Patterning of small particles by a surfactant-enhanced Marangoni-Bénard instability

Evaporating drops provide a means of organizing particles suspended within them. Here, the manner in which surfactants alter these patterns is studied as a function of the surface state of an insoluble monolayer at the drop interface. The surface state is visualized throughout the drop evolution using fluorescence microscopy. A regime of surfactant coverage is identified that creates conditions that enhance the Marangoni-Bénard instability. This result was not anticipated in prior studies, in which surfactants are predicted to prevent this instability. These data demonstrate that, by tuning the liquid-gas boundary condition, the patterns formed from an evaporating drop can be controlled.     

A level-set continuum method for two-phase flows with insoluble surfactant

A level-set continuum surface force method is presented to compute two-phase flows with insoluble surfactant. Our method recasts the Navier–Stokes equations for a two-phase flow with insoluble surfactant as “one-fluid” formulation. Interfacial transport and interfacial jump conditions are treated using the level-set method and the discrete Dirac function. Based on the density-weighted projection method, a stable semi-implicit scheme is used to decouple the velocity components in solving the regularized Navier–Stokes equations. It allows numerical simulations for a wide range of viscosity ratios and density ratios.Numerical simulations on single drop deformation in a 2D shear flow are presented. Simulations on two drop interaction shows that surfactants can play a critical role in preventing drop coalescence. A fully 3D simulation demonstrating the physical interactions of multiple surfactant-laden drops is presented.     

Terrestrial simulation of drop saturation by a surfactant under microgravity conditions

The paper presents the results of experimental study of mass transfer in a drop of weakly soluble fluid during its saturation with a surfactant from its water solution forming a thin (∼ 1 mm) horizontal layer. The use of the interferometer allowed us to visualize the concentration fields and to define the structure of flows under the conditions of maximum suppression of the buoyancy convection. It has been found that despite the small layer thickness, in the case of initially homogeneous solution penetration of the surfactant into the drop leads to the formation of the local density inhomogeneities at both sides of the interface and to the development of a slow gravitational flow. An increase in the initial concentration of the surfactant in the solution causes a small-scale non-stationary solutocapillary motion at the drop surface. On the other hand, solubility of the drop in water also increases which leads to the formation of a counter-diffusion flux and eventually to the disappearance of the interface. In the case of initially inhomogeneous distribution of the surfactant, a competition between the gravitational and capillary mechanisms of convection gives rise to a large-scale oscillatory flow around the drop.     

Estimating surfactant surface coverage and decomposing its effect on drop deformation

A novel method is introduced to estimate surface coverage and the equation of state of insoluble surfactant on droplets, involving measurement of interfacial tension on a single parent drop and progressively subdivided generations of daughter drops. This has enabled quantitative decomposition of the dilution, tip-stretching, and Marangoni effects of surfactants on drop deformation. For a small viscosity ratio of 0.09, the Marangoni effect dominates, increasing first and then decreasing with surface coverage, the dilution effect is significant at high, and tip-stretching only at low surface coverage. For a viscosity ratio of 2.3, the dilution effect dominates, and neither Marangoni nor tip-stretching effects play an important role.     

活性剂浓度分布对液膜排液过程的影响

针对含不溶性活性剂的垂直液膜排液过程,考虑分离压和表面黏度的作用,应用润滑理论建立液膜厚度、活性剂浓度和表面速度的控制方程组,分析初始活性剂浓度及梯度对排液过程的影响.结果表明：当液膜表面不含活性剂时,其排液历程很短,很快发生破断.当液膜表面添加活性剂时,可以延长液膜存续时间.而当液膜表面活性剂浓度较低时,其诱发的Marangoni效应不足以克服重力的排液作用,其形成的"黑膜"不能稳定存在.随活性剂浓度增大,液膜表面流动速度减小,液膜表面更加"坚固",所形成的"黑膜"非常稳定.当考虑初始活性剂浓度梯度时,其影响主要体现在减缓排液初期的表面速度.     

Stability of core-annular flow of power-law fluids in the presence of interfacial surfactant

The shear-thinning influence on the core-annular flow stability of two immiscible power-law fluids is considered by making a linear stability analysis.The flow is driven by an axial pressure gradient in a straight pipe with the interface between the two fluids occupied by an insoluble surfactant.Given the basic flow for this core-annular arrangement,the analytical solution is obtained with respect to the power-law fluid model.The linearized equations for the evolution of infinitesimal disturbances are deriv...     

凹槽基底上含不溶性活性剂液膜的流动特性

针对凹槽基底上含不溶性活性剂液膜的流动过程,采用润滑理论建立液膜厚度和浓度演化模型,通过数值模拟得到液膜的流动特性及相关参数的影响规律.研究表明:含不溶性活性剂液膜在凹槽基底上流动时,重力和活性剂浓度梯度引起的Marangoni力对液膜的流动起促进作用,表面活性剂通过引起表层液体流动进而牵引内部液体运动,但其作用力相对重力较弱,重力起主导作用;与基底尺寸有关的粘性力则起阻碍作用;提高邦德数G和减小毛细力数C具有减弱液膜变形的作用;增大凹槽高度或减小凹槽斜度,均使Marangoni力增加,促使液膜变形加大.     

Large-scale Marangoni convection in a liquid layer with insoluble surfactant of low concentration

We derive a system of amplitude equations describing the evolution of a large-scale Marangoni patterns in a liquid layer with poorly conducting boundaries in the presence of a small amount of an insoluble surfactant on the free flat interface. The presence of quadratic nonlinear terms in the amplitude equation leads to the selection of hexagonal patterns. The type of hexagons bifurcating into the subcritical region, depends on the parameters of the system.     

A diffuse-interface method for two-phase flows with soluble surfactants

A method is presented to solve two-phase problems involving soluble surfactants. The incompressible Navier-Stokes equations are solved along with equations for the bulk and interfacial surfactant concentrations. A non-linear equation of state is used to relate the surface tension to the interfacial surfactant concentration. The method is based on the use of a diffuse interface, which allows a simple implementation using standard finite difference or finite element techniques. Here, finite difference methods on a block-structured adaptive grid are used, and the resulting equations are solved using a non-linear multigrid method. Results are presented for a drop in shear flow in both 2D and 3D, and the effect of solubility is discussed.     

A conservative SPH method for surfactant dynamics

In this paper, a Lagrangian particle method is proposed for the simulation of multiphase flows with surfactant. The model is based on the multiphase smoothed particle hydrodynamics (SPH) framework of Hu and Adams (2006) [1]. Surface-active agents (surfactants) are incorporated into our method by a scalar quantity describing the local concentration of molecules in the bulk phase and on the interface. The surfactant dynamics are written in conservative form, thus global mass of surfactant is conserved exactly. The transport model of the surfactant accounts for advection and diffusion. Within our method, we can simulate insoluble surfactant on an arbitrary interface geometry as well as interfacial transport such as adsorption or desorption. The flow-field dynamics and the surfactant dynamics are coupled through a constitutive equation, which relates the local surfactant concentration to the local surface-tension coefficient. Hence, the surface-tension model includes capillary and Marangoni-forces. The present numerical method is validated by comparison with analytic solutions for diffusion and for surfactant dynamics. More complex simulations of an oscillating bubble, the bubble deformation in a shear flow, and of a Marangoni-force driven bubble show the capabilities of our method to simulate interfacial flows with surfactants.     

可溶性活性剂驱动的液滴铺展稳定性

对含可溶性活性剂的液滴在预置液膜上的非均匀铺展过程,基于润滑理论建立基态和扰动态下的液滴厚度、表面活性剂浓度和内部浓度的演化模型,应用非模态理论分析演化过程的稳定性,探讨活性剂溶解特性及典型参数对液滴演化特征的影响.研究表明:扰动波的引入有利于增强液滴演化的稳定性,且稳定程度与扰动波的波数呈正相关性;然而随扰动波数的持续增大,液滴演化的稳定性逐渐下降,直至失稳;相对于非溶性活性剂,可溶性活性剂减缓了液滴的铺展程度,增强了演化过程的稳定性;预置液膜厚度、Marangoni数、毛细力数及吸附系数的增大,均有利于液滴稳定演化,其中Marangoni效应和毛细力的影响较大,预置液膜厚度则主要增强液滴厚度演化的稳定性.     

A front-tracking method for computation of interfacial flows with soluble surfactants

A finite-difference/front-tracking method is developed for computations of interfacial flows with soluble surfactants. The method is designed to solve the evolution equations of the interfacial and bulk surfactant concentrations together with the incompressible Navier–Stokes equations using a non-linear equation of state that relates interfacial surface tension to surfactant concentration at the interface. The method is validated for simple test cases and the computational results are found to be in a good agreement with the analytical solutions. The method is then applied to study the cleavage of drop by surfactant—a problem proposed as a model for cytokinesis [H.P. Greenspan, On the dynamics of cell cleavage, J. Theor. Biol. 65(1) (1977) 79; H.P. Greenspan, On fluid-mechanical simulations of cell division and movement, J. Theor. Biol., 70(1) (1978) 125]. Finally the method is used to model the effects of soluble surfactants on the motion of buoyancy-driven bubbles in a circular tube and the results are found to be in a good agreement with available experimental data.     

液滴在不同润湿性表面上蒸发时的动力学特性

基于润滑理论,采用滑移边界条件建立了二维液滴厚度的演化模型和移动接触线动力学模型,利用数值计算方法模拟了均匀加热基底上固着液滴蒸发时的动力学特性,分析了液-气、固-气和液-固界面张力温度敏感性对壁面润湿性和液滴动态特性的影响.结果表明,液滴的运动过程受毛细力、重力、热毛细力和蒸发的影响,重力对液滴铺展起促进作用,而毛细力、热毛细力则起抑制作用;通过改变界面张力温度敏感性系数,可使液滴蒸发过程中的接触线呈现处于钉扎或部分钉扎模式,且接触线钉扎模式下的液滴存续时间低于部分钉扎模式;提高液-气与液-固界面张力温度敏感系数均可改善壁面润湿性能,加快液滴铺展速率;而增大固-气界面张力温度敏感系数则导致壁面润湿性能恶化、延缓液滴铺展过程;通过改变固-气界面张力温度敏感系数更有利于调控处于蒸发状态下的液滴运动.     

表面弹性和分离压耦合作用下的垂直液膜排液过程

针对含不溶性活性剂的垂直液膜排液过程,在考虑表面弹性和分离压耦合作用的基础上,采用润滑理论建立了液膜厚度、表面速度和活性剂浓度的演化方程组,通过数值计算分析了表面弹性和分离压单独作用和耦合作用下的液膜演化特征.结果表明:表面弹性与分离压均对垂直液膜排液过程有显著影响.表面弹性单独作用时,液膜初始厚度随弹性增大,黑膜仅在液膜顶部形成,长度较短且不能稳定存在;分离压单独作用时,活性剂随流体不断汇集在底端,液膜表面无法形成表面张力梯度,不发生逆流现象;当二者耦合作用时,可得到较稳定的液膜,排液前期增加表面弹性可提高液膜的厚度、降低表面速度和促使液体逆流,从而减缓排液过程;后期出现黑膜后,分离压中的静电斥力起主要作用,延缓液膜"老化".     

表面单分子膜的垂悬液滴方法研究

提出一种扩展或收缩气/液界面单分子层的分子密度，用荧光成像技术检测单分子层的扩展或收缩效果的新方法——垂悬液滴分析方法-此方法通过改变液滴体积，对液滴界面上的表面活性剂分子实施扩展和收缩，具有Langmuir-Blodgett(LB槽)的功能-对表面活性剂荧光分子，用液滴的激光感生荧光图像，可以实时测量液滴界面的相对分子密度变化；用偏振荧光分析技术，可获取荧光偶极矩在液滴界面的相对取向-对罗丹明(B)表面活性剂分子ODRB的实验结果表明：1) 表面单分子层分子密度在压缩过程中遵循σ/σ_{0关键词：}     

随活性剂浓度变化的分离压对垂直液膜排液过程的影响

针对含不溶性活性剂的垂直液膜排液过程,基于文献实验结果进一步完善了受活性剂浓度影响的分离压(disjoining pressure)模型,应用润滑理论建立了液膜厚度、活性剂浓度和液膜表面速度的演化方程组,通过数值计算分析了在不同分离压作用下含不溶性活性剂液膜的演化特征.结果表明,垂直液膜的排液过程通常经历两个阶段:首先是厚膜阶段,此时重力对排液过程起主导作用.在随后的薄膜阶段,毛细作用和分离压作用影响逐渐增大,其中分离压将控制液膜的演化历程.分离压对垂直液膜排液过程的影响与活性剂类型及活性剂浓度与静电作用力的关联强度密切相关.当分离压与活性剂浓度正相关时,随斥力关联系数α增大,液膜的排液和变薄过程得以减缓,由此增强了液膜稳定性;当分离压与活性剂浓度负相关时,随斥力关联系数α绝对值增大,液膜排液过程加速,由此加大液膜失稳的风险.     

连续凹槽基底对含非溶性活性剂薄液膜流动特性的影响

针对连续凹槽基底上含非溶性活性剂液膜的流动过程, 采用润滑理论建立了液膜厚度和活性剂浓度演化模型, 利用PDECOL程序数值模拟得到了液膜流动的动力学特征及基底结构参数的影响. 研究表明: 活性剂液膜流经连续凹槽时, 负向台阶处形成凹陷, 正向台阶处形成隆起, 且随时间逐渐增大; 与平整基底相比, 连续凹槽下的活性剂液膜铺展速度加快; 基底凹槽的高度增加或斜度减小可加速液膜破断的可能性; 增大凹槽宽度可促进液膜流动; 减小斜度会使液膜进入第一凹槽前形成隆起特征; 重力在液膜的爬坡和下坡过程中具有相反的作用, 但均使得流动稳定性变差; 分子间作用力形成的结合压可加速液膜流动, 进而引发去湿润现象, 分离压则与之相反. 关键词： 活性剂液膜 非平整基底 铺展 分子间作用力     

Tip streaming from a drop in the presence of surfactants

Drop breakup in a linear extensional flow is simulated numerically using a nonlinear model for the surface tension that accounts for maximum packing at the interface. Surface convection sweeps surfactant to the drop poles, where it accumulates and drives the surface tension to near zero. The drop assumes a transient shape with highly pointed tips. From these tips, thin liquid threads are pulled. Subsequently, small, surfactant-rich droplets are emitted from the termini of these threads. The scale of the shed drops depends on the initial surfactant coverage. Dilute initial coverage leads to tip streaming, while high initial coverage leads to the tip dropping breakup mode.     

A molecular dynamics study on interfacial heat transport of alkanethiol surfactant coated nanofluids-effect of chain length and stiffness

The thermal transport across the alkanethiol surfactant layer at the nanoparticle/base fluid interface in nanofluids was investigated by molecular dynamics simulation, with consideration of the conformation of the surfactant layer with different surfactant chain lengths and backbone stiffness. The variation of temperature drop at nanoparticle-surfactant interface reveals that the interfacial thermal conductance was mediated by the chain length, possibly due to the difference in the adsorption density of surfactant on the surface of the nanoparticles, because of the blocking effect from the bending of the long alkyl chains. The intrinsic thermal conductivity of the surfactant layer increased with decreasing chain length and increasing chain stiffness because of the phonon scattering effect from the bending and cross-linking of the alkyl chains. We quantified the modes of heat flow across the surfactant layer and found that the contribution of intramolecular bonded interaction was much higher than that of atomic translation and nonbonded interaction separately. By analysing the variation of bonded interaction contrition with chain length and stiffness, it is demonstrated that the increased thermal conductivities benefited from the enhanced thermal transfer through the covalent bonds of surfactant molecules. The results can provide insights into the design of thermally conductive surfactants.     

Effect of reaction systems and surfactant additives on the morphology evolution of hydroxyapatite nanorods obtained via a hydrothermal route

Well-dispersed hydroxyapatite (HA) nanorods with different morphologies were synthesized by a hydrothermal method in oleic acid, ethanol and water reaction system, and the surfactant assisted modifications effect was also comparatively studied. The structure and morphology of samples were characterized using X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy and transmission electron microscopy (TEM), respectively. The effect of reaction systems and surfactant additives on the morphology evolution of HA nanorods were discussed in detail. The results showed that the controlled experimental conditions in the systems, such as the content ratio of oleic acid/ethanol, pH value and the content ratio of Ca/P source had an significant effect on the morphology evolution of as-prepared HA nanorods. Further, the selected surfactant additives, such as cetyltriethylammnonium bromide (CTAB), sodium dodecyl sulfate (K12) also play an important role in the formation of the uniform morphology of HA nanorods. Some possible formation mechanisms of the HA nanorods in the present reaction systems is proposed.