The chiral magnetic nanomotors

Abstract

Propulsion of the chiral magnetic nanomotors powered by a rotating magnetic field is in the focus of the modern biomedical applications. This technology relies on strong interaction of dynamic and magnetic degrees of freedom of the system. Here we study in detail various experimentally observed regimes of the helical nanomotor orientation and propulsion depending on the actuation frequency, and establish the relation of these two properties with the remanent magnetization and geometry of the helical nanomotors. The theoretical predictions for the transition between the regimes and nanomotor orientation and propulsion speed are in excellent agreement with available experimental data. The proposed theory offers a few simple guidelines towards the optimal design of the magnetic nanomotors. In particular, efficient nanomotors should be fabricated of hard magnetics, e.g., cobalt, magnetized transversally and have the geometry of a normal helix with a helical angle of 35 • ÷ 45 • .

Magnetization directions and geometries of helical microswimmers for linear velocity-frequency response

“…They showed that microswimmers with such magnetization had much reduced wobbling compared to microswimmers with random easy axis, as well as improved swimming characteristics. In addition, in a theoretical examination of the rotational and swimming behavior of helices with permanent magnetic dipole, Morozov and Leshansky [42] concluded that for such microswimmers, it is optimal to have the magnetic dipole perpendicular to the helical axis to minimize wobbling.…”

“…Man and Lauga [40] showed that during wobbling the precession angle scales as inverse frequency using nu merics and asymptotics of nearly straight helices. Ghosh e t a l. [39,41] and Morozov and Leshansky [42] investi gated the transition of stability from tumbling to wobbling behavior as frequency increases by treating the rotational dynamics as that of ellipsoids numerically and analytically, respectively.…”

“…In the previous work on the rotational dynamics of helical microswimmers mentioned above [39,41,42], the rotational dynamics was approximated by the dynamics of an ellipsoid or rod, ignoring chirality and nonaxisymmetry of the helix. Here we treat the rotational dynamics of a truly helical geometry.…”

Hydrodynamic interactions between two forced objects of arbitrary shape. I. Effect on alignment

“…[8][9][10][11] dealt with isolated asymmetric objects in Stokes flow, which exhibit a chiral response. The object's chiral response is encoded in the offdiagonal block of its self-mobility matrix, referred to as the twist matrix.…”

“…Such richer responses can be exploited to obtain "steerable colloids" -objects whose orientation and transport can be controlled in much more detail. For example, applying a torque by a rotating uniform magnetic field was used to achieve efficient transport of chiral magnetic objects 8 . Another example, which is the main issue of the present work, is the ability to achieve orientational alignment of asymmetric objects by applying an external force [9][10][11] .…”