I Didn’t Know That . . .

Q: When I pour milk or orange juice slowly from a carton, it dribbles down the side. Why?

A: The dribbling is caused primarily by the surface tension of the liquid. At low pouring speeds, the surface tension is stronger than gravitational force and causes the fluid to cling to the container. Once the liquid becomes attached to the outside of the container, it tends to remain there, flowing down the side of the carton in a phenomenon known as the Coanda effect. A similar effect is seen if you place a lighted candle on one side of a bottle and blow at the other side. The wind will travel around the bottle and extinguish the candle.

The effect is named after Romanian Henri Coanda, who built the first jet aircraft in 1910. It had two jet engines, one on each side of the fuselage. To his shock, the flames from the jet clung to the surface of the fuselage on takeoff, scorching it badly. A picture of the Coanda is available at https://www.allstar.fiu.edu/aero/coanda.htm.