Thermodynamics of Hygroresponsive Soft Engines: Cycle Analysis and Work Ratio

Abstract

Soft materials that respond to external stimuli, such as moisture, heat, and light, can convert the environmental energy into mechanical motions, and thus can be considered engines. The energy-conversion process within the soft-materials-based engines is fundamentally different from that of conventional heat engines utilizing gaseous media, but has been rarely studied to date. Here we construct a theoretical framework to analyze the thermodynamic performance of humidity-responsive soft actuators, as a canonical model of soft engines, which operate in a similar manner to such motile plants as wild wheat seeds and pine cones. Considering the free-energy change and the work generation through a four-process cycle, we define a work ratio as the work of actual engines relative to that of a so-called hygro-Carnot cycle going through reversible changes of volume and chemical energy. We propose an explanation why natural motile plants exhibit higher work ratios than common artificial hygroscopic actuators based on the time scales of soft actuation and the environmental humidity change. Our thermodynamic model can be extended to a range of soft engines driven by diffusion of stimulus, e.g., solvent, heat, and ions, into soft media.