Everyone knows the frustration of trying to pour water out of a cup and having it stick to the side and spill all over the place. In my experience, sometimes it seems like it’s the cup at fault, since I can consistently get things like a measuring cup to pour where i want, even slowly. but there’s one mug in particular that no matter what we do, will NEVER POUR IN THE PLACE ITS SUPPOSED TO. This has been confirmed independently by my roommates, what about this cup makes it so bad at pouring?
Water has high surface tension and good adhesive properties. It likes to stick to itself, and it likes to stick to other things.
In order to overcome this, you need to create some kind of physical disruption: something that will “dislodge” the water from sticking to the surface.
If you have a glass with a smooth round rim (like any mug or drinking glass), the rim is smooth enough that it doesn’t disrupt the water: the water sticks to the glass on its way down toward the rim, and then continues sticking past the rim and travels down the side of the glass.
When you think of a lab beaker, they literally have a “beak” i.e. the little half-spout thingy. This channels the water and disrupts the flow, so the water doesn’t have the chance to keep sticking to the surface and it pours out smoothly.
So in conclusion, the reason water tends to flow down the walls of some cups is because their rims don’t provide enough physical disruption in the water’s flow, and the water gets to keep doing what it likes to do: stick to the surface.
The best cups for pouring have a kind of spout or beak where the water gets disrupted and gets channelled into a stream.