The micro-robots in the Penn team's study are thin slices of magnet, about a third of a millimeter in diameter. Despite having no moving parts or sensors of their own, the researchers refer to them as robots because of their ability to pick and place arbitrary objects that are even smaller than they are.

That ability is a function of the specialized environment where these micro-robots work: at the interface between two liquids. In this study, the interface is between water and hexadecane, a common oil. Once there, the robots deform the shape of that interface, essentially surrounding themselves with an invisible "force field" of capillary interactions.

The same capillary forces that draw water from the roots of a tree to its leaves are here used to draw plastic microparticles into contact with the robot, or other particles already stuck to its edges.

Applied Physics Letters – Directed assembly and micro-manipulation of passive particles at fluid interfaces via capillarity using a magnetic micro-robot