Motile active matter—emergent properties by structure and hydrodynamics
Prof. Dr. Roland Winkler (Forschungszentrum Jülich)

The perpetual conversion of either internal chemical energy, or utilization of energy from the environ- ment into directed motion is an integral process in active matter [1]. Its respective out-of-equilibrium nature is the origin of intriguing emerging structural and dynamical properties, which are absent in pas- sive systems. This particularly applies to soft matter systems, e.g., comprised of filaments or polymers, which renders active soft matter a promising new class of materials. The spatiotemporal dynamics of motile active matte systems is controlled by the propulsion mechanism of the active agents in combination with various direct interactions, such as steric repulsion and hydrodynamics. These direct interactions are typically anisotropic, and emerge from different sources, such as spherical and elongated particle shapes, intrinsic flexibility, pusher and puller flow fields, etc. The combination of the various aspects leads to new emergent behavior, with a possible synergistically or antagonistically effect of the various interactions [2].
Our simulation studies of prolate spheroidal microswimmers—called squirmers—in quasi-two- dimensional confinement reveal a suppression of motility-induced phase separation (MIPS) by hydrody- namic interactions in contrast to MIPS in similar non-hydrodynamic active Brownian particles (ABPs) ensembles. The fundamental difference between ABPs and squirmers is attributed to an enhanced re- orientational dynamics of squirmers by hydrodynamic torques during their collisions. In contrast, for elongated squirmers, hydrodynamics interactions enhance MIPS. The transition to a phase-separated state strongly depends on the nature of the swimmers flow field, with an increased tendency toward MIPS for pullers, a reduced tendency for pushers. Thus, hydrodynamic interactions show opposing ef- fects on MIPS for spherical and elongated microswimmers, and details of the propulsion mechanism of biological microswimmers may be very important to determine their collective behavior.
Hydrodynamic interactions play a particular role in systems with a large number of internal degrees of freedom like in filamentous, polymer-like structures. As simulation and analytical studies show, activity leads to swelling of flexible polymers, shrinkage and reswelling of semiflexible polymers, and an enhanced dynamics [3]. In such systems, hydrodynamics enhances shrinkage, modifies swelling significantly, and changes the intramolecular dynamics. The shrinkage, even in the presence of excluded-volume interac- tions, results in an enhanced packing, which might be important for polymer organization in confinement.

[1] J. Elgeti, R. G. Winkler, G. Gompper, Physics of microswimmers—single particle motion and collective behavior: a review, Rep. Prog. Phys., 78, 056601 (2015)

[2] M. Theers, E. Westphal, K. Qi, R. G. Winkler, G. Gompper Clustering of microswimmers: Interplay of shape and hydrodynamics , Soft Matter, accepted for publication; arXiv:1807.01211 (2018)

[3] T. Eisenstecken, G. Gompper, R. G. Winkler, Internal dynamics of semiflexible polymers with active noise, J. Chem. Phys. 146, 154903 (2017)