An electron beam is a stream of tiny particles carrying an electric charge. When such a beam hits a material, it doesn't just heat it or knock out atoms: it also gives the surface a slight push, like a jet of water pushing against your palm. Until now, no one could measure that push directly: it's far too weak, thousands of times less than the weight of a dust mote.
Engineers assembled a sensor from a tiny spring with a mirror on its end. The electron beam presses on the mirror, the spring compresses by fractions of a nanometer, and a laser beam reflected off the mirror reveals the shift. The sensor was calibrated using the pressure of light, whose force is precisely known, so the measurements can be trusted.
It turned out that for electron energies from 2 to 30 kiloelectronvolts, the pressure force grows with the beam current and reaches a peak around 10 kiloelectronvolts. That's as if a water jet pushed your palm hardest at a certain speed, not just at a higher flow rate. This behavior is linked to how electrons penetrate the material and transfer their momentum to it.
The measured force allowed researchers to check how much an electron beam mechanically "pushes" ice during microchip etching. The contribution turned out to be tiny, millions of times smaller than the total etch depth, and almost independent of beam energy. So the ice is destroyed mainly by chemical action, not by impact.
Now that the push force can be measured, engineers will be able to control electron beams more precisely in chip manufacturing, microscopes, and possibly in thrusters for space probes.
The force of electron beam pressure measured in the experiment is 100 trillion times weaker than the force of a mosquito landing on your skin.
In Arthur C. Clarke's story "The Wind from the Sun," giant sails are pushed by light; here, an electron beam pushes a sensor, the same principle at the nanoscale.