Encoding Latent Properties of Hygromorphic-Thermobimetal Composites as a Passive Mechanism for Adaptive Building Skins

Adaptive facades require complex mechanical systems to control their motion. Programmable materials are increasingly replacing mechanical systems through encoding their latent properties to achieve passive controlled motion response to external stimuli, thus acting as zero-energy adaptive systems. This paper introduces HMTM, a hygromorphic-thermobimetal laminated composite composed of wood, an anisotropic material whose mechanical properties vary according to fiber direction, and metal; an isotropic material with uniform linear expansion. The composite acts as an embedded sensor and actuator that initiates passive motion in response to temperature variation in hot climates. Physical experiments were conducted to deduce a grammar for the passive motion of the composite. The HMTM motion response was captured, analyzed and programmed through a physical-digital interface closed loop using image analysis. The HMTM grammar encodes two types of parameters: (1) embedded parameters related to latent properties of each of the composite’s materials such as dimensional ratio, grain orientation, thickness, lamination and expansion coefficient and (2) control parameters related to composite assembly such as fixation position and area isolation. These parameters collectively affect the output motion response in terms of deflection and motion type through semantic rules that define material configuration. The added value in the paper lies in the mutual benefit of integrating hygroscopic and thermal properties, specifically: (1) the extended actuation of the HMTM composite in hot arid climates where temperature variation is dominant, as opposed to humid climates only and (2) the extended resulting motion configurations such as sliding, rotation and twisting, as opposed to linear configurations.