2A20.30 • Surface Tension: Alveoli with Surfactant

Location: Balloons and pump in balloon drawer, handsoap next to sink

Video shows the demonstration used at the University of Texas at Austin

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Concepts Conveyed:

• Balloon = alveoli; tiny air sacs in lungs.
• Soap = chemical surfactant that is present in our lungs.
• Pumping air mimics inhalation of air.
• The soap reduces the surface and wall tension of the balloon wall; soap creates a larger net force that favors the balloon expanding.

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Instructions/Notes:

• Have two individuals, after securing the balloons on the pumps, pump both balloons at the same time and same rate. Make sure to angle the balloons slightly downward.
• One balloon has the soap inside, the other does not.
• The balloon with the soap with expand faster than the balloon without the soap despite the two balloons having the same initial volume and rate of air pumping.

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Demo Staff:

• Put 2 drops of handsoap inside only one of the two balloons.
• Spread the soap throughout the inside of the balloon by rubbing the balloon until there is a thin layer spread evenly. If there is excess soap, squeeze and remove in the sink.
• Ensure both balloons are the same size, and preferably, use identical balloon pumps as well.

Last updated on October 28, 2025