  Transformation optics originating from the invariance of Maxwell's equations
under the coordinate mapping has enabled the design and demonstration of many
fascinating electromagnetic devices that were unconceivable or deemed
impossible before [1-11], and has greatly contributed to the advancement of
modern electromagnetism and related researches assisted with the development of
metamaterials [12-15]. This technique has been extended to apply to other
partial differential equations governing different waves [16-23] or flux
[24-28], and has produced various novel functional devices such as acoustic
cloaks [20-23] and Schrodinger's 'hat' [19]. In the present work we applied the
coordinate transformation to the time-dependent heat diffusion equation [24-28]
and achieved the manipulation of the heat flux by predefined diffusion paths.
In the experiment we demonstrated a transient thermal cloaking device
engineered with thermal metamaterials and successfully hid a centimeter sized
strong 'scatter' (thermal disturber), i.e., a vacuum cavity. To facilitate
reliable fabrication we adopted the rescaled thermal diffusion equation for
various ingredient materials with nearly constant product of the density and
heat capacity, and took the anisotropic thermal diffusivities as the key
parameters for the design. Our results unambiguously show the practical
possibility to implement the complex transformed thermal media with high
accuracy and acquire some unprecedented thermodynamic functions, which we
believe will help to broaden the current research and pave a new way to
manipulate heat for novel device applications.
