特殊浸润性表面的液滴凝结

以铝片为基底, 经电化学腐蚀和沸水处理制备了多级微纳米结构; 通过气相沉积和涂油分别制备了超疏水表面、 疏水超润滑(slippery)表面和亲水slippery表面; 探究了表面不同的特殊浸润性(超亲水、 超疏水、 疏水slippery和亲水slippery)对液滴凝结的影响. 结果表明, 超亲水表面的液滴凝结属于膜状冷凝, 超疏水表面和slippery表面的液滴凝结均属于滴状冷凝. 超疏水表面液滴合并时, 合并的液滴会不定向弹离表面. 疏水slippery表面和亲水slippery表面由于表面浸润性的不同导致液滴成核密度和液滴合并的差异, 亲水slippery表面凝结液滴的最大体积远大于疏水slippery表面凝结液滴的最大体积. 4种表面的雾气收集效率由大到小依次为亲水slippery表面>疏水slippery表面>超亲水表面>超疏水表面.     

Binary Coalescence of Water Drops in Organic Media in Presence of Ionic Surfactants and Salts

Binary coalescence of water drops in o‐xylene and toluene, and ethylene glycol drops in toluene were studied in this work. The effects of cationic and anionic surfactants on coalescence time were studied. Cetyl trimethyl ammonium bromide (CTAB) and cetyl pyridinium bromide (CPyBr) were used as cationic surfactants. Sodium dodecyl benzene sulfonate (SDBS) was used as the anionic surfactant. The effects of salts (NaCl and CaCl₂) containing monovalent and divalent ions on coalescence were investigated. The coalescence time was found to follow distributions in each of these experiments. The minimum and maximum values of the distributions were largely different. The stochastic model developed earlier by us was used to fit the distributions. The effects of the physical properties of the system (such as density, size of the drops, interfacial tension, and surface excess of adsorbed surfactant) on the model parameters were discussed.     

Silica particle-stabilized emulsions of silicone oil and water: aspects of emulsification

A study of the emulsification of silicone oil and water in the presence of partially hydrophobic, monodisperse silica nanoparticles is described. Emulsification involves the fragmentation of bulk liquids and the resulting large drops and the coalescence of some of those drops. The influence of particle concentration, oil/water ratio, and emulsification time on the relative extents of fragmentation and coalescence during the formation of emulsions, prepared using either batch or continuous methods, has been investigated. For batch emulsions, the average drop diameter decreases with increasing particle concentration as the extent of limited coalescence is reduced. Increasing the oil volume fraction in the emulsion at fixed aqueous particle concentration results in an increase in the average drop diameter together with a dramatic lowering of the uniformity of the drop size distribution as coalescence becomes increasingly significant until catastrophic phase inversion occurs. For low oil volume fractions (phi(o)), fragmentation dominates during emulsification since the mean drop size decreases with emulsification time. For higher phi(o) close to conditions of phase inversion, coalescence becomes more prevalent and the drop size increases with time with stable multiple emulsions forming as a result.     

Discussion of the Drop Rest Phenomenon at Millimeter Scale and Coalescence of Droplets at Micrometer Scale

Adsorption of surfactants at water-oil interfaces is of great importance in the coalescence of drops and stability of emulsions. In this work, we have studied the adsorption of nonionic surfactants Span 80 at water-oil interfaces and its influence on the drop rest phenomenon and W/O emulsion stability in a pulsed DC electrical field. The variation of interfacial tension with the concentration of surfactant was studied and the data were fitted using a surface equation of state derived from the Langmuir adsorption isotherm. A stochastic model for coalescence was used to fit the coalescence time distributions. The significance of the model parameters was discussed. The stability of the emulsion was evaluated by conductivity methods. The researches in this article indicated that both of the rest time distribution of the drops at the interface and stability of the emulsion in the electrical field was significantly affected by surfactant concentration.     

不完全相反转乳化过程分散相水滴形态发展研究

相反转乳化技术是制备高分子树脂水基分散体系的新方法[1～4].相反转指多组分体系(如油/水/乳化剂)中的连续相在一定条件下相互转化的过程,如在油/水/乳化剂体系中,其连续相由水相向油相(或从油相变为水相)的转变.在连续相转变区,体系的界面张力最低,因而分散相的尺寸最小.同理,可利用相反转技术直接将高分子树脂乳化为尺寸很小的水基微粒,即制备高分子树脂的水基分散体系.由于高分子树脂的粘弹性及相反转过程的复杂性,对高分子树脂的相反转乳化过程的机理研究较少.杨振忠[5]等通过调节高分子非离子型乳化剂浓度,可以有效地控制相反转完善程…     

超疏水表面上冷凝液滴发生弹跳的机制与条件分析

使用液滴合并前后的体积和表面自由能守恒作为两个限制条件,确定了合并液滴的初始形状,即为偏离平衡态的亚稳态液滴,具有缩小其底半径而向平衡态液滴转变的推动力.进而分析了液滴变形过程中的推动力和三相线(TPCL)上的滞后阻力,建立了液滴变形的动态方程并进行了差分求解.如果液滴能够变形至底半径为0mm的状态,则根据该状态下液滴重心上移的速度确定液滴的弹跳高度.不同表面上冷凝液滴合并后的变形行为的计算结果表明,光滑表面上的液滴合并后,液滴只能发生有限的变形,一般都在达到平衡态之前就停止了变形,因此冷凝液滴不会发生弹跳;粗糙表面上的Wenzel态液滴的三相线上的滞后阻力更大,因而液滴更难以变形和弹跳;具有微纳二级结构表面上只润湿微米结构,但不润湿纳米结构的部分Wenzel态液滴能够变形至Cassie态,但没有明显的弹跳;只有在纳米或微纳二级结构表面上的较小Cassie态液滴合并后,液滴易于变形至底半径为0mm的状态并发生弹跳.因此,Cassie态合并液滴处于亚稳态,并且其三相线上的移动阻力很小,是导致冷凝液滴弹跳的关键因素.     

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.     

Contact line dynamics in drop coalescence and spreading

The dynamics of coalescence of two water sessile drops is investigated and compared with the spreading dynamics of a single drop in partially wetting regime. The composite drop formed due to coalescence relaxes exponentially toward equilibrium with a typical relaxation time that decreases with contact angle. The relaxation time can reach a few tenths of seconds and depends also on the drop size, initial conditions, and surface properties (contact angle, roughness). The relaxation dynamics is larger by 5 to 6 orders of magnitude than the bulk hydrodynamics predicts, due to the high dissipation in the contact line vicinity. The coalescence is initiated at a contact of the drops growing in a condensation chamber or by depositing a small drop at the top of neighboring drops with a syringe, a method also used for the studies of the spreading. The dynamics is systematically faster by an order of magnitude when comparing the syringe deposition with condensation. We explain this faster dynamics by the influence of the unavoidable drop oscillations observed with fast camera filming. Right after the syringe deposition, the drop is vigorously excited by deformation modes, favoring the contact line motion. This excitation is also observed in spreading experiments while it is absent during the condensation-induced coalescence.     

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.     

两个液滴之间的聚并

对于系统中不含杂质时两个液滴在不互溶液体中的聚并过程进行理论分析，得到聚并所需时间与两相物理性质一范德华力的关系，该结果也适用于气泡在液体中的聚并，只要知道系统的物性数据和液滴半径，就可以计算聚并时间，理论预测与实验结果符合较好。     

An NMR study of the freezing of emulsion-containing drops

Various nuclear magnetic resonance (NMR) techniques were used to monitor the freezing behaviour of suspended 2-mm-diameter drops. The drops were composed of hydrocarbon oils emulsified in either water or water/sucrose mixtures. As such they were good model systems for the study of spray freezing, sharing structural similarities with potential products such as ice cream. In particular, simple 1H NMR spectroscopy was used to monitor and individually quantify the freezing or solidification behaviour of the various constituent species of the drops. In addition, the effect of freezing on the emulsion droplet size distribution (and hence emulsion stability) was also measured based on NMR self-diffusion measurements. The effect of freeze/thaw cycling was also similarly studied. The nucleation temperature of the emulsion droplets was found to depend on the emulsion droplet size distribution: the smaller the droplets, the lower the nucleation temperature. Emulsion droplet sizing indicated that oil-in-sucrose-solution emulsions were more stable, showing minimal coalescence, whereas oil-in-water emulsions showed significant coalescence during freezing and freeze/thaw cycling.     

Shear-induced coalescence of emulsified oil drops

Crude oil droplets, when suspended in water, possess negative surface charges which give rise to double-layer repulsive forces between the drops. According to conventional DLVO theory, the magnitude of this repulsion (based on the measured zeta potential) is more than sufficient to prevent coalescence of the droplets. Indeed, when two such droplets were brought together on direct (i.e., "head-on") approach, coalescence was rarely observed. Upon oblique approach, however, the same droplets were seen to coalesce readily. An oblique encounter must necessarily give rise to lateral relative motion-or shearing-between the droplet surfaces. It is speculated that, if the charge distributions at the droplet surfaces were heterogeneous, lateral shearing would facilitate many encounters between surface patches of different zeta potentials across the intervening water film. If the repulsion across any local region were sufficiently weak to allow formation of an oil bridge across the water film, coalescence of the drops would follow inevitably. With the hypothesis of surface heterogeneity, it is not necessary to invoke any additional colloidal interactions (such as "hydrophobic forces") to account for the observed droplet-droplet coalescence. This finding may have important implications for the underlying mechanisms of emulsion stability in general and the commercial extraction of bitumen from oil sands in particular.     

Coalescence of particle-laden drops with a planar oil-water interface

The coalescence mechanism of a particle-laden drop resting at an oil-water interface has been studied. Two mechanisms for drop coalescence are observed; (i) complete coalescence, in which the drop experiences total coalescence in one event, and (ii) partial coalescence, where a drop is observed to separate during coalescence, producing a smaller secondary drop that rebounds and comes to rest at the planar oil-water interface. For particle-laden drops of approximately 4mm in diameter, we show the critical condition for partial to complete coalescence to be dependent on the particle concentration, and the interparticle interaction energy. Colloidal silica spheres dispersed in 10(-4) M KNO(3) electrolyte solution are highly charged and remain dispersed in the drop. By increasing the solids concentration, we measure the transition from partial to complete coalescence at 20 wt.%. However, this critical condition can be reduced by increasing the interparticle interaction energy. In 1 M KNO(3) electrolyte solution, the particle surface charge is sufficiently screened such that particle clusters readily form in the water drop. With particle clustering, transition from partial to complete coalescence is measured at 8 wt.% solids.     

Coalescence during emulsification. 3. Effect of gelatin on rupture and coalescence

An oil-soluble fluorescent probe, undecyl pyrene (UDP), is used to measure the amount of coalescence that occurs during the emulsification of tri-2-ethylhexyl phosphate using a high-pressure homogenizer. From these measurements, the roles of anionic surfactant (SDS) and gelatin in stabilizing drops against coalescence and promoting drop rupture during emulsification are deduced. It is found that gelatin aids in reducing coalescence, whereas SDS aids in rupture of drops. The effect of variables such as gelatin MW, surfactant type, and pH on coalescence and final drop size is investigated.     

Population balance models and Monte Carlo simulation for nanoparticle formation in water-in-oil microemulsions: implications for CdS synthesis

We address controlled CdS nanoparticle formation by tuning experimental synthesis conditions. To this end, a bivariate population balance equation (PBE) model has been developed based on time scale analysis, to explain the mechanism of nanoparticle formation in self-assembled templates. It addresses the process of mixing two water-in-oil (w/o) microemulsions, each containing a pre-dissolved reactant in the microemulsion drops. Brownian collision and coalescence of two water drops of nanometer size results in mixing and exchange of reactant molecules, leading to chemical reaction. The water insoluble reaction product nucleates to form a nanoparticle in an individual drop, which subsequently grows internally by consuming the excess product and by coalescence-exchange with other drops. Finite rates of nucleation and coalescence-exchange are accounted for in the PBE, while the rates of reaction and internal growth of nanoparticles are found to be instantaneous. Experimentally proven binomial redistribution of reactant and product molecules upon drop coalescence is implemented in the present work. This results in a very good prediction of experimental data of the mean aggregate number (MAN) and hence size of CdS nanoparticles. Both our model and Monte Carlo (MC) simulation quantitatively capture the reported variation of MAN with molar excess of Cd2+ concentration and microemulsion drop size. Our results together with previous experimental data establish that usage of stoichiometrically five times or more of excess Cd2+ concentration can cause surface adsorption and desirable enhanced emission intensity of CdS nanoparticles, without altering particle size. We also propose a simplified and computationally efficient univariate PBE model. The univariate model gives very fast (in minutes) and accurate estimates (for low reactant concentrations) of the number and mean size of CdS nanoparticles. Time-scale analysis offers a good a priori choice of the appropriate model based on range of reactant concentrations.     

Spreading kinetics of water drops on self-assembled monolayers of thiols: significance of inertial effects

Spreading of 5-15 microL water drops on self-assembled monolayers of 1-hexadecanethiol and 11-mercapto-1-undecanol, both homogeneous and mixed compositions, formed on gold-coated silicon wafers or glass slides was recorded with a high-speed video camera. The time (t) evolution of the drop base diameter (D) during spreading was analyzed by a power law-correlation: D approximately t(n). The n value was found to increase from n = 0.3-0.5 for water drops on hydrophobic surfaces characterized by the advancing water contact angle of thetaA = 94-104 degrees to n = 0.5-0.8 on less hydrophobic surfaces (thetaA = 45-66 degrees ). These experimental values were found to be of similar magnitude as the literature values reported for small drops and bubbles, which spread over a variety of different substrates including water and water-ethanol drops on self-assembled monolayers of alkylsilanes, air bubbles in water on glass, molten metals on solid metals and ceramics, hydrocarbon drops on water, and others. Inertial effects, which are often not accounted for in the analysis of spreading results, appear to have an impact on the spreading kinetics of small drops in at least the first few milliseconds of the spreading phenomenon.     

A model for carbon nanotube-DNA hybrid using one-dimensional density of states

The Oil Transfer Technique (OTT) was developed by Taisne et al. [1] to measure coalescence during emulsification and has been applied since in several studies. One of the main drawbacks of this technique is that it only gives a qualitative measure of coalescence. This paper proposes a new evaluation method of OTT experimental results for estimating qualitative coalescence rates, e.g. for investigating the scaling of coalescence with emulsification parameters (such as homogenizing pressure, and emulsifier concentration). The method is based on comparison with simulated OTT experiments using bivariate Population Balance Equation models. Simulations have been performed under a wide variety of conditions in order to investigate the influence of assumptions on coalescence and fragmentation kernels. These investigations show that the scaling of coalescence rates could be determined accurately when the scaling of efficient residence time of drops in the active region of homogenization is known. The proposed evaluation method is also exemplified by analyzing OTT data from two previously published studies.     

Nonionic block copolymer antifoams

Aqueous dispersions of alkoxylated alcohol block copolymer (BCP) drops are investigated as antifoams. A model aqueous nonionic surfactant solution of Polysorbate 20 and an industrial white water suspension are used as foaming systems. Visual evidence obtained using a two-bubble technique involving a CCD camera coupled with high magnification lenses clearly revealed the role of BCP droplets in the bubble coalescence process. The enhancement of bubble coalescence decreased as the temperature increased from 25 to 60 degrees C, which is due to the corresponding decrease in the rigidity associated with the weak interfacial structure and reduced viscosity of the BCP drops. The antifoaming efficiency measured in the macroscopic recirculation foam column increased with temperature from about 13 to 26 degrees C (attaining a maximum) and decreased as temperature increased further. Oscillatory thermo-rheometric measurements showed a sudden increase in the storage modulus (G') by several orders of magnitude, indicating gel formation initiated at about 13 degrees C and having a maximum at around 26 degrees C for an aqueous solution of the BCP above a critical concentration of around 20 wt %. Results obtained using small-angle X-ray scattering, micro-differential scanning calorimetry, and proton nuclear magnetic resonance confirmed the existence of ordered gel-like structures. Furthermore, macroscopic tests using a sparged air foam column showed a significant increase in antifoaming efficiency when highly hydrophobic particles are embedded in the BCP drops dispersed in water.     

A study of oil droplet coalescence

Oil droplets dispersed in water can be readily studied when they are coated with surfactants, which lower their interfacial tension and enhance their stability. Pure oil droplets are more difficult to study because of their high interfacial tension, which facilitates coalescence and the adsorption of contaminants. In this study, we have characterised the surface charging properties of a water insoluble oil, bromododecane, which has a density close to water. The small density difference allows us to study relatively large drops of this oil and to analyse its coalescence behaviour. The results obtained with this simple, surfactant-free, system suggest that an additional attractive force, such as the long range hydrophobic interaction, might be required to explain oil droplet coalescence behaviour.