Symmetry breaking propulsion of magnetic microspheres in nonlinearly viscoelastic fluids

Microscale propulsion impacts a diverse array of fields ranging from biology and ecology to health applications, such as infection, fertility, drug delivery, and microsurgery. However, propulsion in such viscous drag-dominated fluid environments is highly constrained, with time-reversal and geometric symmetries ruling out entire classes of propulsion. Here, we report the spontaneous symmetry-breaking propulsion of rotating spherical microparticles within non-Newtonian fluids. While symmetry analysis suggests that propulsion is not possible along the fore-aft directions, we demonstrate the existence of two equal and opposite propulsion states along the sphere's rotation axis. We propose and experimentally corroborate a propulsion mechanism for these spherical microparticles, the simplest microswimmers to date, arising from nonlinear viscoelastic effects in rotating flows similar to the rod-climbing effect. Similar possibilities of spontaneous symmetry-breaking could be used to circumvent other restrictions on propulsion, revising notions of microrobotic design and control, drug delivery, microscale pumping, and locomotion of microorganisms. A self-propelling agent at small Reynolds numbers usually requires a fore-aft asymmetry in order to circumvent the scallop theorem. Here Rogowski et al. show that this need not be true for motion in non-linear viscoelastic fluids, where an initial symmetry may be broken spontaneously.

Automated summary

This study demonstrates the spontaneous symmetry-breaking propulsion of rotating spherical microparticles within non-Newtonian fluids, providing new possibilities for microscale propulsion technology. The existence of two equal and opposite propulsion states along the sphere's rotation axis challenges previous symmetry analysis and offers insights into novel propulsion mechanisms for microswimmers.