I notified that when I pour hot tea from a cup into another cup, sometimes it pours well and sometimes it just starts going down the edge of the cup.
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The tendency for the liquid to go down the side of the cup rather than pour in an arc is called adhesion. The liquid adheres to the side of the cup.
Three factors affect what is happening here.
1. Adhesion can be altered by the smoothness of the surface, both obvious and microscopic, the material, and contamination. For example, a cup that has a tiny bit of soap on the surface might not have the same level of adhesion as one that is perfectly clean, or a surface that was scuffed such a tiny amount you cannot see it might adhere differently than a surface that was smoother.
2. The energy of the pour. The faster the liquid leaves the cup, the greater the chance that it will overcome adhesion. Note that adhesion on the *inside* of the cup can affect this velocity by holding the liquid back.
3. Tiny differences in the surface of the cup at the point of pouring might change the direction the liquid leaves the cup at slightly. If a slightly different shape, a mold line on a plastic cup, or a tiny ding changes the way the liquid leaves the cup, it might make it harder or easier to pour. This is one of the reasons for spouts on measuring cups and pitchers.
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