Free-form thermal cloaks in three dimensions.

Cloaking from environmental detection has been a central goal in thermal engineering. The transformation theory has predicted its existence, requiring graded and highly anisotropic material properties. Its realization with general geometry was achieved recently in the two-dimensions. However, engineering free-form omnidirectional thermal cloaks in three-dimensional (3D) space remains far from being understood. Here, we physically realize omnidirectional three-dimensional thermal cloaking with arbitrarily complex shapes via a lattice composite formed by a 3D de-homogenization approach. The composite accurately delivers graded and anisotropic 3D thermal conductivities required for cloaking and features concise, well-connected structures fabricable with additive manufacturing. Numerical and experimental investigations demonstrate the successful concealing of complex objects from multi-directional thermal detection, such as cloaking an apple inside a pear. Integration of the strategy with spherical harmonics enables creating 3D cloaks with record-breaking complexity. Our findings mark the physical realization of the transformation theory in arbitrary 3D shape and provide a general, efficient, and practical route to realizing 3D thermal meta-devices.