Flexible paramagnetic membranes in fast precessing fields

Elastic membranes composed of paramagnetic beads offer the possibility of assembling versatile actuators operated autonomously by external magnetic fields. Here we develop a theoretical framework to study shapes of such paramagnetic membranes under the influence of a fast precessing magnetic field. The competition of the elastic and magnetic energies that arise as a result of their bending and the induced dipolar interactions between nearest neighbors beads, respectively, determine their shape. In the harmonic approximation, the elastic energy is quadratic in the surface curvatures. We account for the magnetic energy by introducing a continuum limit energy, quadratic in the projections of the surface tangents onto the precession axis. We derive the Euler-Lagrange equation governing the equilibria of these membranes, as well as the corresponding stresses. We apply this framework to examine paramagnetic membranes with quasi-planar, cylindrical and helicoidal geometries. In all cases we found that their shape, energy and stresses can be modified by means of the parameters of the magnetic field, mainly by the angle of precession.