Mostly in Greek mythology, humans swear on Olympian gods such as Zeus while gods swear on the river Styx(Iliad 14.271).
Then, is there any case where humans swear on the river Styx instead of Olympian gods? If not, why they didn't do that? Considering that the gods adopt Styx as a witness, it would be more powerful oath when humans use Styx as a witness too. Or is it impossible for humans to swear on the River Styx in Greek mythology?