。
Oil and water don't blend together.
Literal
Oil [and-と] water [topic-は] not-blend-together.
混じり合う (まじりあう) combines 混じる ('to mingle') with ~合う (reciprocal). Japanese distinguishes 混ざる (mix passively), 混ぜる (mix actively), 混じる (mingle), and 混和する (technical miscibility) — a richness English covers with one word.