Magnetic Knot in a Rubber Beam express
An ordinary rubber strip filled with fine iron filings doesn't just bend in a magnetic field: it ties itself into a stable knot. This effect was predicted and reproduced in experiments by physicists working with magnetoactive elastomers: a silicone base with carbonyl iron particles.
Under a uniform magnetic field, the beam stays almost straight along its entire length, but at one spot a sharp helical twist appears, as if someone had twisted a rope hard. This twist doesn't straighten out because, in that zone, the elastic moment (the rubber's tendency to regain its shape) is exactly compensated by the magnetic moment. The iron particles inside the twist align not along the field but at an angle — because of the beam's shape, the magnetization 'pushes' against its edges.
Such a knot is stable: if you nudge it slightly, the forces bring it back into place. This means the beam can be used as a switch — two straight sections with a twisted zone between them. By changing the field, you can move the knot along the beam or make it disappear. For soft robotics and magnetic actuators, this is a new way to control shape without complex mechanisms.
The knot in a rubber beam is a rare example of magnetic energy and elasticity combining into a stable structure. It resembles a knot in a shoelace that doesn't come undone on its own because friction and tension are balanced. Here, instead of friction, it's a magnetic field, and instead of a shoelace, it's a smart material.
Original abstract
as written by the authors arXiv:2602.23682
A novel effect involving the formation of a stable kink-like torsion deformation in a magnetoactive elastomer (MAE) beam subjected to a uniform magnetic field is theoretically predicted and experimentally confirmed. The phenomenon was demonstrated using an elastomer beam containing soft magnetic carbonyl iron microparticles within a silicone matrix. The torsion kink acts as a transition boundary between two undeformed homogeneous states of the beam. We show that the elastic moment is compensated by a magnetoelastic moment in the kink region, where the local magnetization of the beam is non-collinear to the applied magnetic field due to shape anisotropy. We It is established that within the kink region, the MAE beam exists in a non-uniformly elastically deformed, low-symmetry magnetic state. Outside the kink, the beam's magnetization is collinear with the magnetic field, corresponding to an undeformed, high-symmetry homogeneous magnetic state.
Carbonyl iron is produced by decomposing iron pentacarbonyl — a liquid that boils at 103 °C. The resulting particles are so pure that they are used in antennas and magnetic fluids.