Interactions between microswimmers, membranes, and biofilms

Locomotion by swimming is an important ingredient for the survival strategies of microorganisms in their natural habitats, where they encounter diverse chemical substances, other organisms, and confining surfaces. This interplay leads to complex physical phenomena that determine a broad range of biological processes. A profound understanding of the underlying interactions is of utmost importance for microbiology as well as future progress in the nanotechnology of the 21st century. This includes the development of artificial self-propelled particles as tools for controlled drug-delivery systems and bioremediation processes. The research project entitled Interactions between microswimmers, membranes, and biofilms is concerned with the interactions of microswimmers with confining boundaries, such as biological membranes and biofilms. These soft materials exhibit peculiar elastic properties and respond sensitively to external stresses, which evoke a deformation of their boundary. In addition, biofilms are composed of collections of microorganisms, which adhere to each other and communicate using signal molecules. Recent studies have shown that these chemicals can be transported outside the biofilm boundary and thereby crucially impact the behavior of nearby microswimmers. One major goal of the research project represents the theoretical characterization of the hydrodynamic interactions between microswimmers and elastic surfaces. In particular, we want to elucidate, how the flow induced by swimming microorganisms can deform the elastic boundary and how this deformation impacts the behavior of the microswimmer. In this context, different swimming mechanisms are considered. Furthermore, the project aims at characterizing the dynamic behavior of microswimmers in the presence of chemical signals, which are transmitted by the biofilm. Particular emphasis lies on the investigation of the complex interplay of the elastic surface, the chemical signals, and the microswimmer motion. To address these research objectives advanced analytical methods are employed and a simulation software is developed. A detailed comparison of analytic results with simulations will strengthen our predictions. The theoretical results should motivate experiments, specifically at the host university, to obtain a quantitative understanding of real biological systems. A particular feature of the research project is the consideration of soft, deformable surfaces, which occur in almost all biological systems. Our analytical framework is expected to set a new milestone towards a profound understanding of microswimmer motion in real biological settings. The additional consideration of chemical interactions has great potential to improve todays physical understanding of biofilms and their impact on their surrounding environment.

The first part of the project was concerned with the motion of self-propelled agents in complex environments. Locomotion by swimming is important for the survival of many microorganisms in their natural habitats, where they encounter a plethora of external stimuli, crowded environments, and confining surfaces with peculiar material properties. Understanding the fundamental physical mechanisms underlying the motion of the microswimmers in these complex environments is of utmost importance for microbiology and for future progress in nanotechnological applications. Using computer simulations and theory, we unraveled a geometric criterion for the optimal spreading of microswimmers in porous materials, such as soils and tissues. Our results demonstrate that intrinsic mechanisms, which allow agents to change their swimming direction, are indispensable for microorganisms to move in their natural habitats, as they allow them to escape dead-end-pores. These findings may be important for the design of future cargo-carriers, which are expected to, for example, penetrate the tight structures of tumors. Furthermore, we studied the physics of dense suspensions of self-propelled filaments, as model system for elongated bacteria or biofilaments omnipresent inside cells. Most importantly, we found that individual filaments move faster in dense suspensions of others than in a free environment. This counter-intuitive finding is in striking contrast to the slow-down of individuals in traffic jams or in crowds of people and illustrates the rich physics of active matter at the microscale. Using a combination of computer simulations, a scaling theory, and analytical predictions, we could fully characterize the swimming behavior of these active agents in these highly-entangled environments. In addition, the project addressed details of the hydrodynamic interactions of self-propelled agents with elastic and periodic, corrugated surfaces. The second part of the project was concerned with the hydrodynamic coupling of externally-driven particles with corrugated and randomly structured surfaces. These fluid-structure interactions are central to a large variety of microfluidic applications, ranging from the mixing of complex particulate suspensions to the sorting and focusing of biological samples. Combining experiments and theory, we revealed the full three-dimensional helical particle dynamics near corrugated surfaces and elucidated the fundamental physical mechanisms leading to drift of the particle along the surface corrugations. We further identified an optimal drift and universal transport behavior as a function of the relevant length scales of our system. We have extended our work for studying particulate suspensions, which are subject to Brownian motion, through corrugated channels in pressure-driven flows and derived analytical expressions for their effective spreading and drift. Our findings could enable optimization and enhanced sensitivity in existing microfluidic approaches for particle focusing, fractionation, and separation.