drink juice with? What about if you were drinking a thick, goopy smoothie?  

Hand dropping blue ink into a clear glass of water, creating swirling patterns.

Write down your guess, or hypothesis, for which straw do you think will show the strongest capillary action.

Use scissors to trim your three straws so they are all the same length.

Practice holding all three straws in one hand. Make sure they are all lined up next to each other in your hand so that the bottoms and tops of each straw are at the same height.

A hand holding three pastel-colored straws, two green and one yellow, with a pair of scissors in the background.

Place all three straws in the water, but don’t let them touch the bottom of the glass.

Count to thirty.

Write down your observations.



How far up did the water go in each straw?

Was your hypothesis correct?

A green stopwatch overlays an image of a glass filled with blue liquid and submerged straws.

Expected Observations 

The water travels highest in the narrow straw. Some colored water should have made it up the regular straw, but barely any of the liquid should have traveled up in the biggest straw.

Real-World Connection:
NDT & Liquid Penetrant Testing 

In liquid penetrant testing, liquids that are placed on the surface of the object being inspected get drawn into potential flaws or defects that would be invisible to the naked eye. This makes those flaws or defects stand out so that they can be fixed. Technicians use this science to inspect aircraft parts, pipelines, and more—without taking them apart.

Learn More About Liquid Penetrant Testing
A technician applies a developer to a weld joint on a pipeline using spray during a dye penetrant inspection.

Apply What You've Learned

  • Define capillary action in your own words. 

  • Draw a diagram showing how liquid moves through straws of different sizes. 

  • Briefly explain how liquid penetrant testing uses capillary action to find flaws on the surfaces of parts. 

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