Responsive materials deform under external stimuli but often move unidirectionally, limiting their application in energy conversion and harvesting. Astonishingly, membranes composed of hygroscopic 2D nanoflakes, such as titanium carbide (MXene) and graphene oxide (GO), exhibit spontaneous and sustained oscillations when exposed to a steady flow of water vapor. While this behavior implies significant potential of the stacked nanoflake assemblies (SNA) for propulsion and electricity generation, the underlying mechanism of this autonomous reciprocating motion remains poorly understood, hindering its practical applications. Herein, we address this challenge by identifying the key factors governing the dynamics of moisture-driven oscillations, which finally fuse into three characteristic dimensionless parameters of the system. These parameters allow for the controllable tuning of the oscillation frequency, amplitude, and static bending angle. Furthermore, our study reveals that the oscillation is sustained by a negative feedback loop between moisture transport and mechanical motion. Guided by these insights, we achieve the rational design and optimization of SNA membranes for practical applications in continuous propulsion and energy harvesting from a weak ambient humidity gradient.
Zhang et al. (Mon,) studied this question.