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Ternary Free Energy Lattice Boltzmann Model with Tunable Surface Tensions and Contact Angles  [PDF]
Ciro Semprebon,Timm Krüger,Halim Kusumaatmaja
Physics , 2015,
Abstract: We present a new ternary free energy lattice Boltzmann model. The distinguishing feature of our model is that we are able to analytically derive and independently vary all fluid-fluid surface tensions and the solid surface contact angles. We carry out a number of benchmark tests: (i) double emulsions and liquid lenses to validate the surface tensions, (ii) ternary fluids in contact with a square well to compare the contact angles against analytical predictions, and (iii) ternary phase separation to verify that the multicomponent fluid dynamics is accurately captured. Additionally we also describe how the model here presented here can be extended to include an arbitrary number of fluid components.
Transition temperatures and contact angles in the sequential-wetting scenario of n-alkanes on (salt) water  [PDF]
Volker C. Weiss,Joseph O. Indekeu
Physics , 2003,
Abstract: Alkanes of medium chain length show an unusual wetting behavior on (salt) water, exhibiting a sequence of two changes in the wetting state. When deposited on the water surface at low temperatures, the liquid alkane forms discrete droplets that are interconnected only by a molecularly thin film. On increase of the temperature, there occurs a sudden jump of the film thickness and, at this first-order transition, a mesoscopically thick layer of liquid alkane is formed. Heating the system further leads to a divergence of the film thickness in a continuous manner. While the long-range forces between substrate and adsorbate are responsible for the critical transition, which occurs at the temperature at which the Hamaker constant changes sign, it is primarily the short-range components of the forces that bring about the first-order transition. We calculate the Hamaker constant of the system and show how, within a modified Cahn theory, accurate predictions of the first-order transition temperatures can be obtained for n-alkanes (pentane and hexane) on water and even on brine. Furthermore, we study the variation of the contact angle as a function of temperature.
Influence of the cross-section shape in the capillaries on the wetting properties demonstrated by the calculated wetting angles
Hartge K.H.,Bachmann J.,Woche S.K.
International Agrophysics , 2004,
Abstract: Earlier results had shown that the contracting force of water films between parallel solid surfaces increased when they were deformed to give longer menisci by subdividing the water volume. Since deformations of this kind will occur regularly when soils are tilled or wheeled with machinery the question was raised how such events would change soil capillary properties. Circular capillaries with a concave inner surface were compared to the ones with a convex inner surface. They were formed by the fixing of either three or four rods with circular diameter together. Perimeters and crosssectional areas were calculated and the height of capillary rise was measured by two independent methods. For comparison, the results of wetting angles were used. They were calculated from an average inner diameter of the capillaries and separately measured using the height of menisci. The present results showed that in the convex capillaries, higher angles were calculated in comparison to those independently measured at the outsides of rods and tubes. The angles calculated from the capillary height measured in the circular capillaries were smaller than those measured at the outside of the tubes. It was concluded that the wetting angles measured directly with an optical equipment were affected by the solid phase geometry. The above result emphasized that contact angles resulted from a combination of several distinct factors. The curvature of the contact line is one of them.
Dynamic wetting by nanofluids: Lattice Boltzmann method study

- , 2018, DOI: 10.13738/j.issn.1671-8097.017084
Abstract: 纳米流体动态湿润特性与纳米颗粒的微观运动密切相关。由于缺乏纳米尺度的实验观测技术及相关理论描述,纳米流体动态湿润的研究极具挑战,相关机理仍未明晰。本文采用格子-Boltzmann方法研究纳米颗粒在纳米尺度下(10-9 m)的微观运动及颗粒沉积所导致的基液流体表面张力、流变性改变及结构分离压力对宏观动态湿润(10-3 m)的影响机制。结果表明纳米颗粒对基液的表面张力的改性影响纳米流体平衡湿润特性,决定纳米流体是完全浸润还是部分浸润。而纳米颗粒对基液流体流变性的改变影响纳米流体动态湿润过程的铺展指数。纳米颗粒在液滴底部的沉积对动态湿润过程影响较小,而在接触线区域的沉积显著改变纳米流体的动态湿润行为。本研究尝试从跨尺度的角度阐释纳米颗粒微观运动对宏观动态湿润行为的影响,探索从微观层面调控纳米流体动态湿润的新方法。
Nanofluids have been reported to exhibit attractive and tunable dynamic wetting behaviors due to the complex nanoparticle kinetics. Studies of the dynamic wetting by nanofluids are of great challenge because the wetting behavior crosses several length and time scales. The mechanism of dynamic wetting by nanofluids is still unclear due to limitations of nanoscale experimental techniques and fundamental theories. A lattice Boltzmann method with some simple but effective treatments with consideration of nanofluid surface tension and rheology modification, as well as the nanoparticle sedimentations, was used to investigate the effects of nanoparticle kinetics at the nanoscale (10-9 m) on the dynamic wetting behaviors occurring at the macroscopic scale (10-3 m). The individual effect analysis shows that the surface tension modification is related to the equilibrium wettability of nanofluids (complete wetting or partial wetting), while the spreading exponents of nanofluid droplets depend on the rheology modification. The additional hydrophobic nanoparticles, which function like surfactants, facilitate the wettability and result in a complete wetting behavior. Only considering the rheological modification cannot predict the wettability of nanofluids at equilibrium stages. However, without considering the rheology modification, the spreading exponents of nanofluid droplets are underestimated. The shear-thinning non-Newtonian behavior can further enhance the dynamic wetting of nanofluids. The nanoparticle deposition at the bottom of the droplet has few effects on the dynamic wetting for both the complete wetting and partial wetting. However, the nanoparticle deposition in the vicinity of contact line strongly facilitates the contact line motion, especially for the partial wetting droplet at the late spreading stage when the contact angle is small, which can be explained by the additional structural disjoining pressure due to the nanoparticle self-assembly. The study provides multiscale understanding and guidelines to tune the nanofluid dynamic wetting behaviors.
The Wetting Behavior of Fresh and Aged Soot Studied through Contact Angle Measurements  [PDF]
Yiyi Wei, Qing Zhang, Jonathan E. Thompson
Atmospheric and Climate Sciences (ACS) , 2017, DOI: 10.4236/acs.2017.71002
Abstract: In this work, contact angle measurements for soot samples collected from a kerosene lantern, wood-burning fireplace, and municipal bus engine exhaust lines are reported. Contact angles for both freshly collected soot and samples treated with various doses of O3 (g), HNO3 (g), and H2SO4 (g) are considered. Use of a literature method has allowed estimation of the enthalpy of immersion (Himm) for the soot samples based on contact angle observed. Contact angles for freshly collected soot were 65 - 110 deg. indicating its hydrophobic nature. Chemical processing of soot usually resulted in smaller contact angles and large increases in immersion enthalpy. However, the dose of ozone, nitric or sulfuric acid vapor required to achieve alteration of the soot surface appeared to be considerably larger than that expected to be experienced by authentic atmospheric samples during the soot particles lifetime. The most significant variability of soot contact angle was observed for the municipal bus exhaust samples, suggesting that combustion chemistry may significantly affect wetting behavior.
A mesoscopic model for microscale hydrodynamics and interfacial phenomena: Slip, films, and contact angle hysteresis  [PDF]
Carlos E. Colosqui,Michail E. Kavousanakis,Athanasios G. Papathanasiou,Ioannis G. Kevrekidis
Physics , 2012, DOI: 10.1103/PhysRevE.87.013302
Abstract: We present a model based on the lattice Boltzmann equation that is suitable for the simulation of dynamic wetting. The model is capable of exhibiting fundamental interfacial phenomena such as weak adsorption of fluid on the solid substrate and the presence of a thin surface film within which a disjoining pressure acts. Dynamics in this surface film, tightly coupled with hydrodynamics in the fluid bulk, determine macroscopic properties of primary interest: the hydrodynamic slip; the equilibrium contact angle; and the static and dynamic hysteresis of the contact angles. The pseudo- potentials employed for fluid-solid interactions are composed of a repulsive core and an attractive tail that can be independently adjusted. This enables effective modification of the functional form of the disjoining pressure so that one can vary the static and dynamic hysteresis on surfaces that exhibit the same equilibrium contact angle. The modeled solid-fluid interface is diffuse, represented by a wall probability function which ultimately controls the momentum exchange between solid and fluid phases. This approach allows us to effectively vary the slip length for a given wettability (i.e. the static contact angle) of the solid substrate.
Effect of the forcing term in the pseudopotential lattice Boltzmann modeling of thermal flows  [PDF]
Q. Li,K. H. Luo
Physics , 2014, DOI: 10.1103/PhysRevE.89.053022
Abstract: The pseudopotential lattice Boltzmann (LB) model is a popular model in the LB community for simulating multiphase flows. Recently, several thermal LB models, which are based on the pseudopotential LB model and constructed within the framework of the double-distribution-function LB method, were proposed to simulate thermal multiphase flows [G. H\'azi and A. M\'arkus, Phys. Rev. E 77, 026305 (2008); L. Biferale et al., Phys. Rev. Lett. 108, 104502 (2012); S. Gong and P. Cheng, Int. J. Heat Mass Transfer 55, 4923 (2012)]. The objective of the present paper is to show that the effect of the forcing term on the temperature equation must be eliminated in the pseudopotential LB modeling of thermal flows. First, the effect of the forcing term on the temperature equation is shown via the Chapman-Enskog analysis. For comparison, alternative treatments that are free from the forcing-term effect are provided. Subsequently, numerical investigations are performed for two benchmark tests. The numerical results clearly show that the existence of the forcing-term effect will lead to significant numerical errors in the pseudopotential LB modeling of thermal flows.
Contact angle determination in multicomponent lattice Boltzmann simulations  [PDF]
Sebastian Schmieschek,Jens Harting
Physics , 2009, DOI: 10.4208/cicp.201009.271010s
Abstract: Droplets on hydrophobic surfaces are ubiquitous in microfluidic applications and there exists a number of commonly used multicomponent and multiphase lattice Boltzmann schemes to study such systems. In this paper we focus on a popular implementation of a multicomponent model as introduced by Shan and Chen. Here, interactions between different components are implemented as repulsive forces whose strength is determined by model parameters. In this paper we present simulations of a droplet on a hydrophobic surface. We investigate the dependence of the contact angle on the simulation parameters and quantitatively compare different approaches to determine it. Results show that the method is capable of modelling the whole range of contact angles. We find that the a priori determination of the contact angle is depending on the simulation parameters with an uncertainty of 10 to 20%.
Drops on soft solids: Free energy and double transition of contact angles  [PDF]
Luuk A. Lubbers,Joost H. Weijs,Lorenzo Botto,Siddhartha Das,Bruno Andreotti,Jacco H. Snoeijer
Physics , 2013, DOI: 10.1017/jfm.2014.152
Abstract: The equilibrium shape of liquid drops on elastic substrates is determined by minimising elastic and capillary free energies, focusing on thick incompressible substrates. The problem is governed by three length scales: the size of the drop $R$, the molecular size $a$, and the ratio of surface tension to elastic modulus $\gamma/E$. We show that the contact angles undergo two transitions upon changing the substrates from rigid to soft. The microscopic wetting angles deviate from Young's law when $\gamma/Ea \gg 1$, while the apparent macroscopic angle only changes in the very soft limit $\gamma/ER \gg 1$. The elastic deformations are worked out in the simplifying case where the solid surface energy is assumed constant. The total free energy turns out lower on softer substrates, consistent with recent experiments.
Achieving tunable surface tension in the pseudopotential lattice Boltzmann modeling of multiphase flows  [PDF]
Q. Li,K. H. Luo
Physics , 2013, DOI: 10.1103/PhysRevE.88.053307
Abstract: In this paper, we aim to address an important issue about the pseudopotential lattice Boltzmann (LB) model, which has attracted much attention as a mesoscopic model for simulating interfacial dynamics of complex fluids, but suffers from the problem that the surface tension cannot be tuned independently of the density ratio. In the literature, a multi-range potential was devised to adjust the surface tension [Sbragaglia et al., Phys. Rev. E 75, 026702 (2007)]. However, it was recently found that the density ratio of the system will be changed when the multi-range potential is employed to adjust the surface tension. A new approach is therefore proposed in the present work. The basic strategy is to add a source term to the LB equation so as to tune the surface tension of the pseudopotential LB model. The proposed approach can guarantee that the adjustment of the surface tension does not affect the mechanical stability condition of the pseudopotential LB model, and thus provides a separate control of the surface tension and the density ratio. Meanwhile, it still retains the mesoscopic feature and the computational simplicity of the pseudopotential LB model. Numerical simulations are carried out for stationary droplets, capillary waves, and droplet splashing on a thin liquid film. The numerical results demonstrate that the proposed approach is capable of achieving a tunable surface tension over a very wide range and can keep the density ratio unchanged when adjusting the surface tension.
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