# Research

Our group is currently interested in the following research topics

We are always open for new challenges and collaborations!

## Transport behavior of active agents in complex environments

To optimize their survival microorganisms have evolved a large variety of ingenious strategies allowing them to efficiently forage food and proliferate in seemingly unhabitable niches, omnipresent in their diverse habitats, such as soils, the ocean, or the human body. Many microorganisms are able to self-propel through their complex surroundings, providing them an indispensable advantage over transport via ordinary diffusion. We are interested to unravel the intricate physics of this active transport behavior, which displays a range of unusual, out-of-equilibrium phenomena.

One aspect of our research deals with the motion of self-propelled agents in complex surroundings, which can be characterized by geometrical disorder, chemical fields, and hydrodynamics flows.

### Swimming in porous media

Efficient navigation through disordered, porous environments poses a major challenge for microorganisms and future synthetic cargo-carriers, which hold great potential for novel biomedical and environmental applications. In our  work, we model the motion of active agents by self-propelled stiff polymers that employ a run-reverse mechanism at random times. Our findings show that spreading of active agents in porous media can be optimized by tuning their run lengths. We propose a geometric criterion and demonstrate that the optimal spreading occurs when the run lengths are comparable to the longest available pore length of the porous medium. These reversal mechanisms should be included in the strategies for microrobots to traverse porous environments and are relevant for efficient drug-delivery systems and bioremediation tools.

Currently, we are interested in studying how this dynamical behavior changes with respect to:

• the shape of microorganisms,
• hydrodynamic flows, and
• chemical fields.

Relevant publication

C. Kurzthaler*, S. Mandal*, T. Bhattacharjee, H. Löwen, S. S. Datta, and H. A. Stone, Nat. Commun.12 (2021) *equally contributed

Press coverage

Princeton press Researchers find the best way for bacteria to navigate maze-like environments

HHU Düsseldorf press Wie Bakterien aus Labyrinthen herausfinden

### Highly-entangled suspensions of self-propelled filaments

Crowded is slower’ is a well studied problem in many fields ranging from caging in colloidal glasses to the evacuation of crowds to traffic jams. In striking contrast to these observations, we reveal that the diffusivity of self-propelled anisotropic agents, such as biofilaments and elongated bacteria, can be enhanced by orders of magnitude due to crowding. This counterintuitive crowded is faster’ behavior is corroborated by a scaling theory based on the concept of a confining tube pioneered by Doi and Edwards. Our findings propose a novel way to control the mobility of individual self-propelled anisotropic agents in disordered environments by tuning the degree of crowding.

Relevant publication:

S. Mandal*, C. Kurzthaler*, T. Franosch, H. Löwen, Phys. Rev. Lett.125 (2020) *contributed equally

Press coverage:

Physics Filaments Move Quickly Through a Crowd

### Hydrodynamic interactions of microswimmers with boundaries

We study hydrodynamic couplings and motion of microorganisms near elastic and corrugated surfaces using a combination of analytical and numerical methods. In the future, we aim at identifying the dwelling times of agents near these complex boundaries, which may provide insights into the onset of the formation of bacterial communities

Relevant publications:

C. Kurzthaler, H. A. Stone, Soft Matter 17 (2021)

A. Daddi-Moussa-Ider, C. Kurzthaler et al., Phys Rev. E100 (2019)

## Dynamics of synthetic and biological microswimmers

The foundation for investigating, for example, the swimming motion of agents in complex surroundings or their collective behaviors, lies in understanding and quantifying their dynamical behavior at the single-particle level in dilute environments. The spatiotemporal behavior of active (as well as passive or externally driven) particles is encoded in the intermediate scattering function, which constitutes the Fourier transform of the probability density of the particle's displacements. In particular, we have worked on characterizing the dynamical behavior of catalytic Janus colloids using exact solutions for the paradigmatic active Brownian particle model (right, lower panel) and the run-and-tumble behavior of E. coli bacteria in terms of renewal processes. We compare our theoretical predictions to observations of these synthetic and biological micro-swimmers measured by our experimental collaborators.

We are interested in developing theoretical predictions for quantifying the chemotactic behavior of bacteria in homogeneous and complex nutrient landscapes.

Relevant publications

C. Kurzthaler, S. Leitmann, T. Franosch, Sci. Rep.6 (2016)

C. Kurzthaler, T. Franosch, Soft Matter13 (2017)

C. Kurzthaler et al., Phys. Rev. Lett.121 (2018)

Press coverage

U Innsbruck press Einfaches Modell für komplexes Phänomen

## Fluid-structure interactions in microfluidics

Natural and microfluidic environments display a variety of confining surfaces with structured topographies, which modify the surrounding flow fields and and thereby impact the motion of nearby particles via hydrodynamic forces.

Sedimentation near corrugated surfaces: experiments & theory

We study the motion of spherical particles near corrugated surfaces, as are widely used in microfluidic applications, in low-Reynolds-number flows. Comparing quantitatively experiments and theory we show that spheres sedimenting near corrugated surfaces exhibit three-dimensional helical trajectories.

Sedimentation near randomly, structured surfaces

In addition to periodic, wavy surface structures, we study a non-Brownian spherical particle sedimenting nearby a random, rough surface by using an analytical theory and numerical simulations. Roughness of the wall induces fluctuations in the velocity of the fall, leading to a quadratic increase of the variance of the displacements at long times.

Relevant publications:

D. L. Chase*, C. Kurzthaler*, H. A. StoneProc. Natl. Acad. Sci. U.S.A. 119, e2202082119 (2022)

C. Kurzthaler, L. Zhu, A. Pahlavan, H. A. Stone, Phys. Rev. Fluids 5, 082101(R) (2020)

## Statistical physics of semiflexible polymers

In addition to the dynamics of a self-propelled particle, the shape of its trajectory encodes interesting statistical physics and, in particular, is reminiscent of the contour of a semiflexible polymer, e.g., cytoskeletal filaments. Exploiting this analogy, we calculated analytically the experimentally accessible force-extension relations of semiflexible polymers both in 2D and in 3D, which demonstrates a continuous buckling behavior, in contrast to the Euler buckling instability of a rigid rod. We have also derived analytically the associated probability densities in 2D.

Relvant publications:

C. Kurzthaler, Soft Matter 14, 7634 (2018)

C. Kurzthaler, T. Franosch, Soft Matter 14, 2682 (2018)

C. Kurzthaler, T. Franosch, Phys. Rev. E 95 (2017)