What is Coanda effect?

The Coanda effect occurs due to the presence of a curved surface immersed in a flowing fluid when the surface curves away from the flow. The flow 1) causes a lowered pressure at the surface and 2) entrains fluid to flow towards the surface. The effect can be seen as smoke streams in a wind tunnel follow the shape of, e.g. a streamlined automobile.

Another example is a water flow adhering to the back of a spoon. This is caused primarily by the Van der Waals (surface tension) forces between the water and the surface of the spoon.