Using Non-uniform Magnetic Pressures to Actuate Flexible Arrays of Magnetic Nanoparticles*

Because continuous magnetic materials tend to be brittle, embedding magnetic elements within a soft matrix is a common technique for microscopic actuation of flexible structures. Such an approach typically uses weakly-interacting paramagnetic elements in a moderately complex gradient field to exert enough force on the magnetic elements to actuate the structure. Using quasi-2D arrays of self-assembled magnetic nanoparticles, we push this technique to the extreme limits of densely packed magnetic elements in the thinnest, most flexible, possible layer to show how the actuation behavior can depend essentially on strong magnetic interactions between the particles comprising the sheet. In particular, we show that actuation can be achieved even in a uniform external field due to pressures arising from the interparticle interactions. We demonstrate how to infer the non-uniform magnetization state of the sheet from its mechanical configuration measured by confocal microscopy. From this, we then infer the local pressures in the bulk and along the edges, and demonstrate how these combine to give the total deflection of the sheet, comparing experimental results with MD simulations. Finally, we show how these magnetic interactions can lead to a twisting behavior in the sheet in response to a changing orientation of the applied field. The combined magnetic and elastic response in these sheets makes them ideal for targeted applications in microscopic mechanics.