Bug-sized aircraft flies by light

Because solid-state machines produce less friction and heat and need replacement parts less frequently, they have already started to replace devices with moving parts, such as computer storage drives and aircraft (see “The growing catalog of solid-state machines,” Physics Today online, Commentary & Reviews, 20 December 2018 ). Now University of Pennsylvania researchers John Cortes (now at Lawrence Livermore National Laboratory), Igor Bargatin, and their colleagues have built a small solid-state flying prototype. It’s made of micron-thick metamaterial plates and is powered by light-driven fluid flow. At atmospheric pressure, the machine hovers a fraction of a millimeter above the ground, and at reduced pressures it levitates in midair with payloads heavier than itself.

Cortes made the flying metamaterial plate, shown above with a black silicon ring payload and a ruler for scale, by coating a silicon layer with nanometer-thick alumina and a mat of carbon nanotubes and then etching channels into it. The resulting 60-μm-thick sample, shown below hovering above a glass substrate, was captured with a microscope camera. The inset illustrates how the cardboard-like channels redirect air: As LED light from below warms the plate’s thermally conductive surface, gas molecules colliding with it heat up and are forced downward by thermal creep, a flow of gas arising from a temperature gradient. At atmospheric pressure, the sample plate lifts 0.5 mm above the substrate for about a second before it moves out of the light path.

Future versions of the small plates could potentially carry microsensors to take measurements on planets with thinner atmospheres or in Earth’s upper atmosphere where the pressure is too low for airplanes and balloons and too high for satellites. During tests at upper-atmosphere pressures, a 0.1 mg plate rose as high as 10 mm off the ground and lifted a payload three times its mass. (J. Cortes et al., Adv. Mater. 32, 1906878, 2020 .)