Artificial materials manipulate heat flow

Heat flux is easy to understand, at least at its most basic level. But compared with the analogous flow of electric current, heat is much more difficult to control or put to good use. Now, borrowing techniques from the field of metamaterials, Yuki Sato and his postdoc Supradeep Narayana, of the Rowland Institute at Harvard University, have succeeded in manipulating heat flux. They set themselves the challenge of designing a hollow thick cylinder that, when embedded in a material with a uniform thermal gradient perpendicular to the cylinder axis, could drastically alter the heat flux inside without disturbing the flux outside. They settled on making the cylinder out of alternating thin layers of two materials with different thermal conductivities. With a suitable choice of the two materials, the stacked layers induce almost no distortion in the surrounding thermal gradient. By arranging the layers in concentric circles, the researchers created a thermal shield, which isolates the region inside from any measurable thermal flux at all. Arranging the layers as radial spokes did the opposite: increasing the thermal gradient inside the cylinder. But when they arranged the layers in a spiral pattern, as shown in panel a of the figure, they achieved perhaps the most counterintuitive effect of all: locally inverting the direction of the heat flux, so that it flows from right to left inside the cylinder in response to a left-to-right flux outside. Panel b shows the temperature map as imaged with an IR camera: The inside edge of the cylinder is slightly warmer on the right than on the left. (S. Narayana, Y. Sato, Phys. Rev. Lett. 108, 214303, 2012.)—Johanna Miller