Inducing Chirality and Chiral Instabilities in Ferrogels with Homogenous Magnetic Fields

Soft magnetic actuators are materials that undergo bending, elongation, contraction or torsion in magnetic fields. In this paper we investigate ferrogel-based cylindrical actuators that undergo deterministic chiral deformations and chiral instabilities in uniform applied magnetic fields. Such fields cannot induce body forces but can induce body torques. These actuators are in the form of hydrogel cylinders containing magnetic particles (MPs) that have been field structured into particle chains in a direction normal to its cylindrical axis during gelation. When one end of the cylinder is fixed and a uniform magnetic field is applied normal to the cylindrical axis, but at an angle to the particle chains, the cylinder twists. A subsequent rotation of the cylinder base can lead to a metastable state and further deformation can lead to a chiral instability wherein the sign of the chirality switches. This actuation is studied as a function of the type of polymer used in the hydrogel preparation (alginate or acrylamide), the type of MPs used (silica coated iron particles or permalloy flakes particles) and the volume fraction of the magnetic phase. The torsional deformations, as well as certain hysteresis effects, are compared to a theoretical model developed herein.