Water poured from an open bucket by tilting it?
Answers
It is easy to test by putting some food coloring in the water and observing it while pouring the water out. Which is what I did to check my assumptions.
While the exact details may depend on such factors as material of the bucket, shape of its edge, quality of the water, the rate of pouring, etc., the overall picture is that the water from the surface flows out first.
This makes sense, considering, simplistically, that the surface layer of molecules loses lateral support as the bucket is tilted and the surface rises above the edge. The water from the layer below, would have to rise in addition to moving sideways, but there does not seem to be any forces for that, since this layer still should have lateral support from the wall and all forces acting on it are balanced.
There are many subtleties and uncertainties in this process due to the surface tension, viscosity, etc. For instance, due to the surface tension, the water has to rise a little above the edge of the bucket before it starts flowing, so it takes more than just "one layer of molecules" to create a sufficient lateral force disbalance, but, based on the test results, this does not seem to have much effect on the overall sequence.