Getting drowned in a swirl: Deformable bead-spring model microswimmers in external flow fields

Abstract

Deformability is a central feature of many types of microswimmers, e.g. for artificially generated self-propelled droplets. Here, we analyze deformable bead-spring microswimmers in an externally imposed solvent flow field as simple theoretical model systems. We focus on their behavior in a circular swirl flow in two spatial dimensions. Linear (straight) two-bead swimmers are found to circle around the swirl with a slight drift to the outside with increasing activity. In contrast to that, we observe for triangular three-bead or square-like four-bead swimmers a tendency of being drawn into the swirl and finally getting drowned, although a radial inward component is absent in the flow field. During one cycle around the swirl, the self-propulsion direction of an active triangular or square-like swimmer remains almost constant, while their orbits become deformed exhibiting an "egg-like" shape. Over time, the swirl flow induces slight net rotations of these swimmer types, which leads to net rotations of the egg-shaped orbits. Interestingly, in certain cases, the orbital rotation changes sense when the swimmer approaches the flow singularity. Our predictions can be verified in real-space experiments on artificial microswimmers.

Active colloidal molecules

“…This definition is not completely sharp. Previously the attractive bond has been postulated to be permanent (like a springlike force) [25,26] but we generalize it here a bit towards other strong bonding attraction energies much larger than the thermal energy k B T . Typically the bonding force is then larger than or comparable to forces arising from selfpropulsion.…”

Optimal motion of triangular magnetocapillary swimmers

“…Minimization of expression (20) under similar assumptions as of Eq. (18) yields for the equilibrium L 12 19)) and ∼ m y (given by Eq.…”

“…The motion at low frequencies is, furthermore, dominated by the sizable deformations of the interface and the capillary potential has a much more complex structure than described by Eqs. (17) and (20), making the determination of the spring constant even more challenging, given the current setup of the simulation. Importantly, the deformations of the interface introduce an additional time scale τ int…”

Active particles in noninertial frames: How to self-propel on a carousel

“…Moreover one has to define whether the self-propulsion occurs relative to the moving or rest frame, and the same needs to be specified for the fluctuations (white noise). It is a bit surprising that -except for very recent work of active particles on a rotating spherical surface [21] -this issue was not yet considered and explored in microswimmer physics; what has been addressed is overdamped motion of swimmers in an external flow field [22][23][24][25] but this flow field is typically different from that of a purely rotating fluid.…”