Illustration of a micron sized particle translating in a viscoelastic fluid without (top) and with rotation (bottom).
A spinning particle is deflected perpendicular to the driving force (bottom right) when immersed in a viscoelastic medium

Memory Induced Magnus Effect

Spinning objects which move through air or liquids experience a Magnus force. Opposed to large objects where Magnus forces are strong they are only weak at small scales and eventually vanish for overdamped micron-sized particles in simple liquids. Here we demonstrate an about one-million-fold enhanced Magnus force of spinning colloids in viscoelastic fluids. Such fluids are characterized by a time-delayed response to external perturbations which causes a deformation of the fluidic network around the moving particle.

We use a rotating external magnetic field to rotate magnetic microparticles and gravitational force to translate them. For a purely viscous solution we observe no influence of rotation on the particle trajectory, as the motion follows the direction of the external force. In an experiment in viscoelastic fluids, we find a deflection perpendicular to the external force, which depends on the rotation direction of the particles. Compared to the regular Magnus Force in an overdamped system, this effect is over one million times as strong, while also pointing in the opposite direction. For clusters with three particles, we can measure the exact rotation frequency of the cluster, and observe that the memory induced Magnus Force is easily tunable with rotation speed.

We find that when the particle spins the deformation field of the viscoelastic fluid becomes misaligned relative to the particle’s moving direction, leading to a force perpendicular to the direction of travel and the spinning axis. This can be described with a density dipole which is deflected as a result of the particle rotation. The theoretical description of this effect involves other familiar characteristics of viscoelastic fluids, such as the relaxation timescale.

The presence of strongly enhanced memory-induced Magnus forces at microscales opens novel applications for particle sorting and steering, the creation and visualization of anomalous flows and more.

Memory induced Magnus effect

X. Cao, D. Das, N. Windbacher, F. Ginot & C. Bechinger                                                                                

Nat. Phys. (2023)

Collaboration of physicists from the University of Konstanz and the University of Göttingen
Funding provided by: Collaborative Research Centre SFB 1432 "Fluctuations and Nonlinearities in Classical and Quantum Matter beyond Equilibrium", German Research Foundation (DFG)