Cooperative Motion of Microswimmers: The Influence of Ionic and Reactive Screening on Hydrodynamic Interactions in Complex Fluids

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In this project, we will study the effect of electrostatic and reactive screening on the collective dynamics of chemically driven microswimmers in complex media -- specifically, fluids with viscoelastic response -- using computational methods. The ultimate goal is to achieve a better understanding of the interplay of swimmers' contact, (elasto)hydrodynamic, and phoretic (chemical) interactions that underlie the cooperative behavior observed in a large variety of real-world systems. For example, for chemical model systems, which have recently gained experimental attention within the active matter community, an improved understanding of the propulsion mechanisms may give insights into the way bacteria invade the protective viscoelastic mucus lining our vital organs. Our project aims to contribute to this effort by studying currently available model systems from a more fundamental perspective.We have recently considered a variety of self-electrophoretic systems, e.g., Janus colloids in a hydrogen peroxide solution, where we have theoretically demonstrated that their self-propulsion can be strongly modulated by bulk ionic reactions that induce reactive screening. We will extend these single particle results in this SPP funding period to encompass the collective dynamics of these systems. To this end, we will model the active particles using our lattice-electrokinetics (lattice-EK) algorithm, developed during the previous SPP funding period. Lattice-EK exploits the computational efficiency of the lattice-Boltzman (LB) algorithm for hydrodynamics and combines this with powerful numerical solvers for solute concentration and electrostatic fields. This efficiency enables us to simulate the interaction between many chemical swimmers and explore the way collective motion comes about, while giving us full control over all elements included in the modelling. Our lattice-EK is state of the art, as it is capable of simulating moving boundary conditions for charged systems, a first of its kind for this type of algorithm.Our strategy for this SPP funding period is to make further algorithmic improvements to the lattice-EK in order to facilitate the study of collective motion and to better recover the relevant physics. These modifications include adaptive grid refinement and lubrication corrections, as well as bringing together lattice-EK and viscoelastic LB methods. Simultaneously, we will use our software to investigate a number of swimmer systems that are experimentally realized by our SPP collaborators, and systematically characterize the collectivity therein. By following this approach, our project will lead to an improved understanding of the key parameters that bring about the complex out-of-equilibrium behavior of actively driven particles.