Active matter consists of constituents that consume energy in order to self-propel. It can be found in different environments and, in particular, in the microscopic world of micro-organisms and artificial microswimmers, where inertia is negligible. The talk illustrates how the non-equilibrium determines the behavior of active matter.

In dry active matter the frictional coupling to the surrounding is described by a friction coefficient. We use the paradigmatic system of active Brownian particles to discuss how active Szilard engines use information to perform work [1] and explore the unusual dynamic properties of an active bath using microrheology [2].

Microswimmers initiate hydrodynamic flow fields in their aequous environment through which they interact. For this wet active matter, we performed numerical simulations with the method of multi-particle collision dynamics, where we investigate the collective dynamics of spherical and elongated model microswimmers [3,4]. Depending on the neutral or pusher/puller swimmer type, they exhibit a variety of fascinating emergent collective dynamics, including plumes, convective rolls, and active turbulence.

[1] P. Malgaretti and H. Stark, Szilard engines and information-based work extraction for active systems, Phys. Rev. Lett. 129, 228005 (2022).

[2] M. Knezevic, L.E. Aviles Podgurski, and H. Stark, Oscillatory active microrheology of active suspensions, Sci. Rep. 11, 22706 (2021).

[3] F. Rühle and H. Stark, Emergent collective dynamics of bottom-heavy squirmers under gravity, Eur. Phys. J. E 43, 26 (2020).

[4] A.W. Zantop and H. Stark, Emergent collective dynamics of pusher and puller squirmer rods: Swarming, clustering, and turbulence, Soft Matter 18, 6179 (2022).