What is this liquid flow behavior called?

Question

Every morning, when I take a clean, dry metal tumbler and fill it with hot coffee, the first sip is always different than the rest that follow.

The first sip requires some coaxing, for me to tip the tumbler a little further than I will have to tip it for the rest of the sips. The path of the coffee, I guess, hasn't been established. And so the coffee will have to figure out a way to get out.

Another explanation I can figure is that this is somehow capillary action. But I don't fully understand how that works.

Can anyone let me know what this behavior is, that I need to tip the tumbler a little extra on the first sip (and thereby burn my lips and tongue)?

EDIT: I should have mentioned - for any of my tumblers, they are capped with a plastic lid with a small opening. The first sip seems to travel the opening with difficulty, and the rest of the sips are effortless.

Answer 1

Here are two suggestions,

For the first sip the top of the cup is dry so the shape of the first part of the coffee to leave is more spherical, being held to the rest of the coffee by surface tension. On the following sips the top of the cup is wet, so the coffee isn't held back the same by the surface tension.

The second suggestion is to do with waves on the coffee surface. If the coffee is poured into the cup then left a few minutes, e.g. whilst tidying up, putting things away etc... any waves on the surface have diminished.

After the first sip when the cup is put back on the table small waves start again. If the second sip is shortly after the first, these waves allow coffee to come over the rip when the cup is at a less steep angle.

This (second) phenomenon seems to occur when pouring milk, it seems to be easier to control the flow to obtain a small amount of milk in coffee, by tapping the milk bottle on the table before pouring.