arXiv subjects

Lara Koehler

Publications and source records attributed to Lara Koehler.

5 recordsLinked to original sources

arXiv2026

Passive memory reshapes active persistence

Many active systems move in complex environments whose mechanical response is slow and history dependent. To address this regime, we study the collective dynamics of self-sustained active particles in non-Markovian media within a generalized Langevin framework with memory. We focus on the competition between the timescales of active persistence and viscoelastic relaxation in the environment. Using a minimal interacting model with an exponential memory kernel, we show that environmental memory qualitatively reshapes motility-induced phase separation of self-propelled active particles. When the memory timescale is comparable to the active persistence time, delayed response generates an effective anti-persistence that suppresses clustering and produces a broad metastable regime with slow nucleation dynamics. By contrast, for long memory timescales, reduced friction at short times enhances the effective propulsion velocity and restores phase separation. Our results demonstrate that the surrounding medium can actively regulate the emergence, stability, and dynamics of collective organization in active matter.

cond-mat.soft↗

arXiv2026

Flow-wave coupling synchronizes oscillations in growing active matter

Oscillatory biochemical signals and mechanical forces must coordinate robustly during development, yet the principles governing their mutual coupling remain poorly understood. In syncytial embryos and cell-free extracts, mitotic waves propagate across millimeter scales while simultaneously generating cytoplasmic flows, suggesting a two-way interaction between chemical oscillators and mechanics. Here, we combine experiments in Xenopus Laevis cytoplasmic extracts with a minimal particle-based model to reveal a mechanochemical feedback that stabilizes phase wave propagation. In contrast to previous models of oscillatory active matter, an asymmetric size cycle, slow growth and rapid shrinkage, combined with size-dependent mechanical interactions generates a net particle displacement and flows aligned with the wave direction, which in turn drive a synchronization transition. Our results show that mechanical forces actively maintain the coherence of biochemical waves, providing a general mechanism for long-range order in oscillating active matter.

physics.bio-ph↗

arXiv2025

Topological defect engineering enables size and shape control in self-assembly

The self-assembly of complex structures from engineered subunits is a major goal of nanotechnology, but controlling their size becomes increasingly difficult in larger assemblies. Existing strategies present significant challenges, among which the use of multiple subunit types or the precise control of their shape and mechanics. Here we introduce an alternative approach based on identical subunits whose interactions promote crystals, but also favor crystalline defects. We theoretically show that topological restrictions on the scope of these defects in large assemblies imply that the assembly size is controlled by the magnitude of the defect-inducing interaction. Using DNA origami, we experimentally demonstrate both size and shape control in two-dimensional disk- and fiber-like assemblies. Our basic concept of defect engineering could be generalized well beyond these simple examples, and thus provide a broadly applicable scheme to control self-assembly.

cond-mat.soft↗

arXiv2024

Transient contacts between filaments impart its elasticity to branched actin

Branched actin networks exert pushing forces in eukaryotic cells, and adapt their stiffness to their environment. The physical basis for their mechanics and adaptability is however not understood. Indeed, here we show that their high density and low connectivity place them outside the scope of standard elastic network models for actin. We combine high-precision mechanical experiments, molecular dynamics simulations and a mean-field elastic theory to show that they are instead dominated by the proliferation of interfilament contacts under compression. This places branched actin in the same category as undercoordinated, fibrous materials such as sheep's wool. When the network is grown under force, filaments entangle as if knitted together and trap contacts in their structure. Trapped contacts play a similar role as crosslinkers in rigidifying the network, and are thus key to its active adaptive mechanics.

cond-mat.soft↗

arXiv2024

How do particles with complex interactions self-assemble?

In living cells, proteins self-assemble into large functional structures based on specific interactions between molecularly complex patches. Due to this complexity, protein self-assembly results from a competition between a large number of distinct interaction energies, of the order of one per pair of patches. Current self-assembly models however typically ignore this aspect, and the principles by which it determines the large-scale structure of protein assemblies are largely unknown. Here, we use Monte-Carlo simulations and machine learning to start to unravel these principles. We observe that despite widespread geometrical frustration, aggregates of particles with complex interactions fall within only a few categories that often display high degrees of spatial order, including crystals, fibers, and oligomers. We then successfully identify the most relevant aspect of the interaction complexity in predicting these outcomes, namely the particles' ability to form periodic structures. Our results provide a first extensive characterization of the rich design space associated with identical particles with complex interactions, and could inspire engineered self-assembling nanoobjects as well as help understand the emergence of robust functional protein structures.

cond-mat.soft↗