arXiv subjects

Florian Raßhofer

Publications and source records attributed to Florian Raßhofer.

4 recordsLinked to original sources

arXiv2026

Local composition controls pattern formation in conserved active emulsions

Phase separation in passive systems leads to uncontrolled droplet growth, limiting structural control in soft materials and cells. We identify a generic mechanism to arrest coarsening based on chemical interconversion between molecular species with different diffusivities. Sharp-interface theory and simulations show that when the faster-diffusing species becomes enriched inside droplets, composition gradients emerge that oppose mass influx. This transport asymmetry stabilizes droplet sizes even without interaction asymmetries, offering a minimal route to regulate structure formation in active emulsions.

cond-mat.soft↗

arXiv2026

Non-equilibrium phase coexistence in conserved chemically active mixtures

Chemical activity is known to affect phase coexistence and coarsening in liquid mixtures, most commonly through reaction-induced changes of intermolecular interactions. Here, we analyze a scenario in which chemical reactions regulate particle transport while leaving thermodynamic interactions unchanged. We study an incompressible mixture of thermodynamically identical solutes with unequal diffusivities that interconvert through driven chemical reactions. Using linear stability analysis and finite-element simulations, we show that the system can phase-separate into solute-rich and solute-poor domains via two qualitatively different pathways. When interactions are too weak to induce phase separation, patterns arise through a generalized mass-redistribution instability and coarsen uninterruptedly. When interactions favor phase separation, coarsening can be arrested if chemical activity locally enriches faster-diffusing solutes within dense domains. In the limit of fast chemical turnover, the system always coarsens, and phase coexistence is governed by an effective free energy that explicitly depends on kinetic parameters. Beyond this limit, we develop a sharp-interface theory that predicts the onset of arrested coarsening, stationary droplet sizes, and nucleation conditions under chemical driving. Taken together, our results establish kinetic regulation as a minimal and robust mechanism to control phase coexistence and coarsening in chemically active mixtures.

cond-mat.soft↗

arXiv2025

Capillary wave formation in conserved active emulsions

The dynamics of phase-separated interfaces shape the behavior of both passive and active condensates. While surface tension in equilibrium systems minimizes interface length, non-equilibrium fluxes can destabilize flat or constantly curved interfaces, giving rise to complex interface morphologies. Starting from a minimal model that couples a conserved, phase-separating species to a self-generated chemical field, we identify the conditions under which interfacial instabilities may emerge. Specifically, we show that non-reciprocal chemotactic interactions induce two distinct types of instabilities: a stationary (non-oscillatory) instability that promotes interface deformations, and an oscillatory instability that can give rise to persistent capillary waves propagating along the boundaries of phase-separated domains. To characterize these phenomena, we develop a perturbative framework that predicts the onset, wavelength, and velocity of capillary waves, and quantitatively validate these predictions through numerical simulations. Beyond the linear regime, our simulations reveal that capillary waves undergo a secondary instability, leading to either stationary or dynamically evolving superpositions of different wave modes. Finally, we investigate whether capillary waves can facilitate directed mass transport, either along phase boundaries (conveyor belts) or through self-sustained liquid gears crawling along a solid wall. Taken together, our results establish a general framework for interfacial dynamics in active phase-separating systems and suggest new strategies for controlling mass transport in soft matter and biological condensates.

cond-mat.soft↗

arXiv2024

The role of mobility in epidemics near criticality

The general epidemic process (GEP), also known as susceptible-infected-recovered model (SIR), describes how an epidemic spreads within a population of susceptible individuals who acquire permanent immunization upon recovery. This model exhibits a second-order absorbing state phase transition, commonly studied assuming immobile healthy individuals. We investigate the impact of mobility on disease spreading near the extinction threshold by introducing two generalizations of GEP, where the mobility of susceptible and recovered individuals is examined independently. In both cases, including mobility violates GEP's rapidity reversal symmetry and alters the number of absorbing states. The critical dynamics of the models are analyzed through a perturbative renormalization group approach and large-scale stochastic simulations using a Gillespie algorithm. The renormalization group analysis predicts both models to belong to the same novel universality class describing the critical dynamics of epidemic spreading when the infected individuals interact with a diffusive species and gain immunization upon recovery. At the associated renormalization group fixed point, the immobile species decouples from the dynamics of the infected species, dominated by the coupling with the diffusive species. Numerical simulations in two dimensions affirm our renormalization group results by identifying the same set of critical exponents for both models. Violation of the rapidity reversal symmetry is confirmed by breaking the associated hyperscaling relation. Our study underscores the significance of mobility in shaping population spreading dynamics near the extinction threshold.

cond-mat.stat-mech↗