Helium I and Helium II

Helium I vs Helium II — the temperature state world of liquid helium (specifically Helium-4)

When you cool Helium-4 (⁴He) down below its boiling point (4.2 K at atmospheric pressure), it becomes a liquid — and things get wild when it goes even colder.

Helium I (He I)

The normal liquid phase of helium-4

Exists between 4.2 K and 2.17 K

Acts like a typical liquid — flows, has viscosity, etc.

Still super cold, but nothing too unusual in terms of behavior

Helium II (He II)

Appears when liquid helium-4 is cooled below 2.17 K (called the lambda point)

This is superfluid helium — one of the most bizarre states of matter

Wild properties of Helium II:

Property Description

Zero viscosity

Flows with no resistance — can crawl up container walls!

Thermal conductivity

Off-the-charts — hundreds of times greater than copper

Fountain effect

Can shoot itself out of a capillary tube due to quantum effects

Two-fluid model

Behaves like two liquids in one: one normal, one superfluid

Quantum mechanical behavior

Macroscopic quantum phenomena — visible on human scales