Fluids flowing near a surface tend to follow the shape of the surface. Using Schlieren optics, we can see this behavior. It is known as the Coanda Effect and its explanation depends on viscosity, the frictional forces between the molecules of a fluid (be it liquid or gas). The Coanda effect is the culprit behind many everyday incidents as well as more esoteric phenomena, such has levitating a ball in a stream of air.
Details of our Schlieren optics set-up can be found on our website: https://sciencedemonstrations.fas.harvard.edu/presentations/schlieren-optics
