Arbitrarily slow, non-quasistatic, isothermal transformations (Vol. 47 No. 5-6)

Joule or free expansion of an ideal gas into a volume at lower pressure is an example of an irreversible isothermal process. This nonequilibrium example is often used in thermodynamics texts to demonstrate that an arbitrarily slow process need not be reversible. Cyclic operation of engines that involve a free expansion therefore requires work.

Here, the authors explore experimentally the origin of thermodynamic irreversibility at the level of a single-particle “gas”. A feedback trap confines a silica particle in a virtual bistable potential, creating a system analogous to two vessels connected by a valve, where the volume of one vessel is adjustable via piston. The authors operate two types of cyclic transformations; both start and end in the same equilibrium state, and both use the same basic operations—but in different order. One transformation required no work, while the other required work, no matter how slowly it was carried out.

Why the difference? As the illustration shows, the result of carrying out a protocol backwards in time may not match the initial state. This property is not possible to notice in a single repetition, unlike in a macroscopic system where free expansion is followed by a “whoosh”.

M. Gavrilov and J. Bechhoefer, Arbitrarily slow, non-quasistatic, isothermal transformations, EPL 114, 50002 (2016)
[Abstract]