arXiv subjects

Axel Voigt

Publications and source records attributed to Axel Voigt.

At least 19 recordsLinked to original sources

arXiv2026

A Stream-Function Formulation for the Surface Stokes-Helfrich Model - Surface Finite Element Discretization and Validation

We introduce a stream-function formulation for the Surface Stokes-Helfrich model and study an Arbitrary Lagrangian-Eulerian (ALE) Surface Finite Element (SFEM) discretization in space and a semi-implicit discretization in time of these equations. In contrast with special cases of a surface Stokes model on stationary or prescribed evolving surfaces the surface pressure remains as an unknown and needs to be reconstructed in each time step. As for the surface Stokes model on a prescribed evolving surface the formulation also requires an additional gradient potential. A detailed computational comparison with an ALE-SFEM discretization in space and a similar semi-implicit discretization in time of the corresponding velocity-pressure formulation is explored on a representative problem formulation for simply-connected surfaces, demonstrating the validity of the approach.

math.NA↗

arXiv2026

Surface Phase-Field-Crystal-Helfrich model for out-of-plane deformations in thin crystalline sheets with lattice mismatch

Thin, flexible crystalline sheets exhibit unique elastic properties due to their ability to undergo out-of-plane deformations. Understanding this behavior requires a description that couples in-plane elasticity, out-of-plane deformation, and their coupling, taking the crystalline structure and its defects into account. We develop a multiscale description for these systems by extending the surface Phase-Field-Crystal-Helfrich model. The extension permits a spatially varying equilibrium lattice spacing, enabling the representation of localized lattice eigenstrain to mimic lattice mismatch in heterostructures. We validate the extended model against analytical predictions from classical F\"oppl-von K\'arm\'an equations for uniaxial compression and from Eshelby's inclusion problem. Using this validated framework, we then show how locally induced compressive stresses drive out-of-plane deformation in the sheets.

cond-mat.mtrl-sci↗

arXiv2026

Scalar Truesdell Time Derivative and $(L^{2},H^{-1})$ -- Surface Gradient Flows

We address surface gradient flows which allow for energy dissipation by evolving the surface and a scalar quantity on it, simultaneously. A proper choice of the time derivative and the gauge of surface independence guarantees energy dissipation and ensures conservation of the scalar quantity. The resulting system of partial differential equations couples geometric evolution equations for the evolution of the surface in normal directions, equations for tangential movement and scalar-valued equations on the evolving surface. We discuss the general setting and the special case of surface tension flows and numerically demonstrate the importance of tangential movement on the evolution.

math-ph↗

arXiv2026

Non-reciprocal Binary-fluid Turbulence

Although effective non-reciprocal interactions have been investigated in a variety of fields, their consequences have not been explored in hydrodynamical turbulence. We initiate such an exploration by introducing non-reciprocal binary-fluid tubulence and uncover its properties by developing a two-dimensional (2D) Non-Reciprocal Cahn-Hilliard-Navier-Stokes (NRCHNS) model. We show that, as we increase the strength of the non-reciprocal terms, this model displays a hitherto unanticipated type of turbulence, with an inverse cascade of energy and an energy spectrum $E(k)\sim k^{-5/3}$, reminiscent of the well-known inverse cascade in forced, 2D fluid turbulence, but distinct from it, in so far as it develops a non-reciprocal flux $\mathbf J$. We demonstrate how NRCHNS turbulence suppresses $J(t) = |\mathbf J|$, as the Reynolds number increases. We compare and contrast 2D NRCHNS turbulence with its fluid-turbulence counterpart by examining spectra, fluxes, spectral balances, flow topologies, and signatures of multifractality.

physics.flu-dyn↗

arXiv2026

Self-organized flows break morphological symmetry in active/passive systems

We consider a phase-separating mixture of active and passive fluids and explore morphological asymmetries of the emerging dominantly bicontinous dynamic emulsion. Two-dimensional numerical simulations reveal that the geometric and topological asymmetries can solely be explained by self-organized flows in the active region. As in inertial turbulence an inverse energy cascade in the active region leads to the formation of condensates. The size of these mesocales vortices is determined by the locally available space in the emulsion. As these condensates accumulate energy they impact the fluctuation of the surrounding interface and thus form a tight coupling between the flow field and the dynamic morphology. While explored for active/passive systems the symmetry-breaking mechanism can be generalized to heterogeneous active systems and proposes a way to control the morphology of various functional soft materials.

cond-mat.soft↗

arXiv2025

Hydrodynamic liquid crystal models for lipid bilayers

Coarse-grained continuous descriptions for lipid bilayers are typically based on minimizing the Helfrich energy. Such models consider the fluid properties of these structures only implicitly and have been shown to nicely reproduce equilibrium properties. Model extensions that also address the dynamics of these structures are surface (Navier--)Stokes--Helfrich models. They explicitly account for membrane viscosity. However, these models also usually treat the lipid bilayer as a homogeneous continuum, neglecting the molecular degrees of freedom of the lipids. Here, we derive refined models which consider in addition a scalar order parameter representing the molecular alignment of the lipids along the surface normal. Starting from hydrodynamic surface liquid crystal models, we obtain a hydrodynamic surface Landau--Helfrich model for asymmetric lipid bilayers and a surface Beris--Edwards model for symmetric lipid bilayers. The fully ordered case for both models leads to the known surface (Navier--)Stokes--Helfrich models. Besides more detailed continuous models for lipid bilayers, we therefore also provide an alternative derivation of surface (Navier--)Stokes--Helfrich models.

cond-mat.soft↗

arXiv2025

Vorticity-induced surfing and trapping in porous media

Microorganisms often encounter strong confinement and complex hydrodynamic flows while navigating their habitats. Combining finite-element methods and stochastic simulations, we study the interplay of active transport and heterogeneous flows in dense porous channels. We find that swimming always slows down the traversal of agents across the channel, giving rise to robust power-law tails of their exit-time distributions. These exit-time distributions collapse onto a universal master curve with a scaling exponent of $\approx 3/2$ across a wide range of packing fractions and motility parameters, which can be rationalized by a scaling relation. We further identify a new motility pattern where agents alternate between surfing along fast streams and extended trapping phases, the latter determining the power-law exponent. Unexpectedly, trapping occurs in the flow backbone itself -- not only at obstacle boundaries -- due to vorticity-induced reorientation in the highly-heterogeneous fluid environment. These findings provide a fundamentally new active transport mechanism with direct implications for biofilm clogging and the design of novel microrobots capable of operating in heterogeneous media.

cond-mat.soft↗

arXiv2025

A spectrum of p-atic symmetries and defects in confluent epithelia

Topological defects provide a unifying language to describe how orientational order breaks down in active and living matter. Considering cells as elongated particles confluent, epithelial tissues can be interpreted as nematic fields and its defects have been linked to extrusion, migration, and morphogenetic transformations. Yet, epithelial cells are not restricted to nematic order: their irregular shapes can express higher rotational symmetries, giving rise to p-atic order. Here we introduce a framework to extract p-atic fields and their defects directly from experimental images. Applying this method to MDCK cells, we find that all symmetries generate defects.No strong positional or orientational correlations are found between nematic and hexatic defects, suggesting that different symmetries coexist largely independently. These results demonstrate that epithelial tissues should not be described by nematic order alone, but instead host a spectrum of p-atic symmetries. Our work provides the first direct experimental evidence for this multivalency of order and offers a route to test and refine emerging p-atic liquid crystal theories of living matter.

cond-mat.soft↗

arXiv2025

Self-avoiding fluid deformable surfaces

We propose a numerical method for fluid deformable surfaces governed by surface Stokes flow and Helfrich bending energy under active growth, aiming to model shape evolution of the epithelium in developmental processes. To prevent self-intersections, which commonly arise under large deformations or low enclosed volume to area ratios, we incorporate the nonlocal tangent-point energy to penalize non-embedded configurations. The resulting formulation is discretized using higher order surface finite elements, with a parallelizable assembly strategy for the nonlocal terms. To tailor mesh quality to the geometric evolution, we propose a curvature-adaptive mesh redistribution strategy that improves mesh resolution in regions of high curvature. Numerical examples include the discocyte-to-stomatocyte transition and the inversion of a sphere within a spherical confinement. Both demonstrate the robustness of the method in capturing large deformations, self-avoidance, symmetry-breaking and growth-induced morphology changes.

physics.flu-dyn↗

arXiv2025

A numerical approach for the dynamics of active viscoelastic surfaces

The dynamics of active viscoelastic surfaces plays an important role in biological systems. One prominent example is the actin cortex, a thin bio-polymer sheet underneath the outer membrane of biological cells which combines active molecular force generation with viscoelastic behavior characterized by elastic properties at short timescales and viscous properties at longer timescales. We consider a surface Maxwell model within dominant rheology and an additional active term to model the dynamics of the actin cortex. This captures both, shear and dilational surface dynamics. We propose a monolithic numerical approach based on the surface finite element method (SFEM), validate the results for special cases and experimentally demonstrate convergence properties. Moreover, imposing a ring-shaped region of an enhanced active stress mimicking the contractile ring during cytokinesis, we observe different types of emergent patterns and shape dynamics depending on the viscoelastic properties. While viscous surfaces show a ring, which slips to one side of the surface, viscoelasticity provides a stabilization mechanism of the ring, thus forming a requirement for subsequent cell division. This study provides an example that viscoelastic properties are key ingredients to understand biological materials.

physics.bio-ph↗

arXiv2025

A topological approach to the Cahn-Hilliard equation and hyperuniform fields

Hyperuniform structures are disordered, correlated systems in which density fluctuations are suppressed at large scales. Such a property generalizes the concept of order in patterns and is relevant across diverse physical systems. We present a numerical characterization of hyperuniform scalar fields that leverages persistent homology. Topological features across different lengths are represented in persistence diagrams, while similarities or differences between patterns are quantified through Wasserstein distances between these diagrams. We apply this framework to numerical solutions of the Cahn-Hilliard equation, a canonical model for generating hyperuniform scalar fields. We validate the approach against known features of the Cahn-Hilliard equation, including its scaling properties, convergence to the sharp interface limit, and self-similarity of the solutions. We then generalize the approach by studying Gaussian random fields exhibiting different degrees and classes of hyperuniformity, showing how the proposed approach can be exploited to reconstruct global properties from local topological information. Overall, we show how hyperuniform characteristics systematically correlate with distributions of topological features in disordered correlated fields. We expect this analysis to be applicable to a wide range of scalar fields, particularly those involving interfaces and free boundaries.

cond-mat.soft↗

arXiv2025

Solid-State Dewetting of Polycrystalline Thin Films: a Phase Field Approach

Solid-state dewetting is the process by which thin solid films break up and retract on a substrate, forming nanostructures. While dewetting of single-crystalline films is understood as a surface-energy-driven process mediated by surface diffusion, polycrystalline films exhibit additional complexity due to the presence of grain boundaries. Most theoretical and computational studies have focused on single-crystalline dewetting. Here, we present the application of the grandpotential multi-phase-field model to the dewetting of thin polycrystalline films in three dimensions, reproducing the key phenomenology of this process. By considering isotropic interface/surface energy, we illustrate its consistency with predictions based on energetic arguments and the morphological evolution towards equilibrium. We also provide novel analytical criteria for the onset of three-dimensional dewetting, serving as fundamental theoretical benchmarks, and highlight the critical role of triple junctions. Moreover, we unveil the dewetting behavior of polycrystalline patches, extending the scenarios of their single-crystalline counterparts.

cond-mat.mtrl-sci↗

arXiv2025

Hyperuniformity in ternary fluid mixtures: the role of wetting and hydrodynamics

Phase separation in multicomponent fluids is central to understanding the organization of complex materials and biological structures. The Cahn-Hilliard-Navier-Stokes (CHNS) equations offer a robust framework for modeling such systems, capturing both diffusive dynamics and hydrodynamic interactions. In this work, we investigate hyperuniformity, characterized by suppressed large-scale density fluctuations, in ternary fluid mixtures governed by the ternary CHNS equations. Using large-scale direct numerical simulations, we systematically explore the influence of wetting conditions and hydrodynamic effects on emergent hyperuniformity. Similar to binary systems we observe that the presence of hydrodynamics weakens the hyperuniform characteristics. However, also the wetting properties have an effect. We find that in partial wetting regimes, all three components exhibit comparable degrees of hyperuniformity. In contrast, for complete wetting scenarios, where one component preferentially wets the other two, the wetting component displays a significant reduction in hyperuniformity relative to the others. These findings suggest that wetting asymmetry can act as a control parameter for spatial order in multicomponent fluids.

cond-mat.soft↗

arXiv2025

Towards a two-scale model for morphogenesis -- How cellular processes influence tissue deformations

We propose a two-scale model to resolve essential features of developmental tissue deformations. The model couples individual cellular behavior to the mechanics at tissue scale. This is realized by a multiphase-field model addressing the motility, deformability and interaction of cells on an evolving surface. The surface evolution is due to bending elasticity, with bending properties influenced by the topology of the cellular network, which forms the surface. We discuss and motivate model assumptions, propose a numerical scheme, which essentially scales with the number of cells, and explore computationally the effect of the two-scale coupling on the global shape evolution. The approach provides a step towards more quantitative modeling of morphogenetic processes.

cond-mat.soft↗

arXiv2025

Surface Minkowski tensors to characterize shapes on curved surfaces

We introduce surface Minkowski tensors to characterize rotational symmetries of shapes embedded in curved surfaces. The definition is based on a modified vector transport of the shapes boundary co-normal into a reference point which accounts for the angular defect that a classical parallel transport would introduce. This modified transport can be easily implemented for general surfaces and differently defined embedded shapes, and the associated irreducible surface Minkowski tensors give rise to the classification of shapes by their normalized eigenvalues, which are introduced as shape measures following the flat-space analog. We analyze different approximations of the embedded shapes, their influence on the surface Minkowski tensors, and the stability to perturbations of the shape and the surface. The work concludes with a series of numerical experiments showing the applicability of the approach on various surfaces and shape representations and an application in biology in which the characterization of cells in a curved monolayer of cells is considered.

math.NA↗

arXiv2024

The influence of higher order geometric terms on the asymmetry and dynamics of membranes

We consider membranes as fluid deformable surface and allow for higher order geometric terms in the bending energy. The evolution equations are derived and numerically solved using surface finite elements. The higher order geometric terms related to the Gaussian curvature squared have a tendency to stabilize tubes and enhance the evolution towards equilibrium shapes, thereby facilitating rapid shape changes. This is demonstrated in axisymmetric settings and fully three-dimensional simulations.

cond-mat.soft↗

arXiv2024

Suppression of hyperuniformity in hydrodynamic scalar active field theories

The coarsening dynamics at late times in phase-separating systems lead to universally hyperuniform patterns. This is well known for scalar field theories, such as the Cahn-Hilliard equation, but has also been shown for dry scalar active field theories. We demonstrate the role of hydrodynamic interactions in influencing hyperuniformity in a wet active system described by active model H. Our direct numerical simulations reveal that, while (passive) model H shows hyperuniformity in the coarsening regime, the interplay of activity and hydrodynamic interactions suppresses hyperuniformity in active model H, especially when the activity generates contractile stress in the fluid.

cond-mat.soft↗

arXiv2024

A Non-Isothermal Phase-Field Crystal Model with Lattice Expansion: Analysis and Benchmarks

We introduce a non-isothermal phase-field crystal model including heat flux and thermal expansion of the crystal lattice. The thermal compatibility condition, as well as a positive entropy-production property, is derived analytically and further verified by numerical benchmark simulations. Furthermore, we examine how the different model parameters control density and temperature evolution during dendritic solidification through extensive parameter studies. Finally, we extend our framework to the modeling of open systems considering external mass and heat fluxes. This work sets the ground for a comprehensive mesoscale model of non-isothermal solidification including thermal expansion within a positive entropy-producing framework, and provides a benchmark for further meso- to macroscopic modeling of solidification.

cond-mat.mtrl-sci↗