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Shuyu Sun

Publications and source records attributed to Shuyu Sun.

At least 19 recordsLinked to original sources
arXiv2026

Molecular Insights into Gas Nanofilms Confined Between Bulk Liquid Phases

Nanometer-thick fluid films play a critical role in confined multiphase processes, yet the thermodynamics and stability of free gas nanofilms remain poorly understood compared with their liquid counterparts. Here, molecular dynamics (MD) simulations are employed to systematically investigate gas nanofilms confined between bulk liquid phases using Lennard-Jones argon as a model system. The results show that the surface tension decreases exponentially with decreasing film thickness, accompanied by an increasing magnitude of the negative disjoining pressure. Upon thinning, the planar gas film undergoes a distinct morphological transition from a stable planar state to a transient or persistent spherical bubble through the formation and growth of a liquid bridge. The film surface area strongly affects its thermodynamic properties, with larger areas producing stronger thickness dependence and larger deviations from classical density functional theory (cDFT) predictions. The closer agreement between MD and cDFT at smaller surface areas suggests that the discrepancy primarily arises from thermal capillary-wave fluctuations, which are included in MD but omitted in mean-field cDFT. Moreover, at small film thicknesses, the magnitude of the disjoining pressure increases with decreasing temperature, consistent with the enhanced sensitivity of the confined gas phase to thickness variations and contrasting with the trend generally reported for liquid nanofilms. These findings provide molecular insights into the thermodynamics and stability of gas nanofilms, with implications for confined multiphase transport and droplet coalescence.

physics.chem-ph↗
arXiv2026

Accounting for Nanofilm Contributions in Interfacial Free Energy Calculations Using Classical Density Functional Theory

Fluid nanofilms play a fundamental role in nanoscale interfacial thermodynamics, yet their treatment in classical density functional theory (cDFT) remains incomplete. We investigate nanofilm interfacial properties using a cDFT framework built upon the perturbed-chain statistical associating fluid theory (PC-SAFT) for both fluid-fluid and fluid-solid interfacial systems. Comparison with molecular simulations shows that the long-standing numerical discrepancies regarding free-standing nanofilms persist in the PC-SAFT functional predictions, supporting the view that they stem from thermal capillary-wave fluctuations rather than deficiencies of the density functional itself, as such fluctuations are inherently neglected in the mean-field approximation adopted by standard cDFT. Importantly, we establish a thermodynamically consistent framework for interfacial free energy (IFE) calculation, which explicitly incorporates nanofilm thermodynamic contributions and redefines the effective fluid volume by excluding the solid-phase region. The proposed method exhibits excellent consistency with another method based on the relationship between IFE and disjoining pressure for fluid systems, while both fluid-fluid and fluid-solid IFEs differ substantially from those predicted by the conventional method. We find that neglecting nanofilm contributions induces prominent size-dependent variations in the IFE and contact angle of hemicylindrical droplets, which is inconsistent with the extensive literature. Different methods also lead to opposite signs of the line tension for a hemispherical argon nanodroplet on a strongly lyophobic surface. The proposed framework provides a unified molecular-level basis for understanding interfacial processes involving nanofilms, including wetting, nucleation, adsorption, and other phenomena that rely on the accurate evaluation of IFEs.

physics.chem-ph↗
arXiv2026

A time-decoupling scheme for mixed-dimensional poroelastic models with fractures

We propose a locking-free decoupling method for a mixed-dimensional poroelasticity model with fractures. By introducing the total pressure, the fractured Biot system is reformulated as a four-field formulation involving the displacement, total pressure, matrix pressure, and fracture pressure. We establish an energy dissipation law for the continuous model, which shows its consistency with the second law of thermodynamics. Based on this formulation, a time-decoupled scheme is developed. At the initial time step, a fully coupled scheme is employed, while for subsequent time steps, the flow problem is solved first, followed by the mechanics problem. A stabilization term is incorporated into the mechanical equation to help reduce the restrictions imposed on the model parameters in the stability analysis. Energy stability is established for the semi-discrete scheme. For the spatial discretization, the displacement and total pressure are approximated by the Taylor--Hood element, while Lagrange finite elements are used for the matrix and fracture pressures. A fully discrete decoupled scheme is then constructed. Energy stability and error estimates are derived for the fully discrete scheme, and the method is shown to be locking-free. Numerical experiments are presented to support the theoretical results.

math.NA↗
arXiv2026

Singular and nearly singular quadrature in arbitrary dimensions

The accurate numerical evaluation of integrals with singular or nearly singular kernels remains a fundamental challenge in scientific computing, particularly as the spatial dimension increases. While existing approaches, including singularity extraction, adaptive element subdivision, and analytical integration schemes, have achieved considerable success in one and two dimensions, their extension to higher dimensions is often hindered by severe near-singular behaviour and the resulting numerical instabilities. In this work, we propose a systematic approach for the high-accuracy evaluation of singular integrals and nearly singular integrals in arbitrary dimensions, based on variable transformation techniques derived from the Gauss hypergeometric function. We further analyze the structure of singularities and near-singularities in both the Duffy domain and the physical Cartesian domain, and establish a unified treatment via the sinh transformation. A convergence analysis based on the Bernstein ellipse parameter reveals that the sinh transformation ($\beta=1$) consistently outperforms alternative hypergeometric transformations ($\beta=2,3$), which introduce secondary near-singularities despite achieving formal cancellation. Numerical experiments in dimensions two through five, together with an analysis of quadrature-induced consistency errors in variational discretisations, demonstrate that the proposed approach achieves exponential convergence and substantially outperforms direct Gauss--Legendre quadrature. The method offers a practical and theoretically grounded framework for high-dimensional nearly singular integration with broad applicability in scientific computing.

cs.CE↗
arXiv2026

Coupling-Informed Transport Maps for Bayesian Filtering in Nonlinear Dynamical Systems

A likelihood-free transport filtering method is proposed based on the couplings between state and observation variables. By exploiting a block-triangular structure in the transport map, the analysis step of filtering is reformulated as the minimization of the maximum mean discrepancy (MMD) between the true joint measure and its transport-based approximation. To circumvent the non-convexity in the MMD optimization, we introduce a training-free transport filter method via gradient flows, which leads to an analytic computation for the transport map that implies the steepest descent direction of the MMD. The proposed approach accurately approximates non-Gaussian filtering posteriors and avoids particle collapse. We provide a convergence analysis for the expectation of the MMD between the approximated posterior and the truth posterior. Finally, we extend the method to high-dimensional problems through domain localization. Numerical examples demonstrate the superior performance of our approach over conventional filtering methods in nonlinear, non-Gaussian scenarios.

stat.ML↗
arXiv2026

Resolving Discrepancies in Disjoining Pressure Predictions for Liquid Nanofilms from Molecular Simulations

Literature values of disjoining pressure in liquid nanofilms from different molecular simulation methods show significant discrepancies. We demonstrate that these arise from neglecting long-range dispersion interactions and inconsistent definitions of film thickness in the original Peng method. A key insight is that long-range dispersion affects surface tension in a thickness-dependent manner, increasing it at large thickness but suppressing its enhancement at small thickness due to disjoining-pressure-induced normal compression and lateral expansion. This leads to crossover behavior in the surface tension of water nanofilms. Since disjoining pressure is obtained from the derivative of surface tension with respect to thickness, this nontrivial dependence strongly impacts its accuracy. With proper treatment of dispersion interactions and a consistent thickness definition, the revised Peng method agrees with the Bhatt method and yields more accurate Hamaker constants.

physics.chem-ph↗
arXiv2026

An Energy-Stable, Bound-Preserving and Locally Conservative Numerical Framework for Multicomponent Gas Flow in Poroelastic Media

In this paper, we propose a robust and efficient numerical framework for simulating multicomponent gas flow in poroelastic media, with a focus on preserving fundamental thermodynamic principles and ensuring computational reliability. The model captures the complex nonlinear coupling between multicomponent transport and solid deformation, while addressing critical numerical challenges such as mass conservation, energy stability, and molar density boundedness. To achieve this, we develop a stabilized discretization approach that guarantees the preservation of the original energy dissipation law and ensures the boundedness of each gas component's molar density. Furthermore, the proposed method incorporates an adaptive time-stepping strategy that dynamically adjusts the time step size based on the system's dynamics, significantly enhancing computational efficiency without compromising stability or accuracy. For spatial discretization, a mixed finite element method combined with an upwind scheme is employed for the flow and transport equations to ensure local mass conservation, while a discontinuous Galerkin (DG) method is utilized for discretizing the momentum equation of poroelasticity to effectively overcome numerical locking phenomena. Numerical experiments are presented to demonstrate the performance, robustness, and applicability of the method in simulating multicomponent gas flow under various scenarios.

math.NA↗
arXiv2026

Bound-Preserving Adaptive Time-Stepping Method with Energy Stability for Simulating Compressible Gas Flow in Poroelastic Media

In this paper, we present an efficient numerical method to address a thermodynamically consistent gas flow model in porous media involving compressible gas and deformable rock. The accurate modeling of gas flow in porous media often poses significant challenges due to their inherent nonlinearity, the coupling between gas and rock dynamics, and the need to preserve physical principles such as mass conservation, energy dissipation and molar density boundedness. The system is further complicated by the need to balance computational efficiency with the accuracy and stability of the numerical scheme. To tackle these challenges, we adopt a stabilization approach that is able to preserve the original energy dissipation while achieving linear energy-stable numerical schemes. We also prove the convergence of the adopted linear iterative method. At each time step, the stabilization parameter is adaptively updated using a simple and explicit formula to ensure compliance with the original energy dissipation law. The proposed method uses adaptive time stepping to improve computational efficiency while maintaining solution accuracy and boundedness. The adaptive time step size is calculated explicitly at each iteration, ensuring stability and allowing for efficient handling of highly dynamic scenarios. A mixed finite element method combined with an upwind scheme is employed as spatial discretization to ensure mass conservation and stability. Finally, we conduct a series of numerical experiments to validate the performance and robustness of the proposed numerical method.

math.NA↗
arXiv2026

The Existence, uniqueness, and regularity of weak solutions for a thermodynamically consistent two-phase flow model in porous media

Thermodynamically consistent models for two-phase flow in porous media have attracted significant attention in recent years. In this paper, we prove the existence, uniqueness and regularity of the weak solution to such a recent model proposed in [25,35]. To this end, firstly, we introduce a fully implicit time semi-discrete approximation and a fully discrete approximation for an appropriate weak formulation of the thermodynamically consistent model. Next, by using the zeros of a vector field theorem, we prove the existence of the weak solution for the fully discrete approximation. Then the existence of weak solutions for the fully implicit time semi-discrete approximation and the weak formulation of the model are derived by the weak convergence technique and the energy stability estimate. Subsequently, by the Gr{\" o}nwall inequality, we prove the uniqueness result under the smoothness assumption on the chemical potential. Finally, combined with the regularity theory of elliptic partial differential equations (PDE), the regularity of the weak solution for the model with complete Neumann boundary conditions is established.

math.AP↗
arXiv2025

A domain decomposition approach to pore-network modeling of porous media flow

We propose a domain-decomposition pore-network method (DD-PNM) for modeling single-phase Stokes flow in porous media. The method combines the accuracy of finite-element discretizations on body-fitted meshes within pore subdomains with a sparse global coupling enforced through interface unknowns. Local Dirichlet-to-Neumann operators are precomputed from finite-element solutions for each pore subdomain, enabling a global Schur-complement system defined solely on internal interfaces. Rigorous mathematical analysis establishes solvability and discrete mass conservation of the global system. Moreover, we constructively recover classical pore-network models by fitting half-throat conductivities to local Dirichlet-to-Neumann maps, providing a principled bridge between mesh-based and network-based frameworks. Numerical results are presented to demonstrate the validity and effectiveness of the overall methodology.

math.NA↗
arXiv2025

An Optimally Convergent parallel splitting Algorithm for the Multiple-Network Poroelasticity Model

This paper presents a novel parallel splitting algorithm for solving quasi-static multiple-network poroelasticity (MPET) equations. By introducing a total pressure variable, the MPET system can be reformulated into a coupled Stokes-parabolic system. To efficiently solve this system, we propose a parallel splitting approach. In the first time step, a monolithic solver is used to solve all variables simultaneously. For subsequent time steps, the system is split into a Stokes subproblem and a parabolic subproblem. These subproblems are then solved in parallel using a stabilization technique. This parallel splitting approach differs from sequential or iterative decoupling, significantly reducing computational time. The algorithm is proven to be unconditionally stable, optimally convergent, and robust across various parameter settings. These theoretical results are confirmed by numerical experiments. We also apply this parallel algorithm to simulate fluid-tissue interactions within the physiological environment of the human brain.

math.NA↗
arXiv2025

CO2 adsorption mechanisms in hydrated silica nanopores: Insights from grand canonical Monte Carlo to classical and ab initio molecular dynamics

Understanding interfacial phenomena in confined systems is important for optimizing CO2 capture technologies. Here, we present a comprehensive investigation of CO2 adsorption in hydrated amorphous silica nanopores through an integrated computational approach combining grand canonical Monte Carlo (GCMC), classical molecular dynamics (MD), and ab initio molecular dynamics (AIMD) simulations. The excess adsorption isotherms reveal a marked hydration dependence, with CO2 uptake decreasing from 7.6 to 2.6 mmol/g as water content increases from 1 to 15 wt%. Analysis of adsorption kinetics demonstrates a distinctive bimodal process, characterized by rapid initial uptake followed by slower diffusion-limited adsorption, with the latter becoming increasingly dominant at higher hydration levels. Classical MD simulations reveal an inverse correlation between hydration and CO2 mobility, with self-diffusion coefficients decreasing across the studied hydration range. Density profile analysis indicates a hydration-induced transition in CO2 distribution from central pore regions to surface-proximate domains, accompanied by restructuring of interfacial water networks. Notably, AIMD simulations capture previously unrecognized chemical processes, including proton transfer mechanisms leading to surface silanol formation with characteristic O-O distances of 2.4-2.5 {\AA}, and spontaneous CO2 hydration yielding carbonate species through water-mediated reaction pathways. These findings demonstrate the dual role of confined water as both a spatial competitor and reaction medium for CO2 capture, providing molecular-level insights with quantum mechanical accuracy for design of carbon capture materials.

cond-mat.mtrl-sci↗
arXiv2024

Estimating Fluid-solid Interfacial Free Energies for Wettabilities: A Review of Molecular Simulation Methods

Fluid-solid interfacial free energy (IFE) is a fundamental parameter influencing wetting behaviors, which play a crucial role across a broad range of industrial applications. Obtaining reliable data for fluid-solid IFE remains challenging with experimental and semi-empirical methods, and the applicability of first-principle theoretical methods is constrained by a lack of accessible computational tools. In recent years, a variety of molecular simulation methods have been developed for determining the fluid-solid IFE. This review provides a comprehensive summary and critical evaluation of these techniques. The developments, fundamental principles, and implementations of various simulation methods are presented from mechanical routes, such as the contact angle approach, the technique using Bakker's equation, and the Wilhelmy simulation method, as well as thermodynamic routes, including the cleaving wall method, the Frenkel-Ladd technique, and the test-volume/area methods. These approaches can be applied to compute various fluid-solid interfacial properties, including IFE, relative IFE, surface stress, and superficial tension, although these properties are often used without differentiation in the literature. Additionally, selected applications of these methods are reviewed to provide insight into the behavior of fluid-solid interfacial energies in diverse systems. We also illustrate two interpretations of the fluid-solid IFE based on the theory of Navascues and Berry and Bakker's equation. It is shown that the simulation methods developed from these two interpretations are identical. This review advocates for the broader adoption of molecular simulation methods in estimating fluid-solid IFE, which is essential for advancing our understanding of wetting behaviors in various chemical systems.

physics.chem-ph↗
arXiv2024

A locally mass-conservative enriched Petrov-Galerkin method without penalty for the Darcy flow in porous media

In this work we present an enriched Petrov-Galerkin (EPG) method for the simulation of the Darcy flow in porous media. The new method enriches the approximation trial space of the conforming continuous Galerkin (CG) method with bubble functions and enriches the approximation test space of the CG method with piecewise constant functions, and it does not require any penalty term in the weak formulation. Moreover, we propose a framework for constructing the bubble functions and consider a decoupled algorithm for the EPG method based on this framework, which enables the process of solving pressure to be decoupled into two steps. The first step is to solve the pressure by the standard CG method, and the second step is a post-processing correction of the first step. Compared with the CG method, the proposed EPG method is locally mass-conservative, while keeping fewer degrees of freedom than the discontinuous Galerkin (DG) method. In addition, this method is more concise in the error analysis than the enriched Galerkin (EG) method. The coupled flow and transport in porous media is considered to illustrate the advantages of locally mass-conservative properties of the EPG method. We establish the optimal convergence of numerical solutions and present several numerical examples to illustrate the performance of the proposed method.

math.NA↗
arXiv2023

A new approach for the implementation of contact line motion based on the phase-filed lattice Boltzmann method

This paper proposes a new strategy to implement the free-energy based wetting boundary condition within the phase-field lattice Boltzmann method. The greatest advantage of the proposed method is that the implementation of contact line motion can be significantly simplified while still maintaining good accuracy. For this purpose, the liquid-solid free energy is treated as a part of the chemical potential instead of the boundary condition, thus avoiding complicated interpolations with irregular geometries. Several numerical testing cases including the droplet spreading processes on the idea flat, inclined and curved boundaries are conducted, and the results demonstrate that the proposed method has good ability and satisfactory accuracy to simulate contact line motions.

math-ph↗
arXiv2023

A well-balanced lattice Boltzmann model for binary fluids based on the incompressible phase-field theory

Spurious velocities arising from the imperfect offset of the undesired term at the discrete level are frequently observed in numerical simulations of equilibrium multiphase flow systems using the lattice Boltzmann equation (LBE) method. To capture the physical equilibrium state of two-phase fluid systems and eliminate spurious velocities, a well-balanced LBE model based on the incompressible phase-field theory is developed. In this model, the equilibrium distribution function for the Cahn-Hilliard (CH) equation is designed by treating the convection term as a source to avoid the introduction of undesired terms, enabling achievement of possible discrete force balance. Furthermore, this approach allows for the attainment of a divergence-free velocity field, effectively mitigating the impact of artificial compression effects and enhancing numerical stability. Numerical tests, including a flat interface problem, a stationary droplet, and the coalescence of two droplets, demonstrate the well-balanced properties and improvements in the stability of the present model.

physics.flu-dyn↗
arXiv2023

Zero-Shot Digital Rock Image Segmentation with a Fine-Tuned Segment Anything Model

Accurate image segmentation is crucial in reservoir modelling and material characterization, enhancing oil and gas extraction efficiency through detailed reservoir models. This precision offers insights into rock properties, advancing digital rock physics understanding. However, creating pixel-level annotations for complex CT and SEM rock images is challenging due to their size and low contrast, lengthening analysis time. This has spurred interest in advanced semi-supervised and unsupervised segmentation techniques in digital rock image analysis, promising more efficient, accurate, and less labour-intensive methods. Meta AI's Segment Anything Model (SAM) revolutionized image segmentation in 2023, offering interactive and automated segmentation with zero-shot capabilities, essential for digital rock physics with limited training data and complex image features. Despite its advanced features, SAM struggles with rock CT/SEM images due to their absence in its training set and the low-contrast nature of grayscale images. Our research fine-tunes SAM for rock CT/SEM image segmentation, optimizing parameters and handling large-scale images to improve accuracy. Experiments on rock CT and SEM images show that fine-tuning significantly enhances SAM's performance, enabling high-quality mask generation in digital rock image analysis. Our results demonstrate the feasibility and effectiveness of the fine-tuned SAM model (RockSAM) for rock images, offering segmentation without extensive training or complex labelling.

cs.CV↗
arXiv2023

Enhancing Rock Image Segmentation in Digital Rock Physics: A Fusion of Generative AI and State-of-the-Art Neural Networks

In digital rock physics, analysing microstructures from CT and SEM scans is crucial for estimating properties like porosity and pore connectivity. Traditional segmentation methods like thresholding and CNNs often fall short in accurately detailing rock microstructures and are prone to noise. U-Net improved segmentation accuracy but required many expert-annotated samples, a laborious and error-prone process due to complex pore shapes. Our study employed an advanced generative AI model, the diffusion model, to overcome these limitations. This model generated a vast dataset of CT/SEM and binary segmentation pairs from a small initial dataset. We assessed the efficacy of three neural networks: U-Net, Attention-U-net, and TransUNet, for segmenting these enhanced images. The diffusion model proved to be an effective data augmentation technique, improving the generalization and robustness of deep learning models. TransU-Net, incorporating Transformer structures, demonstrated superior segmentation accuracy and IoU metrics, outperforming both U-Net and Attention-U-net. Our research advances rock image segmentation by combining the diffusion model with cutting-edge neural networks, reducing dependency on extensive expert data and boosting segmentation accuracy and robustness. TransU-Net sets a new standard in digital rock physics, paving the way for future geoscience and engineering breakthroughs.

cs.CV↗