Heat engines are able to turn the thermal energy of a material into force and motion. Since the industrial revolution, they've been essential for the development of modern machines and industrial plants.
Heat engines require a heat source and a colder heat sink. Mechanical motion is generated through the movement of a material from the higher temperature heat source to a lower temperature heat sink. Typical heat engines are large, containing a lot of fluid—usually on the order of 1024 fluid particles.
In the 1950s, leading scientists suggested that heat engines could operate at the atomic-level. Over the past decade, scientists and engineers have worked to miniaturize the heat engine. Their efforts resulted in the development of microscale heat engines, but the atomic-level heat engine remained elusive.