Magnetic Microrobots

Simple, microscopic robots, remotely driven by magnetic fields, can use capillary forces to manipulate objects floating at an oil-water interface.

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

Controlled assembly of microscale objects can be achieved by exploiting interactions that dominate at that length scale. Capillary interactions are an excellent candidate for this purpose; microparticles trapped at fluid interfaces disturb the interface shape, migrate, and assemble to minimize the interfacial area. These interactions are independent of microparticle material properties and so can be used to assemble objects of arbitrary materials. By using a magnetic robot as a mobile distortion source, additional control over assembly can be achieved. For example, millimeter-scale magnetic robots that are heavy enough to distort the interface have been used to generate long-range capillary attractions and collect passive particles that are hundreds of micrometers in diameter. However, for smaller robots and particles, gravity is less important, and capillary interactions rely on interface distortions from undulated contact lines. We use a magnetic microrobot to manipulate passive microparticles at the water/hexadecane interface via an interplay of hydrodynamic and capillary interactions. Furthermore, we demonstrate preferred docking at corners of a square microrobot without the need for high resolution position control. We modulate the strength of docking interactions, allowing structure assembly and release. Finally, we design undulated docking stations with multiple stable sites for cargo delivery. The ability to dynamically manipulate microparticles and their structures at fluid interfaces creates new possibilities for manufacturing of complex microstructures.

SOURCES- University of Pennsylvania Engineering, Applied Physics Letters
Written by Brian Wang,