Bernoulli Balloon Blow Apart

The mathematician Daniel Bernoulli discovered a fundamental rule of fluid dynamics: moving air has lower pressure than still air.

In this experiment, you likely expected the balloons to fly apart when you blew between them. Instead, they moved together! This happened because your breath created a high-speed, low-pressure zone in the narrow gap between the balloons. The relatively higher air pressure of the still air on the outside of the balloons then pushed them inward toward the centre.

Low air pressure occurs when air is sped up because the kinetic energy of the molecules is being used primarily for forward motion rather than bouncing off one another. With fewer collisions between the air molecules in that fast stream, the pressure drops. Slower-moving air molecules in the surrounding room have more energy to “push” into that space, forcing the balloons together.

The curved surface of the balloon also plays a role through the Coanda Effect, where a fluid (like air) tends to stay attached to a curved surface. This causes the air to “wrap” around the edges, maintaining the low-pressure zone. This same principle is used to create lift on aeroplane wings and downforce on F1 race cars!

Further Demonstrations

You can see this same effect in action with a funnel and ping-pong balls or by making vortex smoke rings.

Variables to test

Find out more on variables here.

• Surface Area
  Does it matter if you use small or large balloons? Does a larger balloon provide more surface area for the atmospheric pressure to push against?
• Gas Density
  Try blowing between two helium-filled balloons. Since helium is less dense than air, does the reaction happen faster?
• Inertia
  What if the balloons are filled with water? Will your breath still be powerful enough to overcome the inertia (resistance to movement) of the heavy water-filled balloons?