Hands-on Science Carnival Activity Stations: General Physics

Airbrush Painting with the Venturi Effect

[Shopping List: Prepared airbrushing straws; Solo cup paint holders; paint diluted with water to flow easily); buckets, cups or bowls; sticks for stirring; paper or T-shirts to paint; water. Optional: soda bottles.]

1. Pour paint into a Solo cup, not quite to the top, and snap on a cap with a hole in the middle.
2. Specially prepared airbrush assemblies have a small diameter tube inserted into a drinking straw about 1/4" from one end. The end of the tubing should protrude just slightly into the straw, no more than 1/3 to 1/2 of the diameter of the straw. Insert the opposite end of the tubing through the hole in the lid and into the Solo cup filled with paint so that the tube extends well below the surface of the paint.
3. Point the end of the straw with the protruding tubing at a piece of paper or T-shirt and blow air through the opposite end of the straw. Paint should be drawn up from the cup through the tubing and spray out the end of the straw onto the paper. You may need to blow very hard. If you have trouble getting paint to spray, try filling the cup with water instead to practice. Be sure you blow into the straw- do not suck!
4. You should be able to control the spray pattern by blowing faster or slower.
5. You may need to adjust the tubing, particularly at the straw end, to achieve optimal spray. Make sure the paint is drawn up the straw easily as you blow. Check for any obstructions, such as dried paint in the tubing.

What's Happening: Bernoulli's Principle states that when a fluid such as air flows rapidly it creates a region of lower pressure. The faster the airstream, the lower the pressure. When you blow very hard through the straw, you create a low pressure region inside the straw and thus at the top of the tubing. This in turn lowers the air pressure inside the tube and allows the outside air pressure (which is pushing down on the surface of the paint in the cup) to force paint up the tube and into the straw. This "suction" is called the Venturi Effect, and is often used to pump fluids (just as it is doing here). Once the paint emerges from the end of the tube the fast flowing air in the straw breaks it up into very small droplets (this is called atomization) which are carried by the airstream as a very fine mist until they hit the paper. This is exactly how commercial airbrushes work.

Variations and Related Activities: You can demonstrate the Venturi Effect by placing a small diameter straw vertically in a cup of water and blowing air very fast across the top of this straw using a second straw pointed horizontally. Make sure the vertical straw is clear so you can watch the water rise to the top. The faster you blow, the higher the water will rise. Be sure you blow across the top of the vertical straw, not into the straw.

By using a Solo cup filled with water you can make a squirt gun that sprays a fine mist of water. If you have ever blown across the top of an empty soda bottle to make sounds you were also using the Venturi Effect. The fast moving air at the top of the bottle creates low pressure which makes the air inside the bottle begin to oscillate up and down vertically, creating the pressure or sound waves you hear. The rate of oscillation determines the pitch or frequency of the sound, with the dimensions of the bottle controlling which natural or resonant frequencies are the most stable. Try adding water to the bottle to change the length of the air chamber, resulting in a different resonant frequency. By blowing faster (you'll have to blow very hard!) you can make the air inside vibrate faster as well, producing a higher pitch or harmonic of the fundamental frequency.