Water has the highest specific heat capacity of any liquid.

Specific heat is defined as the amount of heat one gram of a substance must absorb or lose to change its temperature by one degree Celsius and for water, this amount is one calorie, or 4.184 Joules.

The mug wall contains inner and outer water chambers, separated by a thermally insulating layer. The inner water chamber is adjacent to the drink chamber.

When the central chamber is filled with a hot beverage, the temperature of the water in the inner chamber quickly rises, and approaches equilibrium with the hot beverage (which cools down slightly as heat energy is transferred to the water in the inner chamber).

The hot beverage only cools by a small amount as it has a much greater volume (and hence specific heat capacity) than that of the water in the inner chamber.

When the valve is closed, the hot water in the inner chamber remains in the inner chamber. (Hot water would rise by convection relative to cold water, but as the only connection to the outer chamber is at the base of the inner chamber, the hot water remains around the beverage.

Little heat energy is transferred to the outer chamber by conduction through the vertical channels, as water is a poor conductor of heat)

When the valve near the top of the mug is opened, hot water from the inner chamber flows through this to the outer chamber (by convection), being replaced by cooler water from the outer chamber moving up into the inner chamber through the vertical channels in the base of the insulating region.

Water that had been heated by the beverage and which had moved into the outer chamber would then cool as heat was lost to the external environment.
When the beverage had been cooled sufficiently to be palatable, the valve at the top of the mug can be closed, preventing further convective flow.

Patent Pending:

• International Patent Application No. PCT/GB2022/051313
• UK Patent Application No. GB2107456.2