to bubble; to form foam or froth on the surface of a liquid.
シャンプーがよく泡立つので、少量でも十分だ。
This shampoo foams well, so a small amount is enough.
ミルクを泡立つまでハンドミキサーで混ぜた。
I mixed the milk with a hand mixer until it became frothy.
海が荒れて、岸辺に白い泡が立っていた。
The sea was rough and white foam was forming along the shore.