Project C7: Dense active suspensions in the chaotic regime
Active matter has become a quickly evolving field spanning from biology and physics to chemistry and engineering. Its defining property is the directed motion—translational, rotational, or both—of its constituents. This directed motion requires the steady input of free energy. Freed from the constraints of thermal equilibrium, active matter exhibits a wide range of novel phenomena; on the level of its single constituents up to emergent many-body collective and dynamic behavior. Extensively studied have been the aggregation of active particles into clusters, swarms, and other highly collective and dynamics states; but also spontaneous flow states where sufficiently high activity triggers the transition from a quiescent to a flowing fluid. At high densities, chaotic behavior has been reported in suspensions of bacteria and in numerical simulations. The aim of this project is to develop a comprehensive multiscale framework that bridges the properties of single particles to the large-scale chaotic flows. This framework will combine fully hydrodynamic simulations on the smallest scale with effective “dry” models on intermedium, and a continuum two-phase model on the coarsest scale. These approaches are complemented by statistical modeling aiming to uncover symmetries that can be exploited to further simply the effective and continuum models. During the first funding period of this project (second funding period of the TRR), we have successfully implemented and numerically investigated a model system of elliptic particles propelled by stress-boundary conditions. This has allowed us to already simulate particle pairs and small suspensions with full hydrodynamic details. In a parallel study, we have investigated the collective behavior of elliptic particles using implicit solvent simulations with effective interactions. Moreover, the basic equations for a detailed statistical analysis have been derived. The objective of the next funding period is to converge the different efforts into a comprehensive theoretical and numerical framework to model suspensions of active particles, bridging the scales from near-field hydrodynamic interactions to the macroscopic flow behavior.
Publications
Thermodynamics of active matter with internal degrees of freedom
R. Bebon, T. Speck
Commun. Physics 9, 162
Link: https://doi.org/10.1038/s42005-026-02662-z
Mutual linearity is a generic property of steady-state Markov networks
Robin Bebon, Thomas Speck
Physical Review Letters 136, 137401 (2026)
Link: https://doi.org/10.1103/jcm3-57d8
Thermodynamically consistent Continuum Theory for Active Ellipsoidal Particle Suspensions
Ben Gozlen, H., Wang, Y. and M. Oberlack
Intnl. J. Multiphase 202, 105839 (2026)
Link: https://doi.org/10.1016/j.ijmultiphaseflow.2026.105839
Impact of currents on non-equilibrium coexistence in chemically driven mixtures
Ellen Meyberg, Joshua F. Robinson, Thomas Speck
The Journal of Chemical Physics 165, 044118 (2026)
Link: https://doi.org/10.1063/5.0336459
Negative drag force on beating flagellar-shaped bodies in active fluids
Timo Knippenberg, Robin Bebon, Thomas Speck, Clemens Bechinger
Physical Review Letters 135, 118301 (2025)
Link: https://doi.org/10.1103/3xbk-sxhh
Steady inhomogeneous shear flows as mechanical phase transitions
Thomas Speck
Physical Review E 111, 015430 (2025)
Link: https://doi.org/10.1103/PhysRevE.111.015430
Thermodynamics of Active Matter: Tracking Dissipation across Scales
Robin Bebon, Joshua F. Robinson, Thomas Speck
Physical Review X 15, 021050 (2025)
Link: https://doi.org/10.1103/PhysRevX.15.021050
Continuum Modeling and Numerical Simulation of Active Suspensions in Curved Channels
H. Ben Gozlen, Y. Wang, and M. Oberlack
Theor. Comput. Fluid Dyn. 39, 32 (2025)
Link: https://doi.org/10.1007/s00162-025-00752-2
Collective Hall current in chiral active fluids: Coupling of phase and mass transport through traveling bands
Frank Siebers, Robin Bebon, Ashreya Jayaram, Thomas Speck
PNAS 121, e2320256121 (2024)
Link: https://doi.org/10.1073/pnas.2320256121
A mixture theory formulation for active suspensions
H. Ben Gozlen, Y. Wang, and M. Oberlack
Proc. Appl. Math. Mech. 24, e202400166 (2025)
Link: https://doi.org/10.1002/pamm.202400166
Effective dynamics and fluctuations of a trapped probe moving in a fluid of active hard discs
Ashreya Jayaram, Thomas Speck
EPL 143, 17005 (2023)
Link: https://doi.org/10.1209/0295-5075/acdf1a
Exploiting compositional disorder in collectives of light-driven circle walkers
Frank Siebers, Ashreya Jayaram, Peter Blümler, Thomas Speck
Science Advances 9, eadf5443 (2023)
Link: https://doi.org/10.1126/sciadv.adf5443
A deterministic two-phase model for an active suspension with non-spherical active particles using the Eulerian spatial averaging theory
B. Deußen, Y. Wang, M. Oberlack
Phys. Fluids 34, 023302 (2022)
Link: https://doi.org/10.1063/5.0077735
Force Generation in Confined Active Fluids: The Role of Microstructure
Shuvojit Paul, Ashreya Jayaram, N Narinder, Thomas Speck, Clemens Bechinger
Physical Review Letters 129, 058001 (2022)
Link: https://doi.org/10.1103/PhysRevLett.129.058001
Second-order invariants of the inviscid Lundgren-Monin-Novikov equations for 2d vorticity fields
V.N. Grebenev, A.N. Grichkov, M. Oberlack, M. Waclawczyk
Z. Angew. Mathematik und Physik 72, 129 (2021)
Link: https://doi.org/10.1007/s00033-021-01562-2
Hunting active Brownian particles: Learning optimal behavior
Marcel Gerhard, Ashreya Jayaram, Andreas Fischer, and Thomas Speck
Physical Review Letters 104, 054614, (2021)
Link: https://doi.org/10.1103/PhysRevE.104.054614
Probability theory of active suspensions
B. Deußen, M. Oberlack, Y. Wang
Physics of Fluids 33 (6), 061902 (2021)
Link: https://doi.org/10.1063/5.0047227
Vorticity Determines the Force on Bodies Immersed in Active Fluids
Thomas Speck, Ashreya Jayaram
Physical Review Letters 126 (13), (2021)
Link: https://doi.org/10.1103/PhysRevLett.126.138002
High-order simulation scheme for active particles driven by stress boundary conditions
B Deußen, A Jayaram, F Kummer, Y Wang, T Speck, M Oberlack
Journal of Physics: Condensed Matter33 (24),244004 (2021)
Link: https://doi.org/10.1088/1361-648X/abf8cf
Quorum-sensing active particles with discontinuous motility
Andreas Fischer, Friederike Schmid, Thomas Speck
Physical Review E101 (1), 012601 (2020)
Link: https://doi.org/10.1103/PhysRevE.101.012601
From scalar to polar active matter: Connecting simulations with mean-field theory
Ashreya Jayaram, Andreas Fischer, Thomas Speck
Physical Review E 101 (2), (2020)
Link: https://doi.org/10.1103/PhysRevE.101.022602
Statistical theory of helical turbulence
B. Deußen, D. Dierkes, and M. Oberlack
Physics of Fluids 32 (6), 065109 (2020)
Link: https://doi.org/10.1063/5.0010874
Implicit-explicit and explicit projection schemes for the unsteady incompressible Navier-Stokes equations using a high-order dG method
Nehzat Emamy, Florian Kummer, Markus Mrosek, Martin Karcher, Martin Oberlack
Computers and Fluids 154, 285-295 (2017)
Link: https://doi.org/10.1016/j.compfluid.2017.06.003