Andrej Vilfan: Odd Viscosity

In chiral active fluids, the breaking of the time-reversal and parity symmetries results in a peculiar transport coefficient called odd viscosity, which does not contribute to the energy dissipation. It can in principle be found at a wide range of scales, from electron fluids in graphene to suspensions of spinning particles to macroscopic tabletop experiments, although its observation in active colloidal systems is still largely limited to two-dimensional layers. I will present some exact analytical solutions of the Stokes equation with odd viscosity and show how the broken symmetries of an active particle, together with the broken symmetry in the active fluid, lead to interesting dynamics [1,2]. A related concept is odd Navier slip, which can occur at chiral active surfaces. I will discuss a microscopic model consisting of flexibly anchored rotating cilia that leads to an effective odd slip at the surface. Finally, I will show that a suspension of particles with odd slip exhibits odd viscosity, establishing a connection between the "odd" phenomena in different dimensions [3].

[1] Y. Hosaka, R. Golestanian and A. Vilfan, Phys. Rev. Lett. 131, 178303 (2023) [2] Y. Hosaka, M. Chatzittofi, R. Golestanian and A. Vilfan, Phys. Rev. Research 6, L032044 (2024) [3] Y. Hosaka, A. Vilfan, in preparation (2026)