Paramagnetic filaments in a fast precessing field: Planar versus helical conformations

TL;DR

This paper analytically investigates the equilibrium shapes of elastic paramagnetic filaments in a precessing magnetic field, identifying conditions favoring planar or helical conformations based on energy minimization.

Contribution

It introduces an analytical framework for predicting filament conformations under magnetic precession, considering both bending and magnetic energies, and characterizes conditions for planar versus helical shapes.

Findings

Planar and helical filament conformations depend on magnetic and bending parameters.

Energy minimization determines the most probable filament shape.

Parameter ranges for different conformations are identified.

Abstract

We examine analytically equilibrium conformations of elastic chains of paramagnetic beads in the presence of a precessing magnetic field. Conformations of these filaments are determined by minimizing their total energy, given in the harmonic approximation by the sum of the bending energy, quadratic in its curvature, and the magnetic dipolar interaction energy, quadratic in the projection of the vector tangent to the filament onto the precession axis. In particular, we analyze two families of open filaments with their ends aligned along the precession axis and described by segments of planar curves and helices. These configurations are characterized in terms of two parameters encoding their features such as their length, separation between their ends, as well as their bending and magnetic moduli, the latter being proportional to the magnitude and precession angle of the magnetic field. Based on energetic arguments, we present the set of parameter values for which each of these families of curves is probable to occur.