Thermal metadevices with geometrically anisotropic heterogeneous composites

highlights•A generalized effective medium theory for thermal systems is proposed.•Performances of theoretical predictions fit well with finite-element simulations.•Thermal metadevices are controllable by adjusting the orientations of inclusions. The classical effective medium theory (EMT) provides a powerful tool in designing heterogeneous-composite functional thermal metadevices. But it is usually appropriate for spatially symmetric or geometrically isotropic systems, which limits the flexibility or adjustability in control. Here, we develop a generalized EMT to predict effective thermal conductivities of heterogeneous systems with elliptical (or ellipsoidal) inclusions. With the generalized EMT, omnidirectional thermal invisibility and directional Janus thermal illusion are designed and experimentally demonstrated just by tuning orientations of shape-anisotropy particles. In particular, we try to clarify the respective scopes of the generalized Maxwell-Garnett and Bruggeman theory based on both theoretical interpretations and numerical simulations. Our work may offer a promising fundamental framework and application prototype in elaborating thermal metadevices with asymmetry, irregularity, or anisotropy in configuration.