Thermo-osmotic metamaterials with large negative thermal expansion

Negative thermal expansion (NTE) is important for compensation of thermal dilatation effects and has significant applications in high-precision devices and instruments. Several materials with intrinsic negative expansion exist but are chemically complex, difficult to manufacture and their thermal expansion coefficients (TECs) are small, typically in the order of 10−5/K–10−6/K. Here we present a metamaterial with a large NTE, with TEC in the order of 10−3/K, enabled by thermo-osmosis of entrapped air molecules through a multitude of nanometer-thin layers. We have generated this material by holographically patterning a biopolymer (dichromated pullulan). The presented manufacturing process is quite simple and capable of generating large-area NTE materials. The concept of achieving (NTE) through thermo-osmosis is universal and can be extended to many other polymers. Our research, for the first time, introduces a relation between NTE and thermo-osmosis.