Once you decide to build a very small version of a rather large machine, you quickly get into a thicket of new problems. Engineers have been trying, for instance, to build miniature motors that could, say, power a robot the size of a grain of rice. Large motors generate mechanical power from a spinning rotor, which spins according to the magnetic charge in a coil of wire. But how do you get the rotor of a motor the size of a grain of sand to spin? Even if you managed to wrap a wire around it, the wire would have to be so thin it would barely conduct electricity.
Masahiro Ota and his fellow mechanical engineers at Tokyo Metropolitan University in Japan came up with an ingenious solution: zap the rotor with a laser. First he made a rotor out of aluminum that works a little bit like a riverboat’s paddle-wheel. It has four blades, each four-one-hundredths of an inch long and as thin as a piece of paper that tilt upwards. When a precisely focussed laser beam strikes the front surface of one of the blades, it gets hot and heats the air in front of the blade. The air expands, pushing the blade and turning the rotor.
Ota has gotten his rotor to turn 12 times a second, but he is now trying to sandwich insulation between the front and back of the blades to maximize the difference in temperature, which would make the rotor turn faster. The greater temperature difference between the front and back sides makes a greater pressure difference, which produces increased torque of rotation, says Ota. Then he wants to shrink the rotor a thousand-fold, down to about the size of a bacterium, which he expects will take about three years. Then he’ll then be ready to figure how to embed the motor into a larger machine that carries its own tiny laser to power it.