Dynamical configurations and bistability of helical nanostructures under external torque

Abstract

We study the motion of a ferromagnetic helical nanostructure under the action of a rotating magnetic field. A variety of dynamical configurations were observed that depended strongly on the direction of magnetization and the geometrical parameters, which were also confirmed by a theoretical model, based on the dynamics of a rigid body under Stokes flow. Although motion at low Reynolds numbers is typically deterministic, under certain experimental conditions the nanostructures showed a surprising bistable behavior, such that the dynamics switched randomly between two configurations, possibly induced by thermal fluctuations. The experimental observations and the theoretical results presented in this paper are general enough to be applicable to any system of ellipsoidal symmetry under external force or torque.

The chiral magnetic nanomotors

“…(17) to the experimental results [8] is shown in Fig. 5 for the critical frequency ν c = A = 11.5 Hz.…”

“…The optimal value of the helix angle is in the range Θ = 35 • ÷ 45 • maximizing the chirality, Ch ≈ 0.2 (v). Despite a large variability in the nanofabrication techniques and experimental setups [4][5][6][7][8][9][10]19], it is interesting to point out that one of the pioneering experimental works in this field, [4], has empirically adopted most of the rules formulated above.…”

“…2). According to [8] (21) yields θ min = 13.8 • . The theoretical estimate proves to be close to the the experimental value of θ min ≈ 14 • found in [8] for the rightmost point in Fig.…”

“…We determine the dimensionless ratio B /(ξ R) numerically as described in the Supporting Information and using the data reported in [8], i.e., R = 0.20 µm the radius of the helix, a = 0.17 µm for the radius of the filament, P = 1 µm for the helical pitch, so that the helical angle Θ = tan −1 (2πR/P ) ≈ 51.5 • . The computed value of the dimensional coefficient 2πB /ξ is equal to 0.094 µm, while for a slightly higher angle Θ = 53.5 • we find 2πB /ξ = 0.081 µm that perfectly fits the experimental results.…”

“…It is not surprising that in the past few years the new technique has attracted considerable attention [7][8][9][10]. Physically, the dynamics of the chiral magnetic nanomotors involve two interrelated phenomena of magnetic and hydrodynamic nature, respectively.…”