Superfluid helium-4 is an extremely slippery substance, which is why I came upon the idea of weaponizing it. One of the planets in my setting is rich in helium, so the weaponization of the element would make sense realistically. The grenade would either leak the superfluid out, or it would detonate and spread it all over the nearby area.
Oncoming enemies would have to concentrate on not slipping, which would leave them vulnerable to oncoming fire from the person and his allies who tossed the grenade. It could also be used to stop individuals who are attempting to flee, by tossing it in their path so they'll slip and fall once they step in the puddle.
My setting has advanced technology like combat exoskeletons and such, but on-the-ground troops who move on foot are still commonplace, so the act of using a superfluid grenade to impair their movement would be useful.
EDIT: My story takes place on Earth for the most-part, so the physics of said grenade would apply to Earth's physics/temperatures.