This paper presents thermally actuated hierarchical metamaterials with large linear and rotational motion made of passive solids. Their working principle relies on the definition of a triangular bi-material unit that uses temperature changes to locally generate in its internal members distinct rates of expansion that translate into anisotropic motions at the unit level and large deployment at the global scale. Obtained from solid mechanics theory, thermal experiments on fabricated proof-of-concepts and numerical analysis, the results show that introducing recursive patterns of just two orders of the hierarchy is highly effective in amplifying linear actuation at levels of nearly nine times the initial height, and rotational actuation of almost 18.5 times the initial skew angle.
Mechanical instability is often harnessed in mechanical metamaterials to generate a diverse range of functionalities, and can be triggered by either a mechanical or a field stimulus, such as temperature. Existing field-responsive metamaterials with snap-through instability, however, need to rely on a mechanical input to realize functional reversibility, a limitation depriving them of the capacity to operate solely via the applied field. This work demonstrates reversible snap-through instability in a bi-material framework that is exclusively driven by environmental temperature. The need for mechanical intervention is bypassed by leveraging the thermally induced contact and mismatched thermal expansion of the constituent materials. A combination of experiments, theory and simulations, unveils the physics underpinning the thermally driven snapping undergoing four successive regimes of deformation: noncontact, full contact, partial contact, and release. The advantages of the concept are showcased in two applications. The first is the development of thermal switches with ternary operation (OFF-ON-OFF) and logic functions, going beyond the capabilities of current binary switches. The second is reversible temporal morphing in deployable structures programmed to snap sequentially in multiple locked configurations at predefined values of temperature, opening the door to applications across sectors, such as deployable antennas, soft robots, and self-reconfigurable medical devices.
In article number 2213371, Damiano Pasini, and co-workers leverage thermally induced contact to introduce a mechanical metamaterial that can undergo reversible snap-through instability and can operate entirely through temperature actuation, hence eliminating the need for any mechanical intervention. The functional reversibility is showcased for the development of a novel thermal switch with an OFF-ON-OFF function, and the temporal morphing of deployable structures suitable for space solar arrays and soft robots.
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