界面流变性质对小液滴聚并过程的影响

对表面活性剂溶液中两个小液滴的聚并现象进行理论分析，并考虑相界面上质量传递对该过程的影响，得到聚并时间与界而张力和界面张力梯度、界面粘度、表面活性剂界面扩散系数、连续相和分散相的主体性质、范德华力及液滴半径的关系．     

黏弹性对介观液珠聚并的影响-液珠-细丝-液珠聚并现象及其分析

在四辊流变仪中,黏弹性高分子介观液滴经反复拉伸和松弛形成了有细丝相连的两个黏弹液珠,研究了黏弹液珠的聚并过程,依形状叫做BSB(bead-string-bead,液珠-细丝-液珠)聚并.BSB现象与常见的通过滴间液膜破裂实现的液滴聚并过程大不相同.根据界面上的Laplace力、液珠移动时的黏性阻力和细丝中黏弹应力之间的平衡,推导出一个力学模型来描述BSB现象,理论分析与实验结果相符较好.细丝直径的变化和稳定性由过程参数和物料参数共同决定,尤其是液滴的黏弹性有较大影响.这一效应对多相高分子与复杂流体加工过程的基础理解富有启发.     

模拟乳状液中微小液滴间的相互作用力

对商品化的DCAT21表面/界面张力仪进行改造, 用于直接测量液滴间相互作用力, 同时用数码摄像头Digital 3.0观察记录两液滴接近, 挤压, 排液, 聚并等过程. 研究发现, 溶液中微小液滴间的相互作用力随距离的变化曲线能够提供分散液滴的行为特征信息: 曲线上不同阶段的斜率反映力的大小; 从液滴接触后到聚并前的挤压距离反映液滴的稳定性. 表面活性剂种类不同, 对两液滴聚并所起的稳定作用不同, 非离子表面活性剂具有较好的稳定作用. 溶液中聚合物分子在薄液膜中形成具有一定强度的层状结构, 阻碍液滴聚并, 受力曲线呈阶梯状.     

界面现象与液滴聚并

报导了一个受溶质Marangoni效应影响的液滴聚结的迟缓现象．采用示踪液滴法，对滴加在正戊醇水溶液表面、受自发界而流控制的硝基乙烷液滴的运动和存在状态进行了研究．结果显示，在一定体系中溶质Marangoni效应可导致液滴的悬浮态，并对液滴的聚并产生显著影响．由于界面状态的不同，示踪液滴也表现出完全不同的动力学特征．基于以上认识对液滴运动速率的计算得到了合理的结果．     

固着纳米流体液滴颗粒自组装三维模拟

通过理论模拟固着纳米流体液滴在不同的蒸发模式与蒸发条件下会形成不同的沉积结构,有助于揭示纳米流体蒸发自组装机理并推动调控自组装过程方法的发展。建立了适用于小接触角纳米流体液滴蒸发的三维动力学蒙特卡罗(KMC)模型,引入随液滴厚度与相对半径变化的弧形化学势函数,并在其中加入了化学势衰减率,模拟了小接触角纳米流体液滴的蒸发过程,预测了枝状连续结构的形成。同时分别讨论了不同参数(初始化学势、纳米颗粒浓度、纳米颗粒扩散率)对液滴的蒸发过程与沉积结构的影响。模拟结果与两种实验结果进行了定性对比,结果吻合良好。     

稳态双曲流场中液/液混合的粘性液滴哑铃分散模型

通过对稳态双曲流场中液／液混合体系分散相液滴所受分散作用力的分析，建立了粘性液滴的哑铃分散模型．趋于将两粘性液滴分开的分散作用力与粘度比、流场类型和强度、液滴半径、哑铃取向和尺寸有关．该模型解释了流场类型与分散作用的关系．流场类型对液滴的分散具有很大影响，在纯应变拉伸流场中分散作用力是简单剪切流场中的两倍，因而对于液滴的分散，拉伸流场较简单剪切流场更有效，这与以前的实验结论符合．当体系粘度比趋于无穷大时本模型转化为刚性哑铃分散模型     

PBD/PDMS共混物分散相的聚并捕获行为

借助显微-剪切装置在线研究了低速剪切场下SiO2纳米粒子含量、分散相聚丁二烯(PBD)浓度和剪切速率对PBD/聚二甲基硅氧烷(PDMS)不相容体系中聚并捕获行为的影响.结果表明,聚并捕获所形成的液滴尺寸与形状规整度由粒子含量、分散相浓度和剪切速率等因素共同决定.在较低的SiO2纳米粒子含量或较高的分散相浓度下,PBD液滴在低剪切场下发生聚并捕获,形成尺寸较大、形状不规则的液滴.增加SiO2纳米粒子含量或减小分散相浓度,能够减小分散相的尺寸并提高分散相的规整度.增加剪切速率能有效地减小分散相的尺寸并提高分散相的规整度.     

液滴数字聚合酶链式反应芯片及其在致病菌检测中的应用

设计与制作了一种基于聚二甲基硅氧烷-玻璃(PDMS-Glass)的多功能集成式液滴数字聚合酶链式反应(ddPCR)芯片,该芯片由产生液滴的PDMS模块和收集液滴的玻璃腔体模块组成。PDMS模块采用双通道的T形结构设计,液滴产生速度快且通量高,在30 min内可生成2×10~6个直径约为20μm的微液滴。玻璃腔体模块中存储的液滴在整个实验过程中无需转移,可直接在原位PCR仪上进行扩增,每个液滴均是一个微反应器,经过多次热循环后,液滴仍能保持良好的稳定性。选用副溶血性弧菌(VP)作为食源性致病菌的研究模型,考察了ddPCR芯片对其基因组DNA的绝对定量能力,结果表明,该ddPCR芯片对VP基因组DNA绝对定量的线性范围宽,可跨越5个数量级(10~1~10~6 copies/μL),定量结果与DNA理论参考浓度间有很好的相关性。     

丙烯酸钠反相乳液聚合

以阴离子乳化剂制备丙烯酸钠反相乳液，用γ射线引发其聚合，用扫描电镜观测了聚合前后粒径的变化；并在聚合过程中改变剂量率观测了聚合速率的变化．这两个实验结果都显示聚合以液滴成核为主，即聚合机理类似于悬浮聚合．与以司盘８０为乳化剂的丙烯酸钠反相乳液聚合相比，体系与司盘体系的聚合动力学基本相似；但是体系中的分散液滴比司盘体系要大，结果使得在动力学上体系更类似于悬浮聚合的动力学．从聚合机理看，反相乳液聚合实际上就是粒子分散得比较小的反相悬浮聚合．     

键合方法对聚二甲基硅氧烷液滴型微流控芯片的影响

液滴型微流控芯片表面性质是影响其性能的重要因素. 研究了不同键合方法对基于聚二甲基硅氧烷(PDMS)的液滴型微流控芯片微管道表面性质的影响, 并分别观察和评价了不同键合方法所制作液滴型微流控芯片应用于制备油包水和水包油两种液滴分散体系的效果. 结果显示热扩散键合方法适用于制作油包水型PDMS液滴型微流控芯片, 而等离子键合方法制作的PDMS芯片适于形成水包油型的液滴分散体系.     

The Dynamics of Marangoni-Driven Local Film Drainage between Two Drops

A study of Marangoni-driven local continuous film drainage between two drops induced by an initially nonuniform interfacial distribution of insoluble surfactant is reported. Using the lubrication approximation, a coupled system of fourth-order nonlinear partial differential equations was derived to describe the spatio-temporal evolution of the continuous film thickness and surfactant interfacial concentration. Numerical solutions of these governing equations were obtained using the Numerical Method of Lines with appropriate initial and boundary conditions. A full parametric study was undertaken to explore the effect of the viscosity ratio, background surfactant concentration, the surface Péclet number, and van der Waals interaction forces on the dynamics of the draining film for the case where surfactant is present in trace amounts. Marangoni stresses were found to cause large deformations in the liquid film: Thickening of the film at the surfactant leading edge was accompanied by rapid and severe thinning far upstream. Under certain conditions, this severe thinning leads directly to film rupture due to the influence of van der Waals forces. Time scales for rupture, promoted by Marangoni-driven local film drainage were compared with those associated with the dimpling effect, which accompanies the approach of two drops, and implications of the results of this study on drop coalescence are discussed. Copyright 2001 Academic Press.     

Effects of Applied Electric Fields on Drop—Interface and Drop—Drop Coalescence*

In this work, coalescence of a single organic or aqueous drop with its homophase at a horizontal liquid interface was investigated under applied electric fields. The coalescence time was found to decrease for aqueous drops as the applied voltage was increased, regardless of the polarity of the voltage. For organic drops, the coalescence time increased with increasing applied voltage of positive polarity and decreased with increasing applied voltage of negative polarity. Under an electric field, the coalescence time of aqueous drops decreases due to polarization of both the drop and the flat interface. The dependency of organic drop-interface coalescence on the polarity of the electric field may be a result of the negatively charged organic surface in the aqueous phase. Due to the formation of a double layer, organic drops are subjected to an electrostatic force under an electric field, which, depending on the field polarity, can be attractive or repulsive. Pair-drop coalescence of aqueous drops in the organic phase was also studied. Aqueous drop-drop coalescence is facilitated by polarization and drop deformation under applied electric fields. Without applied electric fields, drop deformation increases the drainage time of the liquid film between two approaching drops. Therefore, a decrease in the interfacial tension, which causes drop deformation, accelerates drop-drop coalescence under an electric field and inhibits drop coalescence in the absence of an electric field.     

Film Drainage between Colliding Drops at Constant Approach Velocity: Experiments and Modeling

Experiments and modeling of the drainage of the thin liquid film between two deformable spherical drops approaching each other at constant velocity in another liquid are being presented. Two numerical models based on the lubrication theory have been developed considering the cases of immobile or mobile drop interfaces. The absolute film thickness and the thinning rate have been measured using laser interferometry for a wide range of capillary numbers. In all studied cases, the model with immobile interfaces was found to give the best predictions of the experimental time evolution of the film thickness and radial expansion. These results made it possible to derive a typical time scale of the drainage process. Copyright 2000 Academic Press.     

Film drainage between two captive drops: PEO-water in silicon oil

An experimental study of the deformation and drainage of a Newtonian liquid film trapped between two drops is performed for the cases of a constant and slightly rising interaction force. Series of polyethylene oxide (PEO) water solutions are used for the dispersed and polydimethylsiloxane (PDMS) for the continuous phase. The film evolution is observed by an interferometric technique. Experimental data for the film thinning rate and for the film profile allow quantitative comparison with the available drainage models.     

Drop impact on surfactant films and solutions

Drops impacting on horizontal aqueous surfactant films have been analyzed using a high-speed camera. Drops of either water or aqueous surfactant solutions had a diameter of 2.4?±?0.4 mm and impacted with a velocity of 0.1 to 1.3 m/s. As surfactants, anionic sodium dodecyl sulfate and cationic cetyltrimethyl ammonium bromide were used. Pure water drops impacting on freestanding surfactant films showed coalescence, bouncing, partial bouncing, passing, and partial passing. For bouncing, the concentration of surfactant in the surfactant film must exceed the critical micelle concentration. When surfactant was added to the drop, coalescence and partial passing were suppressed. We attribute the different behavior to different hydrodynamic boundary conditions at the surface of pure water and surfactant solution, leading to different repulsive hydrodynamic forces arising when the air has to flow out of the closing gap between the two liquid surfaces. The boundary condition changes as a function of surfactant concentration from a slip to no-slip, leading to stronger hydrodynamic repulsion. In addition, estimates of the characteristic velocities show that diffusion of air into the water is slow and can only account for the very last thinning of the air gap before coalescence.     

The effect of interfacial viscosity on the droplet dynamics under flow field

The effects of interfacial viscosity on the droplet dynamics in simple shear flow and planar hyperbolic flow are investigated by numerical simulation with diffuse interface model. The change of interfacial viscosity results in an apparent slip of interfacial velocity. Interfacial viscosity has been found to have different influence on droplet deformation and coalescence. Smaller interfacial viscosity can stabilize droplet shape in flow field, while larger interfacial viscosity will increase droplet deformation, or even make droplet breakup faster. Different behavior is found in droplet coalescence, where smaller interfacial viscosity speeds up film drainage and droplet coalescence, but larger interfacial viscosity postpones the film drainage process. This is due to the change of film shape from flat‐like for smaller interfacial viscosity to dimple‐like for larger interfacial viscosity. The film drainage time still scales as Ca⁰ at smaller capillary number (Ca), and Ca^1.5 at higher capillary number when the interfacial viscosity changes. The interfacial viscosity only affects the transition between these limiting scaling relationships. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1505–1514, 2008     

Evaporation rates of water from concentrated oil-in-water emulsions

We have investigated the rate of water evaporation from concentrated oil-in-water (o/w) emulsions containing an involatile oil. Evaporation of the water continuous phase causes compression of the emulsion with progressive distortion of the oil drops and thinning of the water films separating them. Theoretically, the vapor pressure of water is sensitive to the interdroplet interactions, which are a function of the film thickness. Three main possible situations are considered. First, under conditions when the evaporation rate is controlled by mass transfer across the stagnant vapor phase, model calculations show that evaporation can, in principle, be slowed by repulsive interdroplet interactions. However, significant retardation requires very strong repulsive forces acting over large separations for typical emulsion drop sizes. Second, water evaporation may be limited by diffusion in the network of water films within the emulsion. In this situation, water loss by evaporation from the emulsion surface leads to a gradient in the water concentration (and in the water film thickness). Third, compression of the drops may lead to coalescence of the emulsion drops and the formation of a macroscopic oil film at the emulsion surface, which serves to prevent further water evaporation. Water mass-loss curves have been measured for silicone o/w emulsions stabilized by the anionic surfactant SDS as a function of the water content, the thickness of the stagnant vapor-phase layer, and the concentration of electrolyte in the aqueous phase, and the results are discussed in terms of the three possible scenarios just described. In systems with added salt, water evaporation virtually ceases before all the water present is lost, probably as a result of oil-drop coalescence resulting in the formation of a water-impermeable oil film at the emulsion surface.