Casimir forces are of fundamental interest because they originate from quantum fluctuations of the electromagnetic field 1 . Apart from controlling the Casimir force via the optical properties of the materials 2-11 , a number of novel geometries have been proposed to generate repulsive and/or non-monotonic Casimir forces between bodies separated by vacuum gaps 12-14 . Experimental realization of these geometries, however, is hindered by the difficulties in alignment when the bodies are brought into close proximity. Here, using an on-chip platform with integrated force sensors and actuators 15 , we circumvent the alignment problem and measure the Casimir force between two surfaces with nanoscale protrusions. We demonstrate that the Casimir force depends non-monotonically on the displacement. At some displacements, the Casimir force leads to an effective stiffening of the nanomechanical spring. Our findings pave the way for exploiting the Casimir force in nanomechanical systems using structures of complex and non-conventional shapes.The prediction of the attraction between two neutral perfect conductors by Casimir 1 was obtained by considering the boundary conditions imposed by two planar surfaces on the quantum fluctuations of the electromagnetic field. Alternatively, the Casimir force is sometimes considered as an extension of the van der Waals force between fluctuating dipoles to solid bodies and in the regime of retardation. This attractive force increases monotonically when the distance between the two planes decreases. As the Casimir force becomes the dominant interaction between electrically neutral surfaces separated by nanoscale gaps, they are of practical importance in nanomechanical devices 16,17 . In experiments, one of the flat surfaces is often replaced by a spherical body 4-11 due to the difficulty in maintaining parallelism at small separations 18 . By introducing corrugations to one of the surfaces, recent experiments 19,20 have demonstrated the non-trivial geometry dependence of the Casimir force. Measuring the Casimir Device fabrication Supplementary Figure S1 | The fabrication procedure of the device (not to scale). a, A cross-sectional view of the silicon-on-insulator wafer. The silicon device layer, the buried oxide layer and the substrate are shown in blue, yellow and grey respectively. b, The silicon oxide etch mask (red) is created using the resist pattern from lithography. c, Silicon in the regions not protected by silicon oxide is removed by DRIE. d, HF selectively etches the silicon oxide isotropically, undercutting of the top silicon structure by ~ 2.7 µm. The middle silicon piece is thin enough to be suspended. The other two pieces have oxide underneath and therefore are anchored to the substrate.

Stretchable transparent electrodes are key elements in flexible electronics and e-skins. However, existing stretchable transparent electrodes, including graphene sheets, carbon nanotube, and metal nanowire networks, weakly adheres to the substrate by van der Waals forces. Such electrodes suffer from poor scratch-resistance or poor durability, and this issue has been one of the biggest problems for their applications in industry. Here we show that, by introducing a Au–S bond between a Au nanomesh (AuNM) and the underlying elastomeric substrate, the AuNM strongly adheres to the substrate and can withstand scratches of a pressure of several megapascals. We find that the strong chemical bond, on the other hand, leads to a stiffening effect and localized rupture of the AuNM upon stretching; thus the stretchability is poor. A prestraining process is applied to suppress the localized rupture and has successfully improved the stretchability: electrical resistance of the prestrained AuNM exhibits modest change by one-time stretching to 160%, or repeated stretching to 50% for 25 000 cycles. This conductor is an ideal platform for robust stretchable photoelectronics. The idea of introducing a covalent bond to improve the scratch-resistance may also be applied to other systems including Ag nanowire films, carbon nanotube films, graphene, and so forth.

We demonstrate a W–Ni–YSZ cermet-based solar absorber with high solar absorptance, low infrared emittance and high thermal stability up to 600 °C.