arXiv subjects

Friederike Schmid

Publications and source records attributed to Friederike Schmid.

At least 19 recordsLinked to original sources

arXiv2026

From Single-Chain Dynamics to Structure Formation: Dynamic Self-Consistent Field Theory and Molecular Dynamics of (Co)polymer Melts across Entanglement Regimes

Dynamic self-consistent field theory (DSCFT) provides an efficient continuum framework for studying structure formation in inhomogeneous polymer systems, but its predictive accuracy depends on the choice of the nonlocal mobilities. Here, we construct mobility functions for moderately and strongly entangled homopolymer and diblock copolymer systems from the relaxation dynamics of single-chain structure factors, based on molecular dynamics (MD) simulations of the Kremer-Grest model and analytical reptation theory. Single- chain mobilities are combined such that the resulting DSCFT accounts for the dependence of fluxes on local chain densities. The theory is then applied to the spinodal decomposition of symmetric homopolymer blends and diblock copolymer melts following a quench into the (micro)phase-separation regime. Predictions of DSCFT are systematically compared with MD simulations. Mobility functions derived from single-chain dynamics are found to reproduce the kinetics of structure formation more accurately than conventional Debye-type mobilities. We additionally investigate the influence of adding stochastic currents (noise) that are correlated according to the fluctuation-dissipation relation. At low noise levels, they enable the generation of equilibrium initial states and facilitate defect annealing. At high noise levels, however, nonolinear effects lead to discrepancies between DSCFT and MD simulations.

cond-mat.soft↗

arXiv2026

Particle-Mediated Tuning of Defect Stability in Lamellar Block Copolymer Systems

We study how colloidal inclusions modify the formation energy of dislocation pairs in lamellar block copolymer systems. Using a hybrid particle/Ginzburg--Landau model, we calculate defect formation energies by comparing defect-free and defect-containing states with and without embedded colloids. Finite-size scaling is used to obtain formation energies in the thermodynamic limit. The effect of colloid insertion depends strongly on particle sizes and surface patterning. Homogeneous particles increasingly stabilize dislocation pairs with increasing particle sizes. Particles larger than one lamellar domain preferentially occupy the dislocation cores, where they replace strained polymer rather than deforming defect-free lamellae. The magnitude of this stabilization depends on surface affinity. Balanced Janus particles instead increase the formation energy, because their competing surface preferences cannot be satisfied simultaneously near the curved core. Varying the patch ratio interpolates between these limits. Surface patterning has little effect for particles smaller than one lamellar domain but changes the formation energy by several tens of $k_BT$ for larger particles. These results provide quantitative guidelines for controlling topological defect stability in lamellar block copolymer systems.

cond-mat.soft↗

arXiv2026

Active Transport as a Mechanism of Microphase Selection in Biomolecular Condensates

The size and organization of biomolecular condensates formed by liquid-liquid phase separation (LLPS) are set by multiple cellular mechanisms that are not yet fully understood. Here we identify a transport-driven mechanism: stochastic binding of phase-separating proteins to cytoskeletal motor proteins, followed by active redistribution along filament networks, generates an effective long-range repulsion that arrests coarsening and selects a finite condensate size. A minimal diffusion-transport model, analyzed by linear stability theory and three-dimensional simulations, reveals a transition from macroscopic to microphase separation at remarkably low binding/release fractions, corresponding to minute motor-bound populations. Tuning motor binding rates $b$ or transport velocities enables sublinear control of condensate sizes ($L \sim b^{-1/4}$) from a few hundred nanometres up to the micron scale. The selected length scale is robust to the intrinsic shot noise of the binding--release reactions. In anisotropic cytoskeletal environments, transport asymmetry drives morphological transitions from spherical to cylindrical condensates, independently of the thermodynamic parameters. This mechanism provides a versatile, spatiotemporally programmable route to condensate organization and informs the design of synthetic active emulsions with tunable architectures.

physics.bio-ph↗

arXiv2026

From real polymers to random graphs: percolation thresholds in associative polymer solutions

Sol-gel transitions are ubiquitous in soft matter and biological systems, yet their thresholds are often poorly captured by classical Flory-Stockmayer theory because spatial organization and loop formation are neglected. Here, we combine molecular dynamics simulations with random graph and random geometric graph models to determine the respective roles of topology and geometry in reversible associative polymer solutions. We show that a coordinate-free random graph recovers the mean-field Flory-Stockmayer limit, whereas a random geometric graph quantitatively reproduces the shifted percolation thresholds observed in molecular dynamics simulations when the detection radius is chosen according to the polymer conformational size. This geometric mapping remains quantitatively valid for linear chains with regularly spaced binding sites over a broad range of chain stiffness. At the microscopic level, we identify primary loops formed already in the pre-gel regime as the dominant source of the deviation from mean-field predictions. Near the gel point, the cluster-size statistics obtained from simulations and random geometric graphs are consistent with the universality class of three-dimensional percolation. These results establish random geometric graphs as a minimal predictive framework for describing topological transitions in reversible associative polymer solutions and show that gelation and network formation can be inferred directly from single-chain conformational information.

cond-mat.soft↗

arXiv2026

Kinetics of Droplet Cloaking and Wetting Ridge Growth on Lubricated Polymer Brushes

We investigate the kinetics of wetting ridge growth and droplet cloaking on lubricant-infused polymer brushes using a combination of experiments, molecular dynamics simulations, and theoretical modeling. We focus on three representative systems: DMSO-water on hexadecane-swollen PLMA (D-H), water on hexadecane-swollen PLMA (W-H), and water on PDMS (W-S). The dynamics are governed by the interplay between interfacial thermodynamics, brush elasticity, and transport of lubricant within the brush. Ridge growth is accompanied by the formation of depletion zones both beneath and outside the drop. This leads to a progressive slowdown governed by the need to transport lubricant through the brush. At sufficiently high swelling, we observe local separation of oil from the brush within the ridge, providing an additional mechanism for lubricant depletion. To rationalize these observations, we develop a continuum diffusion model based on the free energy of the brush and its coupling to the contact line. The model quantitatively captures the growth of the wetting ridge at intermediate and late times, demonstrating that the kinetics are largely controlled by diffusive transport within the brush.

cond-mat.soft↗

arXiv2026

Reconstruction of spin structures from topological charge distributions via generative neural network systems

Localized topological defects inherently possess a multiscale character. While their microstructure configuration depends on the specific physical system, their topological features and mutual interactions can be described on the macroscale in terms of a particle representation. However, determining the physical properties associated with a given defect pattern often requires knowledge of the underlying microscopic structure. In this work, we extend a Wasserstein generative adversarial neural network by incorporating physical constraints and Fourier-space information to generate microscopic spin configurations consistent with prescribed macroscopic patterns and thermodynamic parameters. Using the two-dimensional XY model as a test case, where vortex-antivortex pairs act as long-range interacting defects, we show that the model generates spin configurations that accurately reproduce magnetization, susceptibility, helicity modulus, and spin-spin correlations over a wide range of temperatures below the Kosterlitz-Thouless transition. At the same time, deviations in the specific heat reveal limitations in reproducing higher order energy fluctuations. A complementary analysis based on topological data analysis uncovers subtle differences in global spin-correlation structures at near critical temperatures that are not apparent from conventional correlation functions alone. These results demonstrate both the promise and current limitations of generative approaches for multiscale studies of defect-dominated spin systems and at the same time highlight topological methods as valuable tools for characterizing critical behavior.

cond-mat.stat-mech↗

arXiv2026

A general model for frictional contacts in colloidal systems

In simulations of colloidal matter, frictional contacts between particles are often neglected. For spherical colloids, such an approximation can be problematic, since frictional contacts couple translational and rotational degrees of freedom, which may affect the collective behavior of, e.g., colloids under shear and chiral active matter. Deterministic models for frictional contacts have been proposed in the granular matter community. On the colloidal scale, however, thermal fluctuations are important and should be included in a thermodynamically consistent manner. Here, we derive the correct fluctuation-dissipation relation for linear and nonlinear instantaneous frictional contact interactions. Among other, this generates a new generalized class of dissipative particle dynamics (DPD) thermostats with rotation-translation coupling. We demonstrate effects of frictional contact interactions using the examples of Poiseuille flow and motility induced phase separation in active Langevin particles.

cond-mat.soft↗

arXiv2025

Determining extended Markov parameterizations for vector-valued generalized Langevin Equations

The generalized Langevin equation is used as a model for various coarse-grained physical processes, e.g., the time evolution of the velocity of a given larger particle in an implicitly represented solvent, when the relevant time scales of the dynamics of the larger particle and the solvent particles are not strictly separated. Since this equation involves an integrated history of past velocities, considerable efforts have been made to approximate this dynamics by data-driven Markov models, where auxiliary variables are used to compensate for the memory term. In recent works we have developed two algorithms which can be used for this purpose, provided the dynamics in question are scalar processes. Here we extend these algorithms to vector-valued processes. As a physical test bed we consider an S-shaped particle sliding on a planar substrate, which gives rise to a truly two-dimensional velocity process. The two algorithms provide Markov approximations of this process with 10-20 auxiliary variables and a very accurate fit of the given autocorrelation data over the entire time interval where these data are non-negligible.

cond-mat.stat-mech↗

arXiv2025

From Heteropolymer Stiffness Distributions to Effective Homopolymers: A Conformational Analysis of Intrinsically Disordered Proteins

Synthetic copolymers and biopolymers, such as polypeptides and double-stranded DNA, often exhibit strong variations in bending stiffness along their contour, which can significantly impact conformational behavior at larger scales. To investigate these effects, we employ a discretized heterogeneous worm-like chain model, where the local persistence lengths are drawn from a Gaussian distribution. In the first part, we develop a theoretical model that maps such heterogeneous chains to homogeneous chains with a single effective persistence length. For uncorrelated disorder, our model predicts that this effective stiffness is systematically smaller than the arithmetic mean of the local persistence lengths, indicating that flexible segments have a bigger influence on the overall chain stiffness than rigid segments. We validate our model predictions using off-lattice Monte Carlo simulations, considering both ideal and self-avoiding chains in good solvent, and find excellent agreement in the regime, where the persistence lengths are on the order of a few bond lengths, consistent with typical values observed in polypeptides. In the second part, we performed simulations using various coarse-grained models of intrinsically disordered proteins (IDPs), finding that the simulated IDPs have similar shapes like the corresponding homogeneous and heterogeneous worm-like chains. However, the IDPs are systematically larger than ideal worm-like chains, yet slightly more compact when excluded volume interactions are considered. We attribute these differences to intramolecular interactions between non-bonded monomers, which our theoretical models do not account for.

cond-mat.soft↗

arXiv2025

A kinetic model to simulate charge flow through an electrochemical half cell

A kinetic model of the electron transfer at the electrode / electrolyte solution interface is developed, implemented in a Monte Carlo framework, and applied to simulate this process in idealised systems consisting of the primitive model of electrolyte solutions limited by an impenetrable conducting surface. In the present implementation, a charged, spherical interface surrounding an equally spherical sample of electrolyte solution is introduced to model a single-electrode system, providing the computational analog to the conceptual half-cell picture that is widely used in electrochemistry. The electron transfer itself is described as a simple surface hopping process underlying a first order reaction corresponding to one of the coupled M/M$^+$ and X$^-$/X half reactions. Then, the electron transfer at the interface is combined with the self-diffusion of ions in the electrolyte solutions whose role is to supply reagents and disperse products, allowing the system to settle in a stationary non-equilibrium state. Simulations for the primitive model of electrolyte in contact with a charged impenetrable surface show that, after a brief transient, the samples sustain a steady current through the electrolyte solution. The results quantify the dependence of the current on: the overall charge of the electrode, the electrolyte concentration, the solvent viscosity and the kinetic parameter $k_e$ that represents the rate of the electron transfer for each ion in contact with the electrode. Since the simulated interface is very idealised, strategies to overcome the limitations of the present model are outlined and briefly discussed.

physics.chem-ph↗

arXiv2025

Micelle Forming Linear-Dendritic Block Copolymers: A Theoretical Comparison between Random Hyperbranched and Precise Dendrimer Polymer Architectures

Hyperbranched block copolymers offer a simpler and more efficient synthesis route compared to more traditional dendritic systems, while still providing exceptional control over surface functionality and self-assembly. This makes them ideal candidates for engineering nanoparticles with tailored properties for applications such as drug delivery and sensing. Here we use self-consistent field calculations to compare the micelle structures formed by copolymers with a polydisperse hyperbranched (LHBC), monodisperse dendritic (LDBC), and linear solvophilic blocks. Representative LHBC structures were generated by molecular dynamics simulations mimicking the slow-monomer addition protocol. We find that LHBC micelles are more stable, have a lower critical micelle concentration, and are better at accommodating larger drug payloads than LDBC micelles, and these properties further improve with increasing polydispersity. LHBC micelles also offer more terminal ends for functionalization than LDBC micelles for LDBCs with up to four branching generations, with the number of terminal ends being surprisingly independent of the LHBC polydispersity. Our findings highlight the superiority of LHBC micelles in flexibility and performance over LDBC micelles.

cond-mat.soft↗

arXiv2025

Sol-gel transition in heteroassociative RNA-protein solutions: A quantitative comparison of coarse-grained simulations and the Semenov-Rubinstein theory

Protein RNA-binding domains selectively interact with specific RNA sites, a key interaction that determines the emergent cooperative behaviors in RNA-protein mixtures. Through molecular dynamics simulations, we investigate the impact of the specific binding interactions on the phase transitions of an examplary RNA-protein system and compare it with predictions of the Semenov-Rubinstein theory of associative polymers. Our findings reveal a sol-gel (percolation) transition without phase separation, characterized by double reentrant behavior as the RNA or protein concentration increases. We highlight the crucial role of bridge formations in driving these transitions, particularly when binding sites are saturated. The theory quantitatively predicts the binding numbers at equilibrium in the semidilute regime, but it significantly overestimates the size of the concentration range where percolation is observed. This can partly be traced back to the fact that the mean-field assumption in the theory is not valid in the dilute regime, and that the theory neglects the existence of cycles in the connectivity graph of the percolating cluster at the sol-gel transition. Our study enriches the understanding of RNA-protein phase behaviors, providing valuable insights for the interpretation of experimental observations.

cond-mat.soft↗

arXiv2024

Structure and Dynamic Evolution of Interfaces between Polymer Solutions and Gels and Polymer Interdiffusion: A Molecular Dynamics Study

Letting free polymers diffuse from solution into a crosslinked polymer gel is often a crucial processing step in the synthesis of multiphase polymer-based gels, e.g., core-shell microgels. Here we use coarse-grained molecular dynamics simulations to obtain molecular insights into this process. We consider idealized situations where the gel is modeled as a regular polymer network with the topology of a diamond lattice, and all free polymers and strands have the same length and consist of the same type of monomer. After bringing the gel and the polymer solution into contact, two time regimes are observed: An initial compression of the gel caused by the osmotic pressure of the solution, followed by an expansion due to swelling. We characterize the time evolution of density profiles, the penetration of free polymers into the gel and the connection between the gel and solution phase. The interfacial structure locally equilibrates after roughly 100 chain relaxation times. At late times, the free chains inside the gel undergo a percolation transition if the polymer concentration in the gel exceeds a critical value, which is of the same order as the overlap concentration. The fluctuations of the interface can be described by a capillary wave model that accounts for the elasticity of the gel. Based on this, we extract the interfacial tension of the gel-solution interface. Interestingly, both the interfacial tension and the local interfacial width increase with increasing free polymer concentration - in contrast to liquid-liquid interfaces, where these two quantities are typically anticorrelated.

cond-mat.soft↗

arXiv2024

Relaxation Dynamics of Entangled Linear Polymer Melts via Molecular Dynamics Simulations

We present an extensive analysis of the relaxation dynamics of entangled linear polymer melts via long-time molecular dynamics simulations of a generic bead-spring model. We study the mean-squared displacements, the autocorrelation function of the end-to-end vector, $P(t)$, the single-chain dynamic structure factor, $S(q,t)$, and the linear viscoelastic properties, especially the shear stress relaxation modulus, $G(t)$. The simulation data are compared with the theoretically expected scaling laws for different time regimes of entangled melts, and with analytical expressions that account for different relaxation mechanisms in the tube model, namely, reptation, contour length fluctuation (CLF), and constraint release (CR). CLF involves a $t^{1/4}$ scaling regime in the time-dependence of $(1-P(t))$. With increasing chain length, a gradual development of this scaling regime is observed. In the absence of CR, the tube model further predicts that at long times, the chain dynamics is governed by one central quantity, the ``surviving tube fraction'' $μ(t)$. As a result, one expects $S(q,t) \propto G(t) \propto P(t)$ in that time regime. We test this prediction by comparing $S(q,t)$ and $G(t)$ with $P(t)$. For both quantities, proportionality with $P(t)$ is not observed, indicating that CR has an important effect on the relaxation of these two quantities. Instead, to a very good approximation, we find $G(t)\propto P(t)^{2}$ at late times, which is consistent with the dynamic tube dilation or double reptation approximations for the CR process. In addition, we calculate non-local mobility functions, which can be used in dynamic density functional theories for entangled inhomogeneous polymer blends, and discuss the effect of entanglements on the shape of these functions.

cond-mat.soft↗

arXiv2024

Strong stretching theory of polydisperse curved polymer brushes

We investigate the effect of polydispersity on the properties of curved linear brushes in good solvent and for molten brushes. To this end, we extend the strong stretching theory for polydisperse brushes to curved geometries and investigate the polymer chain end profiles, bending moduli and other properties for experimentally relevant polymer chain length distributions of the Schulz-Zimm type. We also investigate the properties of End Exclusion Zones (EEZ) that may appear in convex geometries under certain conditions, and show that their position in the brush can be engineered by careful selection of the polymer length distribution. Lastly, we propose a method to engineer chain end profiles by engineering the polymer length distribution.

cond-mat.soft↗

arXiv2024

Force renormalization for probes immersed in an active bath

Langevin equations or generalized Langevin equations (GLEs) are popular models for describing the motion of a particle in a fluid medium in an effective manner. Here we examine particles immersed in an inherently nonequilibrium fluid, i.e., an active bath, which are subject to an external force. Specifically, we consider two types of forces that are highly relevant for microrheological studies: A harmonic, trapping force and a constant, "drag" force. We study such systems by molecular simulations and use the simulation data to derive an effective GLE description. We find that, in an active bath, the external force in the GLE is not equal to the physical external force, but rather a renormalized external force, which can be significantly smaller. The effect cannot be attributed to the mere temperature renormalization, which is also observed.

cond-mat.soft↗

arXiv2024

Stability and Elasticity of Ultrathin Sphere-Patterned Block Copolymer Films

Sphere-patterned ultrathin block copolymers films are potentially interesting for a variety of applications in nanotechnology. We use self-consistent field theory to investigate the elastic response of sphere monolayer films with respect to in-plane shear, in-plane extension and compression deformations, and with respect to bending. The relations between the in-plane elastic moduli is roughly compatible with the expectations for two-dimensional elastic systems with hexagonal symmetry, with one notable exception: The pure shear and the simple shear moduli differ from each other by roughly 20%. Even more importantly, the bending constants are found to be negative, indicating that free-standing block copolymer membranes made of only sphere mono-layer are inherently unstable above the glass transition. Our results are discussed in view of experimental findings.

cond-mat.soft↗

arXiv2024

Cloaking Transition of Droplets on Lubricated Brushes

We study the equilibrium properties and the wetting behavior of a simple liquid on a polymer brush, with and without presence of lubricant by multibody Dissipative Particle Dynamics simulations. The lubricant is modelled as a polymeric liquid consisting of short chains that are chemically identical to the brush polymers. We investigate the behavior of the brush in terms of the grafting density and the amount of lubricant present. Regarding the wetting behavior, we study a sessile droplet on top of the brush. The droplet consists of non-bonded particles that form a dense phase. Our model and choice of parameters result in the formation of a wetting ridge and in the cloaking of the droplet by the lubricant, i.e. the lubricant chains creep up onto the droplet and eventually cover its surface completely. Cloaking is a phenomenon that is observed experimentally and is of integral importance to the dynamics of sliding droplets. We quantify the cloaking in terms of its thickness, which increases with the amount of lubricant present. The analysis reveals a well-defined transition point where the cloaking sets in. We propose a thermodynamic theory to explain this behavior. In addition we investigate the dependence of the contact angles on the size of the droplet and the possible effect of line tension. We quantify the variation of the contact angle with the curvature of the contact line on a lubricant free brush and find a negative value for the line tension. Finally we investigate the effect of cloaking/lubrication on the contact angles and the wetting ridge. We find that lubrication and cloaking reduce the contact angles by a couple of degrees. The effect on the wetting ridge is a reduction in the extension of the brush chains near the three phase contact line, an effect that was also observed in experiments of droplets on crosslinked gels.

cond-mat.soft↗