arXiv subjects

Denvid Lau

Publications and source records attributed to Denvid Lau.

7 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

Estimating the disjoining pressure of liquid nanofilms from molecular dynamics simulations via implicit treatment of the bulk liquid phase

The commonly adopted constant bulk liquid density approximation for estimating disjoining pressure in liquid nanofilms, although justified by the low compressibility of liquids, can introduce significant errors in its evaluation. We hypothesize that the bulk liquid density is a thermodynamic state variable that depends on film thickness and local pressure, and should therefore be determined self-consistently to accurately capture interfacial forces under confinement. A thermodynamically consistent molecular dynamics simulation framework is developed by coupling a surface-tension-based formulation with bulk equations of state obtained from independent simulations. An iterative algorithm is employed to simultaneously determine bulk liquid density, film thickness, and disjoining pressure. The method is applied to water and argon nanofilms, and results are benchmarked against conventional constant-density approaches and alternative computational routes based on chemical potential calculation. The proposed framework provides a computationally efficient and thermodynamically rigorous route for evaluating disjoining pressure without requiring explicit full coexisting system simulations. Incorporating density variations significantly improves the accuracy and consistency of predicted disjoining pressures. For water nanofilms, a 3.6% deviation in density can lead to up to 77% overestimation of disjoining pressure at a thickness of 12 {\AA}, whereas the effect is weaker but still non-negligible for argon. The proposed framework restores the inverse-cubic scaling with film thickness predicted by Hamaker theory and reduces discrepancies between independent computational methods. Overall, the results demonstrate that self-consistent treatment of bulk thermodynamics is essential for quantitatively reliable evaluation of surface forces in confined fluid systems.

physics.chem-ph↗

arXiv2026

Scaling atom-by-atom inverse design with nano-topology optimization and diffusion models

The mechanical properties of metallic nanostructures are governed not only by topology but also by crystal symmetry and face-specific surface physics, which are typically absent from continuum topology optimization. We develop an atom-by-atom inverse design framework that combines Nano-Topology Optimization (Nano-TO) with conditional denoising diffusion probabilistic models. Nano-TO treats each atom as a discrete design variable and evaluates stiffness from the symmetric curvature of the total energy, removing residual surface-stress bias. A crystallography-aligned multi-shell sensitivity filter stabilizes the optimization and enables designs containing more than 6.5 x 10^5 atoms. Using aluminum nanocantilevers, we identify a surface-physics-driven topology selection rule: thickness-periodic beams favor brace-dominated trusses, whereas finite-thickness beams favor nearly closed walls that provide efficient shear paths and reduce surface penalties. At sufficiently small scales, these walls become mechanically unstable, and truss-like layouts reappear. In nanopillar studies, atomistic optimization outperforms continuum topology-optimized designs. Finally, conditional diffusion models trained on Nano-TO data generate diverse high-performance candidates near the optimization frontier. These results establish nanoscale inverse design as a coupled problem of topology and surface physics.

physics.app-ph↗

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↗

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↗

arXiv2016

Molecular Mechanics of Chitin-Protein Interface

Chitin and protein are two main building blocks for many natural biomaterials. The interaction between chitin and protein critically determines the properties of the composite biological materials. As living organisms usually encounter complex ambient conditions like water, pH and ions are critical factors towards the structural integrity of biomaterials. It is therefore essential to study the chitin-protein interface under different environmental conditions. Here, an atomistic model consisting of a chitin substrate and a protein filament is constructed, which is regarded as a representative of the chitin-protein interface existing in many chitin-based biomaterials. Based on this model, the mechanical properties of chitin-protein interface under different moisture and pH values are investigated through molecular dynamics simulations. The results reveal a weakening effect of water towards the chitin-protein interface, as well as acidity, i.e. the protonated protein forms a stronger adhesion to chitin than that in the alkaline environment. In addition, the effect from side-chain of protein is studied and it is found that certain kinds of amino acid can form hydrophobic connections to chitin surface, which means that these peptides partly dodge the weakening effect of water. Our observation indicates that terminuses and side-chains in protein are of importance in forming interfacial hydrogen bonds. From our full atomistic models, we can observe some molecular mechanisms about how protein interacts with chitin in different conditions, which may spotlight the engineering on biomaterials with similar interfaces.

cond-mat.mtrl-sci↗