Wenxiang Xu

Effect of surface free energies on the heterogeneous nucleation of water droplet: A molecular dynamics simulation approach

Heterogeneous nucleation of water droplet on surfaces with different solid-liquid interaction intensities is investigated by molecular dynamics simulation. The interaction potentials between surface atoms and vapor molecules are adjusted to obtain various surface free energies, and the nucleation process and wetting state of nuclei on surfaces are investigated. The results indicate that near-constant contact angles are already established for nano-scale nuclei on various surfaces, with the contact angle decreasing with solid-liquid interaction intensities linearly. Meanwhile, noticeable fluctuation of vapor-liquid interfaces can be observed for the nuclei that deposited on surfaces, which is caused by the asymmetric forces from vapor molecules. The formation and growth rate of nuclei are increasing with the solid-liquid interaction intensities. For low energy surface, the attraction of surface atoms to water molecules is comparably weak, and the pre-existing clusters can depart from the surface and enter into the bulk vapor phase. The distribution of clusters within the bulk vapor phase becomes competitive as compared with that absorbed on surface. For moderate energy surfaces, heterogeneous nucleation predominates and the formation of clusters within bulk vapor phase is suppressed. The effect of high energy particles that embedded in low energy surface is also discussed under the same simulation system. The nucleation preferably initiates on the high energy particles, and the clusters that formed on the heterogeneous particles are trapped around their original positions instead of migrating around as that observed on smooth surfaces. This feature makes it possible for the heterogeneous particles to act as fixed nucleation sites, and simulation results also suggest that the number of nuclei increases monotonously with the number of high energy particles. The growth of nuclei on high energy particles can be divided into three sub-stages, beginning with the formation of a wet-spot, increase of contact angle with near-constant contact line, and finally growth with constant contact angle. The growth rate of nuclei also increases with the size of high energy particles.

Modeling of soft interfacial volume fraction in composite materials with complex convex particles

The influence of the soft interfacial volume fraction on physical properties of composite materials has been found to be significant. However, the soft interfacial volume fraction is difficultly determined by traditional experimental methods and simple models proposed so far. This article addresses the problem by means of theoretical and numerical approaches that start at a microscopic scale of composite materials, which are regarded as a three-phase composite structure with polydisperse convex particles, soft interfaces, and a matrix. A theoretical scheme for the soft interfacial volume fraction is proposed by a theory of the nearest-surface distribution functions and geometrical configurations of polydisperse convex particles. The theoretical scheme represents a generalized model for the soft interfacial volume fraction in that it cannot only determine the interfacial volume fraction around convex polyhedral particles but also to derive that around ellipsoidal and spherical particles. In order to test the theoretical scheme, a numerical model that adopts the three-phase composite structure and a numerical Monte Carlo integration scheme is presented. Also, theoretical and numerical results of the soft interfacial volume fraction around ellipsoidal and spherical particles in the literature are further compared. By way of application, it is shown that the developed model provides a quantitative means to evaluate the dependence of the soft interfacial volume fraction on various factors, such as geometrical configurations of particles and the interfacial thickness.

Parking simulation of three-dimensional multi-sized star-shaped particles

The shape and size of particles may have a great impact on the microstructure as well as the physico-properties of particulate composites. However, it is challenging to configure a parking system of particles to a geometrical shape that is close to realistic grains in particulate composites. In this work, with the assistance of x-ray tomography and a spherical harmonic series, we present a star-shaped particle that is close to realistic arbitrary-shaped grains. To realize such a hard particle parking structure, an inter-particle overlapping detection algorithm is introduced. A serial sectioning approach is employed to visualize the particle parking structure for the purpose of justifying the reliability of the overlapping detection algorithm. Furthermore, the validity of the area and perimeter of solids in any arbitrary section of a plane calculated using a numerical method is verified by comparison with those obtained using an image analysis approach. This contribution is helpful to further understand the dependence of the micro-structure and physico-properties of star-shaped particles on the realistic geometrical shape.

Interfacial effect on physical properties of composite media: Interfacial volume fraction with non-spherical hard-core-soft-shell-structured particles

Interfaces are known to be crucial in a variety of fields and the interfacial volume fraction dramatically affects physical properties of composite media. However, it is an open problem with great significance how to determine the interfacial property in composite media with inclusions of complex geometry. By the stereological theory and the nearest-surface distribution functions, we first propose a theoretical framework to symmetrically present the interfacial volume fraction. In order to verify the interesting generalization, we simulate three-phase composite media by employing hard-core-soft-shell structures composed of hard mono-/polydisperse non-spherical particles, soft interfaces, and matrix. We numerically derive the interfacial volume fraction by a Monte Carlo integration scheme. With the theoretical and numerical results, we find that the interfacial volume fraction is strongly dependent on the so-called geometric size factor and sphericity characterizing the geometric shape in spite of anisotropic particle types. As a significant interfacial property, the present theoretical contribution can be further drawn into predicting the effective transport properties of composite materials.

Effect of nano structures on the nucleus wetting modes during water vapour condensation: from individual groove to nano-array surface

The geometrical structures of surfaces are important to the formation and growth of nuclei during water vapor condensation and the related heat and mass transfer performances. In the present research, the nucleus wetting modes on individual grooves and nano-array surfaces were investigated by molecules dynamics simulations. The results proposed a criterion that the nucleus wetting modes on a V-shaped groove are determined by the intrinsic contact angle θ and the cross sectional angle β. As the cross sectional angle decreases to β < 2θ − π, the nuclei can suspend in the groove center and the suspending height increases with decreasing β. For the nano-array surfaces, the nucleus can wet the grooves between adjacent nano arrays during the initial nucleation stage as the initial nuclei are very small and the nucleus surface are fluctuating frequently. The wetting mode may change as nucleation continuous and the nucleus can depart from the groove bottom to form a Cassie mode nucleus on a surface with β < 2θ − π. The apparent contact angle of nucleus also increases sharply with the wetting mode transition. Moreover, the dynamic behaviors of nucleating droplets were also observed on a nano-array superhydrophobic surface that meets the criterion. The droplets on this surface can recover the spherical shape after coalescence and the droplet jumping occurs, indicating a lower surface stiction. The results provide an insight of the interfacial phenomenon between the nucleus and the geometrical structures and propose a guideline to construct nano-array surfaces in the aim of promoting the Cassie mode nucleus.

Transport properties of concrete-like granular materials interacted by their microstructures and particle components

Granular materials as typical soft matter, their transport properties play significant roles in durability and service life in relevant practical engineering structures. Physico-mechanical properti...

Numerical evaluation of overestimation of the interface thickness around ellipsoidal particle

When interfacial layers are viewed as a separate phase, the interface thickness plays an essential role in assessing physico-mechanical properties of particulate materials. However, the interface thickness from sectional analysis is often overestimated, due to the irregularity of surface textures of grains in opaque materials that gives rise to the normal of a cross-sectional plane non-perpendicular to the surface of grains. Hence, the determination of the overestimation degree is very critical to precisely obtain the interface thickness. This article develops a numerical model for the overestimation degree of the interface thickness around an ellipsoidal grain with an arbitrary aspect ratio, by applying an accurate sectional analysis algorithm, and quantitative stereology and geometrical probability theories. Furthermore, on the basis of the developed numerical model, the influence of ellipsoidal particle shape on the overestimation degree is quantitatively characterized.

Ride-comfort-oriented suspension optimization using the pseudo-excitation method

The main purpose of this article is to optimize the stiffness and damping parameters of suspension systems to improve the ride comfort of a coach by using the pseudo-excitation method (PEM) and the method of moving asymptotes (MMA). The finite element model of the coach under investigation has 1778 degrees of freedom. The surface roughness of the highway is regarded as a spatial stationary random process, which is accurately transformed into the superposition of a series of deterministic pseudo-harmonic surface unevennesses by using PEM. By doing so, the proposed method considerably simplifies the solution of the vibration equations, the derivation of sensitivities of various random responses, and the execution of the optimization process. MMA is applied to solve the ride comfort optimization problem. The proposed method is well justified by the numerical example.

Numerical modeling on the influence of particle shape on ITZ’s microstructure and macro-properties of cementitious composites: a critical review

Spherical particle models are often employed in the literature to numerically quantify the relationship among composition, structure, and macro-performance of cementitious composites. However, most particles in cementitious composites are non-spherical particle. To clarify how particle shape affects the microstructure and macro-properties of materials, both Platonic and spheroidal particles packing models are developed. An ITZ layer is constructed around these non-spherical particles via Minkowski sum approach. Dependence of the ratio of apparent to actual ITZ thickness and volume fraction of ITZ on aggregate’s shape is quantitatively presented. Influence of sphericity and volume fraction of particles on the degree of hydration and total porosity of cement paste, effective elastic modulus, water permeability and diffusivity of concrete is presented. All the investigations reveal that particle shape significantly affects microstructure and properties of cementitious composites. Neglecting the influence of particle shape may cause serious deviation away from the reality.