Flaska

Meet Flaska, a vacuum flask who keeps your tea hot by giving heat almost nowhere to go.

Heat leaves a hot drink in three ways: conduction through solid or still material, convection carried by moving air or liquid, and radiation, which needs no material at all. A vacuum flask has two walls with the air pumped out between them. With almost no molecules in the gap, conduction and convection across it nearly stop. The walls are silvered so they reflect most infrared radiation back. James Dewar invented this design in 1892 for storing very cold liquefied gases.

How much heat is at stake? Cooling 0.5 kg of coffee from 90 °C to 60 °C releases 0.5 × 4186 × 30 ≈ 62,800 J. A good flask spreads that loss over many hours instead of under an hour.

The same idea insulates laboratory dewars for liquid nitrogen and cryostats for superconducting magnets.

No flask is perfect. Heat still leaks through the neck, the stopper and the joints between the walls, and the silvering reflects most, not all, radiation. A flask keeps cold drinks cold for the same reason.