Enhanced clamshell swimming with asymmetric beating at low Reynolds number

Multiflagellate Swimming Controlled by Hydrodynamic Interactions

Many eukaryotic microorganisms propelled by multiple flagella can swim very rapidly with distinct gaits. Here, we model a three-dimensional mutiflagellate swimmer, resembling the microalgae. When the flagella are actuated synchronously, the swimming efficiency can be enhanced or reduced by interflagella hydrodynamic interactions (HIs), determined by the intrinsic tilting angle of the flagella. The asynchronous gait with a phase difference between neighboring flagella can reduce oscillatory motion via the basal mechanical coupling. In the presence of a spherical body, simulations taking into account the flagella-body interactions reveal the advantage of anterior configuration compared with posterior configuration, where in the latter case an optimal flagella number arises. Apart from understanding the role of HIs in the multiflagellate microorganisms, this work could also guide laboratory fabrications of novel microswimmers.

Spontaneous oscillation of an active filament under viscosity gradients

We investigate the effects of uniform viscosity gradients on the spontaneous oscillations of an elastic, active filament in viscous fluids. Combining numerical simulations and linear stability analysis, we demonstrate that a viscosity gradient increasing from the filament's base to tip destabilises the system, facilitating its self-oscillation. This effect is elucidated through a reduced-order model, highlighting the delicate balance between destabilising active forces and stabilising viscous forces. Additionally, we reveal that while a perpendicular viscosity gradient to the filament's orientation minimally affects instability, it induces asymmetric ciliary beating, thus generating a net flow along the gradient. Our findings offer new insights into the complex behaviours of biological and artificial filaments in complex fluid environments, contributing to the broader understanding of filament dynamics in heterogeneous viscous media.

Elastohydrodynamic propulsion of a filament magnetically driven at both ends

The elastohydrodynamic interaction between an elastic filament and its surrounding fluid was exploited to develop the first microswimmers. These flexible microswimmers are typically actuated magnetically at one end and their propulsion behavior is relatively well understood. In this work, we move beyond the traditional single-end actuation setup and explore the propulsion characteristics of an elastic filament driven by magnetic torques at both ends. We report the emergence of new modes of propulsion behaviors in different physical regimes, depending on the balance of elastic and viscous forces as well as the arrangement of the magnetic moments at the filament ends. In particular, under the same magnetic actuation, a filament driven at both ends can propel either forward or backward depending on its relative stiffness. Moreover, this new backward propulsion mode can generate a magnitude of propulsion that is unattainable by the traditional single-end actuation setup. We characterize these new propulsion behaviors and provide some physical insights into how they emerge from the complex interplay between viscous and elastic forces and magnetic actuation in various configurations. Taken together, these findings could guide the development of soft microrobots with enhanced propulsion performance and maneuverability for future biomedical applications.