Magnetically driven omnidirectional artificial microswimmers

Abstract

We present an experimental realisation of two new artificial microswimmers that swim at low Reynolds number. The swimmers are externally driven with a periodically modulated magnetic field that induces an alternating attractive/repulsive interaction between the swimmer parts. The field sequence also modulates the drag on the swimmer components, making the working cycle non-reciprocal. The resulting net translational displacement leads to velocities of up to 2 micrometers per second. The swimmers can be made omnidirectional, meaning that the same magnetic field sequence can drive swimmers in any direction in the sample plane. Although the direction of their swimming is determined by the momentary orientation of the swimmer, their motion can be guided by solid boundaries. We demonstrate their omnidirectionality by letting them travel through a circular microfluidic channel. We use simple scaling arguments as well as more detailed numerical simulations to explain the measured velocity as a function of the actuation frequency.

Recent Advances in Nano‐ and Micromotors

“…[28] Two types of magnetic micromotors, namely thrower and rower, according to their mobility performance, were designed. [29] The thrower consisted of two spherical magnetic particles of different sizes that assembled and disassembled with time, resulting in micromotor motion. Alternatively, the rower was based on three magnetic particles of the same size, but with one of them de-attached and re-attached from the other two, again yielding in micromotor movement.…”

“…This adaptation can take place either by exhibiting a flexible planar tail or by taking advantage of aggregates of individual nano/micromotors in the shape of chains . Two types of magnetic micromotors, namely thrower and rower, according to their mobility performance, were designed . The thrower consisted of two spherical magnetic particles of different sizes that assembled and disassembled with time, resulting in micromotor motion.…”

The TAP–Plefka Variational Principle for the Spherical SK Model

“…In the classical Bragg-Williams approximation [8,28,Section 4.1.2] in non-disordered statistical physics one restricts this sup to simple measures G that are parameterized by a mean magnetization m ∈ R N ; in the case of ±1 spins one considers measures under which the spins σ i are independent with mean m i . For any m the corresponding measure gives a lower bound for the free energy, because of the Gibbs variational principle.…”

Modular approach to microswimming

“…2g) can also be rectified by circular channels. 55 With the development of 3D printing and microfluidic manufacturing techniques, 114,115 more chemically or physically modified substrates can be designed to guide modular swimmers to specified destinations. 23 External magnetic/electric field control has become a prominent approach to steering single unit swimmers of asymmetric magnetic properties or anisotropic dielectric properties.…”