1. Fig. 4. Simulated thermal distributions of (a) the designed thermal expander, (b) the referenced Material I (Copper) sample, and (c) the referenced Material II (Copper with holes) sample in a point-line heat field. The corresponding results of experimental observations are shown in (d), (e) and (f), respectively. The black curves depict the structure and location of the thermal expander for reference. The white curves in (a), (b) and (c) are isothermal lines (with steps of 7.3 0 C) corresponding to the color legend. Point-line conversion Uniformly expanding effect Fig. 3. Simulated expanding effects of (a) the designed thermal expander, (b) the referenced Material I (Copper) sample, and (c) the referenced Material II (Copper with holes) sample. The corresponding results of experimental observations are shown in (d), (e) and (f), respectively. The black curves depict the structure and location of the thermal expander for reference. The white curves in (a), (b) and (c) are isothermal lines (with step of 6 0 C) corresponding to the color legends. Thermal Expander Junying Huang and Jiping Huang Department of Physics and State Key Laboratory of Surface Physics, Fudan University, Shanghai 200433, China Introduction As one of the fundamental energy transport phenomena in nature, heat flow automatically balances the thermal distribution around us. Unfortunately, although people have realized that the ability to precisely control heat conduction will lead to an abundant wealth of applications (e. g. thermal computation and thermal memory), heat flow management is still in its infancy. To date, many interesting thermal devices (such as thermal rectifiers, thermal diodes, thermal cloaks, and et al.) have been proposed for the purpose of taming heat flow. However, one kind of thermal device, which could uniformly expand the heat flow without distorting the flow front (described by the shape of isothermal line) as the optical beam expander does for a laser beam, has not been proposed and verified yet. It is apparent that such a kind of thermal device, named as thermal expander, would have deep implication in thermal circuit and thermal management. Here, we report the study on a two-component thermal expander. The realization of the thermal expander is achieved by matching the thermal conductivities and geometries of two materials with a simple condition. The uniformly expanding effect of the thermal expander is demonstrated through both numerical simulation and experimental observation. Furthermore, the thermal expander also shows an advantage in efficiently rectifying a heat flow from crooked flow front to linear flow front, which is also verified by simulation and experiment. Summary In conclusion, a new type of thermal device, named as thermal expander, is proposed and demonstrated through both numerical simulation and experimental observation. The thermal expander performs a uniformly expanding effect on the heat flow. Furthermore, the thermal expander could also efficiently rectify the heat flow from crooked front shape to linear front shape. With these properties, the thermal expander would have deep implications for thermal circuits and thermal management. The realization of a thermal expander only needs that the thermal conductivities and geometries of two materials match a simple condition (Eq. 1). The method is so simple that the thermal expander could show its influence on thermal industry right away. Furthermore, although the thermal expander proposed and demonstrated in our study is in macro-scale, obviously, the method to fabricate a thermal expander can be easily extended to micro-scale and nano-scale regimes where the Eq. 1 is still valid. References [1] Li, N., Ren, J., Wang, L., Zhang, G., Hanggi, P. & Li, B. Rev. Mod. Phys. 84, 1045-1066 (2012). [2] Maldovan, M. Nature (London) 503, 209-217(2013). [3] Chang, C. W., Okawa, D., Majumdar, A. & Zettl, A. Science 314, 1121-1124 (2006). [4] Schittny, R., Kadic, M., Guenneau, S. & Wegener, M. Phys. Rev. Lett. 110, 195901 (2013). [5] Han, T., Bai, X., Gao, D., Thong, J. T. L., Li, B. & Qiu, C.-W. Phys. Rev. Lett. 112, 054302 (2014). [6] Huang, J. P. & Yu, K. W. Phys. Rep. 431, 87-172 (2006). [7] Franssila, S. Introduction to microfabrication (John Wiley & Sons, New York, 2010), 2nd ed. [8] D. A. G. Bruggeman, Ann. Phys. (Leipzig) 24, 636 (1935). Illustration Fig. 1. (a) Schematic diagram of a thermal expander (area depicted by red dashed curves). Blue curves denote the boundaries between regions with thermal conductivities κ 1 and κ 2 (respectively). (b) Simulated expanding effect of thermal expander on heat flow. (c) Simulated result of a heat flow rectified from crooked flow front to linear flow front by thermal expander. The black curves in (b) and (c) depict the structure and location of thermal expander. The white curves are isothermal lines (with steps of 6 0 C for (b) and 6.9 0 C for (c)) corresponding to the color legend. Each isothermal line arrowed describes the shape of flow front on the end-position of thermal expander. Condition Sample for experiment Fig. 2. Blueprint of the designed thermal expander for experiments. The black regions are copper and the white regions are PDMS with thermal conductivities of 400 and 0.15 W (m·K) -1, respectively. The white holes are of diameter 2.2 mm and hexagonally placed with a lattice constant 4 mm.