Numerical simulation of droplet coalescence behavior in gas phase under the coupling of electric field and flow field

The coalescence behavior of droplets in an electric field belongs to the important research contents of electrohydrodynamics. Based on the phase field method of the Cahn–Hilliard equation, the electric field and the flow field are coupled to establish the numerical model of twin droplet coalescence in a coupled field. The effects of flow rate, electric field strength, droplet diameter, and interfacial tension on the coalescence behavior of droplets during the coalescence process were investigated. The results show that the dynamic behavior of the droplets is divided into coalescence, after coalescence rupture, and no coalescence under the coupling of electric field and flow field. The proper increase of the electric field strength will accelerate the coalescence of the droplets, and the high electric field strength causes the droplets to burst after coalescence. Excessive flow rates make droplets less prone to coalescence. Under the coupling field, the larger the droplet interface tension, the smaller the droplet diameter, the smaller the flow rate, and the shorter the droplet coalescence time. The results provide a theoretical basis for the application of electrostatic coalescence in gas–liquid separation technology.     

Dispersed Phase Size During the Blending of Relatively-Inelastic Polymer Melts

The influence of simultaneous drop breakup and drop coalescence on polymer-morphology was studied during the blending of polymer melts in a commercial counter-rotating twin-screw extruder. The polymers employed were PET and nylon 66, and these were chosen to minimize fluid elasticity and drop coalescence effects. Each material was dispersed in the other, and the dispersed phase size was determined using scanning electron microscopy. Variables examined included dispersed phase concentration, shear rate, residence time in the extruder and the addition of a compatibilizer. As has been the experience of others, it was found that coalescence was significant at all concentrations examined, and it increased with increasing dispersed phase concentration. However, coalescence could be drastically reduced with the help of a compatibilizer. It appeared that there were no elastic effects, and the measured drop size seemed to approach the Taylor limit as the dispersed phase concentration was lowered. An unexpected finding was that an increase in shear rate resulted in an increase in droplet size, especially at high concentrations.     

Electrokinetic demixing of aqueous two-phase polymer/salt systems

Electrokinetic demixing of aqueous two-phase polymer/salt systems is demonstrated, resulting in significant enhancement in demixing rates by about 1-4-fold. The effect of field polarity, field strength, volume ratio, and phase composition on phase demixing has been studied. Further the influence of these parameters on phase demixing could be explained based on the hydrodynamic flow-electroosmotic flow (HEF) model.     

Spiral counter-current chromatography of small molecules, peptides and proteins using the spiral tubing support rotor

An important advance in countercurrent chromatography (CCC) carried out in open flow-tubing coils, rotated in planetary centrifuges, is the new design to spread out the tubing in spirals. More spacing between the tubing was found to significantly increase the stationary phase retention, such that now all types of two-phase solvent systems can be used for liquid-liquid partition chromatography in the J-type planetary centrifuges. A spiral tubing support (STS) frame with circular channels was constructed by laser sintering technology into which FEP tubing was placed in 4 spiral loops per layer from the bottom to the top and a cover affixed allowing the tubing to connect to flow-tubing of the planetary centrifuge. The rotor was mounted and run in a P.C. Inc. type instrument. Examples of compounds of molecular weights ranging from <300 to approximately 15,000 were chromatographed in appropriate two-phase solvent systems to assess the capability for separation and purification. A mixture of small molecules including aspirin was completely separated in hexane-ethyl acetate-methanol-water. Synthetic peptides including a very hydrophobic peptide were each purified to a very high purity level in a sec-butanol solvent system. In the STS rotor high stationary phase retention was possible with the aqueous sec-butanol solvent system at a normal flow rate. Finally, the two-phase aqueous polyethylene glycol-potassium phosphate solvent system was applied to separate a protein from a lysate of an Escherichia coli expression system. These experiments demonstrate the versatility of spiral CCC using the STS rotor.     

A microfluidic method to study demulsification kinetics

We present the results of experiments studying droplet coalescence in a dense layer of emulsion droplets using microfluidic circuits. The microfluidic structure allows direct observation of collisions and coalescence events between oil droplets dispersed in water. The coalescence rate of a flowing hexadecane-in-water emulsion was measured as a function of the droplet velocity and droplet concentration from image sequences measured with a high-speed camera. A trajectory analysis of colliding droplet pairs allows evaluation of the film drainage profile and coalescence time t(c.) The coalescence times obtained for thousands of droplet pairs enable us to calculate coalescence time distributions for each set of experimental parameters, which are the mean droplet approach velocity (v(0)), the mean dispersed phase fraction (φ) and the mean hydraulic diameter of a droplet pair (d(p)). The expected value E(t(c)) of the coalescence time distributions scales as E(t(c)) is proportional to (v(0))(-0.105±0.043)(d(p))(0.562±0.287), but is independent of φ. We discuss the potential of the procedure for the prediction of emulsion stability in industrial applications.     

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     

Prediction of the phase structure in polymer blends: what is needed for success?

Rules for prediction of the phase structure in immiscible polymer blends from the knowledge of their composition, component properties and the flow field in a mixing or processing device are discussed. The reliability of qualitative prediction of the dependence of phase structure on system parameters is used as a criterion of plausibility of the rules. No general reliable rule for prediction of the phase structure type (continuity of phases) is available in the literature. Dependence of the droplet break‐up frequency on its size, contribution of simultaneous collisions of three or more droplets to coalescence and the effect of complex flow field on coalescence must be better described for a reliable qualitative prediction of the dependence of the droplet size on the system parameters.     

溶剂对PVDF铸膜液相转化和微孔膜皮-亚两层结构的不同影响

采用皮-亚分步凝固成膜机理分析了3种不同溶剂对聚偏氟乙烯(PVDF)铸膜液相转化和膜结构的影响,采用浊度法测定铸膜液体系的热力学性质,沉淀速度采用光透射仪测定.结果显示,3种膜的皮层分相主要由热力学性质控制,均发生延时液固分相,生成了相互融合的球粒组成的致密皮层.3体系的亚层分相行为由动力学扩散过程控制;对于二甲基亚砜(DMSO)、N,N-二甲基乙酰胺(DMAc)体系亚层发生瞬时液液分相,结晶化对动力学过程影响小,表现为光透射曲线上分相时间t2短,生成了大孔结构为主的亚层,膜厚度、孔隙率和气通量均高、结晶度低;N,N-二甲基甲酰胺(DMF)体系亚层发生延时液液分相,结晶化对动力学过程影响大,t2长,生成蜂窝状孔结构亚层,其膜厚度、孔隙率和气通量较低,但膜的结晶度高.     

A morphological study of membranes obtained from the systems polylactide-dioxane-methanol,polylactide-dioxane-water and polylactide-N-methyl pyrrolidone-water

The influence of liquid–liquid demixing, solid–liquid demixing, and vitrification on the membrane morphologies obtained from several polylactide-solvent-nonsolvent systems has been investigated. The polymers investigated were the semicrystalline poly-L-lactide (PLLA) and the amorphous poly-DL-lactide (PDLLA). The solvent-nonsolvent systems used were dioxane-water, N-methyl pyrrolidone-water and dioxane-methanol. For each of these systems it was attempted to relate the membrane morphology to the ternary phase diagram at 25°C. It was demonstrated that for the amorphous poly-DL-lactide the intersection of a glass transition and a liquid–liquid miscibility gap in the phase diagram was a prerequisite for the formation of stable membrane structures. For the semicrystalline PLLA a wide variety of morphologies could be obtained ranging from cellular to spherulitical structures. For membrane-forming combinations that show delayed demixing, trends expected on the basis of phase diagrams were in reasonable agreement with the observed membrane morphologies. Only for the rapidly precipitating system PLLA-N-methyl pyrrolidone-water were structures due to liquid–liquid demixing obtained when structures due to solid–liquid demixing were expected. Probably, rapid precipitation conditions promote solid–liquid demixing over liquid–liquid demixing, because the activation energy necessary for liquid–liquid demixing is lower than that for crystallization. © 1996 John Wiley & Sons, Inc.     

An Electrorheological Study on the Behavior of Water‐in‐Crude Oil Emulsions Under Influence of a DC Electric Field and Different Flow Conditions

The influence of an applied DC electric field on viscosity and droplet size distribution of different water‐in‐crude oil emulsions was monitored in order to investigate the induction of coalescence of the water droplets. The effects caused by the voltage imposition were studied by rheological analysis and the validity of the obtained results was discussed, comparing with the features of real electrocoalcscer systems. A low field NMR technique (CPMG NMR) and digital video microscopy (DVM) were used to elucidate the behavior of the emulsions. Experiments performed at low shear rate with increasing electric field magnitude showed an increase in viscosity until a critical value. E_CRIT was reached. Thereafter coalescence occurred and viscosity decreased irreversibly below its initial value. The electrorheological behavior of the emulsions can be attributed to the organization (flocculation) of water droplets induced by the electric field, accompanied by an increase in viscosity. The structure breaks down as the shear rate is increased, leading to a decrease in viscosity. Experiments performed at high shear showed only a small decline in the viscosity. Although it was evident that coalescence took place, it did not involve the whole sample, because the electrodes were uncoated. As a direct consequence, the mean value of the droplet size within the emulsion did not change noticeably. Nonetheless this mean value was less recurrent and the formation of droplets of very large diameter occurred.     

How to prepare and stabilize very small nanoemulsions

Practical and theoretical considerations that apply when aiming to formulate by ultrasonication very small nanoemulsions (particle diameter up to 150 nm) with very high stability are presented and discussed. The droplet size evolution during sonication can be described by a monoexponential function of the sonication time, the characteristic time scale depending essentially on the applied power. A unique master curve is obtained when plotting the mean diameter size evolution as a function of sonication energy. We then show that Ostwald ripening remains the main destabilization mechanism whereas coalescence can be easily prevented due to the nanometric size of droplets. The incorporation of "trapped species" within the droplet interior is able to counteract Ostwald ripening, and this concept can be extended to the membrane compartment. We finally clarify that nanoemulsions are not thermodynamically stable systems, even in the case where their composition lies very close to the demixing line of a thermodynamically stable microemulsion domain. However, as exemplified in the present work, nanoemulsion systems can present very long-term kinetic stability.     

Scaling behavior of delayed demixing, rheology, and microstructure of emulsions flocculated by depletion and bridging

This paper describes an experimental comparison of microstructure, rheology, and demixing of bridging- and depletion-flocculated oil-in-water emulsions. Confocal scanning laser microscopy imaging showed that bridging-flocculated emulsions were heterogeneous over larger length scales than depletion-flocculated emulsions. As a consequence, G' as determined from diffusing wave spectroscopy (DWS) corresponded well with G' as measured macroscopically for the depletion-flocculated emulsions, but this correspondence was not found for the bridging-flocculated emulsions. The heterogeneity of bridging-flocculated emulsions was confirmed by DWS-echo measurements, indicating that their structure breaks up into large fragments upon oscillatory shear deformation larger than 1%. Depletion- and bridging-flocculated emulsions showed a different scaling of the storage modulus with the volume fraction of oil and a difference in percolation threshold volume fraction. These differences will be discussed on the basis of the two types of droplet-droplet interactions studied. Gravity-induced demixing occurred in both emulsions, but the demixing processes differed. After preparation of bridging-flocculated emulsions, serum immediately starts to separate, whereas depletion-flocculated systems at polysaccharide concentrations in the overlap regime usually showed a delay time before demixing. The delay time was found to scale with the network permeability, B; the viscosity, eta, of the aqueous phase; and the density difference between oil and water, Deltarho, as tdelay approximately B(-1)etaDeltarho(-1). The results are in line with the mechanism proposed by Starrs et al. (J. Phys.: Condens. Matter 2002, 14, 2485-2505), where erosion of the droplet network leads to widening of the channels within the droplet networks, facilitating drainage of liquid.     

Studies on droplet size distributions during coalescence in immiscible polymer blends filled with silica nanoparticles

The effect of silica nanoparticles on the morphology of （10/90 wt%） PDMS/PBD blends during the shear induced coalescence of droplets of the minor phase at low shear rate was investigated systematically in situ by using an optical shear technique. Two blending procedures were used： silica nanoparticles were introduced to the blends by pre-blending silica particles first in PDMS dispersed phase （procedure 1） or in PBD matrix phase （procedure 2）. Bimodal or unimodal droplet size distributions were observed for the filled blends during coalescence, which depend not so much on the surface characteristics of silica but mainly on blending procedure. For pure （10/90 wt%） PDMS/PBD blend, the droplet size distribution exhibits bimodality during the early coalescence. When silica nanoparticles （hydrophobic and hydrophilic） were added to the blends with procedure l, bimodal droplet size distributions disappear and unimodal droplet size distributions can be maintained during coalescence; the shape of the different peaks is invariably Gaussian. Simultaneously, coalescence of the PDMS droplets was suppressed efficiently by the silica nanoparticles. It was proposed that with this blending procedure the nanoparticles should be mainly kinetically trapped at the interface or in the PDMS dispersed phase, which provides an efficient steric barrier against coalescence of the PDMS dispersed phase. However, bimodal droplet size distributions in the early stage of coalescence still occur when incorporating silica nanoparticles into the blends with procedure 2, and then coalescence of the PDMS droplets cannot be suppressed efficiently by the silica nanoparticles. It was proposed that with this blending protocol the nanoparticles should be mainly located in the PBD matrix phase, which leads to an inefficient steric barrier against coalescence of the PDMS dispersed phase; thus the morphology evolution in these filled blends is similar to that in pure blend and bimodal droplet size distributions can be observed during the early coalescence. These results imply that exploiting non-equilibrium processes by varying preparation protocol may provide an elegant route to regulate the temporal morphology of the filled blends during coalescence.     

Dual effects of fullerene doped to holographic polymer dispersed liquid crystals

Doping of conductive fullerene particles to the formulation of conventional holographic polymer dispersed liquid crystal‐induced dual effects of reducing both droplet coalescence and operating voltage. Fullerene induced an induction period which otherwise does not exist, followed by a gradual increase of diffraction efficiency to a saturation value being increased with increasing fullerene content. The increased diffraction efficiency was caused by the decreased droplet coalescence which was due to the hindered migration of LC by the fullerene particles. On the other hand, doped fullerene particles augmented the conductivity of polymer phase and hence the local electrical field imposed on LC droplet, which overcome the threshold for driving and reduced operating voltage and response times. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5590–5596, 2007     

Numerical simulation of deformation behavior of droplet in gas under the electric field and flow field coupling

The water droplets in the process of electrostatic coalescence are important when studying electrohydrodynamics. In the present study, the electric field and flow field are coupled through the phase field method based on the Cahn–Hilliard formulation. A numerical simulation model of single droplet deformation under the coupling field was established. It simulated the deformation behavior of the movement of a droplet in the continuous phase and took the impact of droplet deformation into consideration which is affected by two-phase flow velocity, electric field strength, the droplet diameter, and the interfacial tension. The results indicated that under the single action of the flow field, when the flow velocity was lower, the droplet diameter was greater as was the droplet deformation degree. When the flow velocity was increased, the droplet deformation degree of a small-diameter droplet was at its maximum size, the large-diameter droplet had a smaller deformation degree, and the middle-diameter droplet was at a minimum deformation degree. When the flow velocity was further increased, the droplet diameter was smaller, and the droplet deformation degree was greater. Under the coupled effect of the electric field and flow field, the two-phase flow velocity and the electric field strength were greater, and the degree of droplet deformation was greater. While the droplet diameter and interfacial tension were smaller, the degree of droplet deformation was greater. Droplet deformation degree increased along with the two-phase flow velocity. The research results provided a theoretical basis for gas–liquid separation with electrostatic coalescence technology.     

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.     

Ostwald ripening in immiscible polyolefin blends

The coarsening in the quiescent melt of the phase-segregated particles of a polymer blend, composed of a narrow molecular weight fraction of an unbranched high-density polyethylene (HDPE) and a highly branched (100 ethyl branches/1000 C atoms) hydrogenated polybutadiene (HPB) was studied. The system was effectively binary, due to the narrow molecular weight and composition distributions of each component. The system was composed of 90 wt % of the HDPE and 10 wt % of the HPB and it formed a two-phase system in the melt at 177°C. The blend was precipitated from xylene solution in order to obtain an initially intimately mixed system. This was the third study in a series of studies of the coarsening of phase-segregated particles in polymer blends. This study was unique in that the system studied was binary in this case while the previous systems were multicomponent. Since the present system was binary, exact thermodynamic calculations of the phase state of this system could be applied with a high level of confidence. The droplet phase particles, which were mainly composed of the HPB, were observed to coarsen on storage in the melt for times of from 5 s to 1 h. At the shortest storage time of 5 s the particles had an average radius of about 0.05 μm and coarsened to about 0.2 μm after 1 h storage in the melt state. Particle dimensions were measured by scanning electron microscopy of n-heptane-etched and gold-coated sections. It was found that the volume of the particles increased linearly with time and that the rate constant of coarsening was K_exp = 1.23 × 10^?18 cm³/s and this agreed fairly well with the rate constant calculated from Ostwald ripening theory of K_ce = 0.86 × 10^?18 cm³/s. In contrast the rate constant for direct particle diffusion and coalescence was K_c = 3.6 × 10^?20 cm³/s. Since this was two orders of magnitude smaller than the rate constant for Ostwald ripening, it was concluded that, although the linear increase of volume with time was also consistent with the particle diffusion and coalescence mechanism, this was not a significant contributor to the coarsening mechanism. The major cause for the insignificance of the particle diffusion and coalescence mechanism was the high melt viscosity of the matrix polymers. The application of the Ostwald ripening theory to this system could be made with a high level of confidence because it was binary. It was found that the phase concentration of the droplet phase apparently underwent a rapid increase during the first 1-2 min of storage in the melt, indicating that the system did not reach phase equilibrium (i.e., did not completely phase-segregate) for about 1-2 min. This further indicated that the long-time coarsening regime was not entered until after this length of time. The particle size distributions remained approximately self-similar over the period of coarsening, as predicted by Ostwald ripening theory. © 1995 John Wiley & Sons, Inc.     

Spontaneously formed trans-anethol/water/alcohol emulsions: mechanism of formation and stability

We studied the spontaneous emulsification and droplet growth mechanism in trans-anethol/water/ethanol solutions, also known as the beverage ouzo, using dynamic light scattering spectroscopy. This simple ternary mixture is a generic example of a system that forms microemulsions spontaneously when brought into the two-phase region. The volume fraction of the dispersed phase was found to profoundly affect the growth rates of the droplets, which is a new finding that has not been predicted by the Lifshitz-Slyozov-Wagner theory. Time-dependent measurements show that the droplet growth is governed by Ostwald ripening (OR), and no coalescence was observed. Furthermore, the OR rates increase with increasing oil concentration at low alcohol content. We attribute this behavior to enhanced droplet-droplet interactions. At high ethanol concentrations, we found that the measured rates decreased as the oil concentration increased. The OR growth mechanism completely correlates with changes in droplet size. The kinetics of droplet growth shows that the ripening has a saturation limit at a droplet radius of about 1.5 mum. Thus, formed emulsions remain stable for months.     

Selective membrane test for crude oil based model emulsions by use of video-enhanced microscopy

 A method for testing water/oil emulsion droplet membranes selectively has been demonstrated. The method uses electric fields to induce attraction, membrane thinning and coalescence between aqueous droplets deposited in an oil continuum. The coalescence process is monitored visually by the use of videomicroscopy. A set of model oils containing indigenous surfactants (asphaltenes) from a crude oil has been studied, and the effects of asphaltene concentration, oil phase aromaticity, aging of oils and interfacial exposure time have been investigated. The strength of the field at the point of coalescence is defined as the critical parameter describing membrane strength. In the current experiments a.c. fields were used and droplet sizes were of the order of 500–600 μm. Received: 8 October 1998 Accepted in revised form: 11 January 1999     

非相容聚合物体系熔融共混过程中分散相粒径变化的预测

以PS PP共混体系为研究对象 ,研究了非相容聚合物体系混炼过程中分散相含量、剪切速率及聚合物弹性等对分散相粒径变化的影响 ,对平衡态分散相粒径的变化进行了预测 ,并对其计算公式进行了新的改进 .研究表明 ,分散相浓度较低时 ,分散相粒径与分散相体积分数呈线性增长关系 ;在较高浓度时 ,分散相粒子的聚结作用明显 ,公式应加以修正 .实验中还观察到 ,对于PS(连续相 ) PP(分散相 )共混体系 ,随着剪切速率的增大 ,分散相粒径先不断减小 ,达到一极小值后 ,却又有所增大