arXiv subjects

Jiaxing Yuan

Publications and source records attributed to Jiaxing Yuan.

10 recordsLinked to original sources

arXiv2026

Hidden amplitude and dynamical information in phase-separation spectra

Framewise variance normalization removes fluctuation amplitude from an observation, but not from the dynamics that generate later patterns. The current normalized spectrum therefore need not determine its own future. We study this effect in three two-dimensional phase-separation models that change transport, add active driving, or suppress large-scale separation. For paired initial fields, their normalized spectra evolve identically in the linear regime. The restoring model provides an exact example: damping does not directly change how the normalized spectrum evolves when both equations are applied to the same field. It nevertheless changes the hidden amplitude, which alters the later nonlinear shape. An exact comparison separates the immediate effect of changing the equation from the effect of reaching a different state through earlier evolution. It also reveals strong cancellation between these effects in the mobility and active models. As a practical test, spectral histories improve classification by about 4-6 percentage points at several later observation windows, although the gain depends on quench depth and spectral construction. Temporal spectra can therefore reveal dynamical consequences of information absent from every individual normalized frame.

cond-mat.soft↗

arXiv2026

Active motility and wetting cooperatively regulate liquid-liquid phase separation

Liquid--liquid phase separation in aqueous two-phase systems is fundamental across physical and biological sciences. While well understood for passive mixtures, how it is regulated by active agents such as motile bacteria remains largely unexplored. By combining experiments on Pseudomonas aeruginosa in a dextran--polyethylene glycol mixture with hydrodynamic simulations, we show that the coupling between bacterial activity and interfacial wetting converts self-propulsion into mechanically effective interfacial stresses, giving rise to a robust sequence of morphologies, including self-spinning droplets, elongated droplet chains, and branched capillary-like clusters. More importantly, it gives activity a dual kinetic role: activity suppresses coarsening in the droplet regime through rotation-induced hydrodynamic repulsion, but accelerates coarsening when dextran is the minority phase, where wetting-mediated attraction drives aggregation. To probe the biological relevance of this mechanism, we further show that a biofilm-associated protein can act as an interfacial ``wetting glue,'' promoting bacterial clustering even in dilute suspensions. Our findings establish activity--wetting coupling, rather than activity or wetting alone, as a mechanism by which active motility regulates pattern morphology and coarsening dynamics, and reveal a physical route to enhancing bacterial aggregation from dilute suspensions.

cond-mat.soft↗

arXiv2026

Wetting-coupled phase separation as an energetic mechanism for active bacterial adhesion

The rapid adhesion of motile bacteria from dilute suspensions poses a fundamental non-equilibrium problem: hydrodynamic interactions bias bacterial motion near surfaces without generating stable confinement, while electrostatic interactions are predominantly repulsive. Here, combining experiments on Pseudomonas aeruginosa and Staphylococcus aureus in a polyethylene glycol/dextran aqueous two-phase system with large-scale hydrodynamic simulations, we identify wetting-coupled liquid--liquid phase separation (LLPS) as an energetic trapping mechanism for bacterial adhesion. When bacteria partition into a phase that preferentially wets the substrate, interfacial free-energy minimization creates a deep energetic trap that stabilizes adhesion and induces lateral clustering via capillary interactions. Crucially, bacterial motility plays a dual role: at low phase volume fractions, activity enhances transport into the wetting layer and promotes accumulation, whereas at higher phase volumes it suppresses adhesion through the formation of self-spinning droplets that generate hydrodynamic lift opposing interfacial trapping. Our results establish wetting-coupled LLPS as a generic physical route governing interfacial organization in active suspensions. This provides a unified energetic framework for bacterial adhesion in complex fluids, with broad implications for deciphering bacterial-cell interactions and controlling biofilm formation.

cond-mat.soft↗

arXiv2024

Network-forming phase separation of oppositely charged polyelectrolytes forming coacervates in a solvent

The formation of coacervates through phase separation of oppositely charged polyelectrolytes (PEs) is critical for understanding biological condensates and developing responsive materials. Traditionally, coacervates are viewed as spherical droplets with growth dynamics resembling liquid-liquid phase separation. However, our fluid particle dynamics simulations incorporating hydrodynamic and electrostatic interactions challenge this perspective. Here, we find that oppositely charged PEs form a percolated network even in semi-dilute solutions, coarsening with a unique growth law, $\ell \propto t^{1/2}$. This self-similarity, absent for neutral polymers in poor solvents, arises because PEs in good solvents exhibit weaker, longer-range attractions due to spatial charge inhomogeneity under global charge neutrality. This results in a lower density of the PEs-rich phase and reduced interfacial tension. Increased charge asymmetry further slows network coarsening. Additionally, coacervate droplets initially display irregular shapes due to weak interfacial tension, transitioning slowly to spherical forms. Our research provides new insights into coacervate morphology and coarsening dynamics.

cond-mat.soft↗

arXiv2024

Collapse/expansion dynamics and actuation of pH-responsive nanogels

Polyelectrolyte (PE) hydrogels can dynamically respond to external stimuli, such as changes in pH and temperature, which benefits their use for smart materials and nanodevices with tunable properties. We investigate equilibrium conformations and phase transition dynamics of pH-responsive nanogels using hybrid molecular dynamics/Monte Carlo simulations with full consideration of electrostatic and hydrodynamic interactions. We demonstrate that PE nanogels exhibit a closed-loop phase behavior with a discontinuous swelling--collapse transition that occurs only at intermediate pH values. A 50~nm nanogel particle close to a critical point functions as a pH-driven actuator with a microsecond conformational response and work density $\approx 10^5~\mathrm{J/m}^3$, an order of magnitude larger than skeletal muscles. The collapse/expansion time scales as $L^{2}$ and the power density scales as $L^{-2}$ where $L$ is the linear size of the gel. Our work provides fundamental insight into phase behavior and non-equilibrium dynamics of the swelling--collapse transition, and our method enables the investigation of charge--structure--hydrodynamic coupling in soft materials.

cond-mat.soft↗

arXiv2024

Collapse and expansion kinetics of a single polyelectrolyte chain with hydrodynamic interactions

We investigate the collapse and expansion dynamics of a linear polyelectrolyte (PE) with hydrodynamic interactions. Using dissipative particle dynamics with a bead-spring PE model, long-range electrostatics and explicit ions we examine how the timescales of collapse $t_\text{col}$ and expansion $t_\text{exp}$ depend on the chain length $N$, and obtain scaling relationships $t_\text{col}\sim N^α$ and $t_\text{exp}\sim N^β$. For neutral polymers, we derive values of $α=0.94\pm0.01$ and $β=1.97\pm0.10$. Interestingly, the introduction of electrostatic interaction markedly shifts $α$ to $α\approx1.4\pm0.1$ for salt concentrations within $c=10^{-4}$ M to $10^{-2}$ M. A reduction in ion-to-monomer size ratio noticeably reduces $α$. On the other hand, the expansion scaling remains approximately constant, $β\approx 2$, regardless of salt concentration or ion size considered. We find $β> α$ for all conditions considered, implying that expansion is always slower than collapse in the limit of long polymers. This asymmetry is explained by distinct kinetic pathways of collapse and expansion processes.

cond-mat.soft↗

arXiv2022

Modeling of a charged dielectric interface: Comparison of the continuum and discrete lattice representations of surface charges

Two main approaches in particle-based simulations for modeling a charged surface are using explicit, discrete charges and continuum, uniform charges. It is well-known that these two approaches could lead to substantially distinct ionic distributions, whereas a systematic exploration of the origin is still absent. In this short communication, we calculate the electrostatic force of a single point charge above a planar substrate characterized by a surface charge density and dielectric mismatch and compare the differences in the electrostatic forces produced by discrete and continuum representations of surface charges. We demonstrate that while the model of uniform surface charges gives a rather simple picture, the model of discrete surface charges can exhibit different scenarios, depending on the respective values of ion-surface distance versus lattice spacing and a self-image interaction parameter.

cond-mat.soft↗

arXiv2022

Accelerated simulation method for charge regulation effects

The net charge of solvated entities, ranging from polyelectrolytes and biomolecules to charged nanoparticles and membranes, depends on the local dissociation equilibrium of individual ionizable groups. Incorporation of this phenomenon, \emph{charge regulation}, in theoretical and computational models requires dynamic, configuration-dependent recalculation of surface charges and is therefore typically approximated by assuming constant net charge on particles. Various computational methods exist that address this. We present an alternative, particularly efficient charge regulation Monte Carlo method (CR-MC), which explicitly models the redistribution of individual charges and accurately samples the correct grand-canonical charge distribution. In addition, we provide an open-source implementation in the LAMMPS molecular dynamics (MD) simulation package, resulting in a hybrid MD/CR-MC simulation method. This implementation is designed to handle a wide range of implicit-solvent systems that model discreet ionizable groups or surface sites. The computational cost of the method scales linearly with the number of ionizable groups, thereby allowing accurate simulations of systems containing thousands of individual ionizable sites. By matter of illustration, we use the CR-MC method to quantify the effects of charge regulation on the nature of the polyelectrolyte coil--globule transition and on the effective interaction between oppositely charged nanoparticles.

cond-mat.soft↗

arXiv2021

HSMA: An O(N) electrostatics package implemented in LAMMPS

We implement two recently developed fast Coulomb solvers, HSMA3D [J. Chem. Phys. 149 (8) (2018) 084111] and HSMA2D [J. Chem. Phys. 152 (13) (2020) 134109], into a new user package HSMA for molecular dynamics simulation engine LAMMPS. The HSMA package is designed for efficient and accurate modeling of electrostatic interactions in 3D and 2D periodic systems with dielectric effects at the O(N) cost. The implementation is hybrid MPI and OpenMP parallelized and compatible with existing LAMMPS functionalities. The vectorization technique following AVX512 instructions is adopted for acceleration. To establish the validity of our implementation, we have presented extensive comparisons to the widely used particle-particle particle-mesh (PPPM) algorithm in LAMMPS and other dielectric solvers. With the proper choice of algorithm parameters and parallelization setup, the package enables calculations of electrostatic interactions that outperform the standard PPPM in speed for a wide range of particle numbers.

physics.comp-ph↗

arXiv2019

Ewald summation for ion-dipole mixture under dielectric confinement

A modified 3D-Ewald summation is presented for accurately simulating the ion-dipole mixture under dielectric confinement. The method is based on the combination of image charges and image dipoles with the conventional Ewald summation and has a scaling O(^3/2). The accuracy and efficiency of our algorithm are examined through numerical examples.

cond-mat.soft↗