Dynamic Interfacial Tension Measurements Research Articles

Ostwald ripening of nanoemulsions stopped by combined interfacial adsorptions of molecular and macromolecular nonionic stabilizers

Aliphatic hydrocarbon oil-in-water nanoemulsions were prepared using mixtures of an amphiphilic nonionic polysaccharide (dextran modified by covalent attachment of hydrocarbon tails) and tetraethylene glycol dodecylether as stabilizers. Nanoemulsions stabilized by amphiphilic polysaccharide alone underwent Ostwald ripening. On the contrary, nanoemulsions prepared with certain mixtures of hydrophobically modified dextran and tetraethylene glycol dodecylether exhibited no significant variation of their average droplet diameter over a period of time that depended strongly on the nature of the dispersed oil. When changing from n-hexane to n-octane and n-dodecane, the period of complete inhibition of Ostwald ripening varied from 3h to several months. Polymer–surfactant interactions and co-adsorption at oil/water interface were demonstrated by dynamic interfacial tension measurements and dilational surface rheology.

Demulsification and Interfacial Properties of Crosslinking Phenol-Amine Resin Block Polyether Demulsifiers

Six novel crosslinking phenol-amine resin block polyether demulsifiers were synthesized for demulsification of surfactant-polymer flooding emulsion. The demulsification performances of these demulsifiers were investigated by conventional graduated bottle test. Their interfacial behaviors at water-oil interface were explored by dynamic interfacial tension and interfacial dilational viscoelasticity measurements. The results show that the demulsification efficiency is dependant on the hydrophilic-hydrophobic balance (HLB) value of these demulsifiers. It was also correlated to the interfacial activity and the dilational elasticity at the water-oil interface. The higher the HLB value of demulsifiers, the better the demulsification efficiency is.

A New Device for Simultaneous Measurements of Dynamic Interfacial Tension and Electrical Potential Difference

Abstract A new device for simultaneous time-resolved measurements of electromotive force (EMF) and interfacial tension (IT) at liquid–liquid interfaces is described. Two examples of applications are presented: 1) measurement of the variations in IT during simulated EMF measurements in model membrane-based ion-selective electrodes, and 2) measurements of transient changes in EMF in a system displaying IT oscillations without any chemical reaction.

From Gibbs- To Langmuir-Type Adsorbed Layers: Alkylated Azacrown Ethers At Liquid–Liquid Interfaces

Adsorption properties of three alkylated diazacrown ether derivatives in a toluene–water system are described. The three derivatives share the same hydrophilic diaza-18-crown-6 moiety, substituted at both nitrogen atoms with decyl (ACE-10), hexadecyl (ACE-16), or tetracosane (ACE-24) alkyl chains. Dynamic interfacial tension measurements at the toluene–water interface revealed a nondiffusional mechanism of adsorption for all three ACEs. The interfacial tension isotherms obtained from the drop-shape analysis were fitted with a reorientation model (for ACE-10 and ACE-16) and Langmuir model (ACE-24), pointing to different structures of the respective spontaneously adsorbed layers: loose (Gibbs-like) for the short-chain derivatives and dense (Langmuir-like) for the long-chain. These results are in full agreement with previous neutron and X-ray reflectivity studies. Despite an apparent cmc, the ACE molecules probably do not form reverse micelles in highly concentrated solutions in nonpolar solvents.

Fast dynamic interfacial tension measurements and dilational rheology of interfacial layers by using the capillary pressure technique

The oscillating drop and bubble analyzer (ODBA) is an experimental set-up based on the measurement of capillary pressure under static and dynamic conditions. It allows studies of slow and fast dynamic surface and interfacial tensions, following different growing and oscillating drop or bubble protocols, as well as determination of the dilational interfacial visco-elasticity of liquid interfacial layers. For the visco-elasticity studies, drops or bubbles are subjected to harmonic oscillations of area or volume in a broad frequency range, and the resulting harmonic capillary pressure response is analyzed by Fourier analysis. Also, transient relaxations can be easily performed, which are of particular importance for isodense systems. The limits of applicability for highly dynamic conditions are given by the hydrodynamics of the fluid flow inside the capillary tip and the pressure cell which depend on fluid properties, capillary tip size, and geometry. For the growing drop protocol, additional phenomena during drop formation and detachment play a significant role. For oscillating drops or bubbles, the highest accessible frequencies also depend on the absolute drop size and applied oscillation amplitude. In this work, the capability of the technique for measurements in the frequency range between 1 and 100Hz are discussed, and elasticity values at up to 80Hz were measured for the liquid–liquid interface.

Mass Transfer Model of Triethylamine across the n-Decane/Water Interface Derived from Dynamic Interfacial Tension Experiments

This publication presents a detailed experimental and theoretical study of mass transfer of triethylamine (TEA) across the n-decane/water interface. In preliminary investigations, the partition of TEA between n-decane and water is determined. Based on the experimental finding that the dissociation of TEA takes place in the aqueous and in the organic phase, we assume that the interfacial mass transfer is mainly affected by adsorption and desorption of ionized TEA molecules at the liquid/liquid interface. Due to the amphiphilic structure of the dissociated TEA molecules, a dynamic interfacial tension measurement technique can be used to experimentally determine the interfacial mass transport. A model-based approach, which accounts for diffusive mass transport in the finite liquid bulk phases and for adsorption and desorption of ionized TEA molecules at the interface, is employed to analyze the experimental data. In the equilibrium state, the interfacial tension of dissociated TEA at the n-decane/water interface can be adequately described by the Langmuir isotherm. The comparison between the theoretical and the experimental dynamic interfacial tension data reveals that an additional activation energy barrier for adsorption and desorption at the interface has to be regarded to accurately describe the mass transport of TEA from the n-decane phase into the aqueous phase. Corresponding adsorption rate constants can be obtained by fitting the theoretical predictions to the experimental data. Interfacial tension measurements of mass transfer from the aqueous into the organic phase are characterized by interfacial instabilities caused by Marangoni convection, which result in an enhancement of the transfer rate across the interface.

글리시돌을 원료로 한 비이온 계면활성제 합성에 관한 연구

글리시돌과 라우릭산으로부터 PGLE와 PGLE3 비이온 계면활성제를 합성하였으며, 합성한 계면활성제들의 구조를 <TEX>$^1H$</TEX> 및 <TEX>$^{13}C$</TEX> NMR 분석을 통하여 확인하였다. PGLE와 PGLE3 비이온 계면활성제의 CMC는 각각 <TEX>$3.59{\times}10^{-2}$</TEX> mol/L, <TEX>$8.80{\times}10^{-1}$</TEX> mol/L이며, CMC에서의 표면장력 값은 각각 26.09 mN/m과 28.68 mN/m이었다. 동적 표면장력 측정 결과에 의하면 PGLE와 PGLE3 비이온 계면활성제 모두, 공기와 수용액의 계면이 계면활성제 단분자에 의하여 비교적 짧은 시간 내에 포화되었으며, 1 wt% PGLE와 PGLE3 계면활성제 시스템들의 접촉각은 각각 <TEX>$25.5^{\circ}$</TEX>와 <TEX>$9.5^{\circ}$</TEX> 나타내었다. 비극성 오일 n-decane과 1 wt% 계면활성제 수용액 사이의 계면장력은 시간에 따라 감소하며, PGLE와 PGLE3 시스템 모두 20분 이내에 평형에 도달하였고, 평형에서의 계면장력 값은 각각 0.42 mN/m와 0.53 mN/m를 나타내었다. PGLE 비이온 계면활성제가 PGLE3 비이온 계면활성제에 비하여 거품 안정성이 큼을 확인하였으며, 이러한 거품 안정성 측정 결과는 표면장력 측정 결과와도 일치하였다. 계면활성제, 물, 비극성 탄화수소 오일로 이루어진 3성분 시스템에 보조계면활성제를 첨가하여 <TEX>$25{\sim}60^{\circ}C$</TEX>의 온도에서 상평형 실험을 수행한 결과, lower phase 마이크로에멀젼 혹은 oil in water 마이크로에멀젼이 excess oil 상과 평형을 이루는 2상 영역만이 관찰되었다. The PGLE and PGLE3 nonionic surfactants were synthesized from the reaction between glycidol and lauryl acid and their structures were confirmed by <TEX>$^1H$</TEX> and <TEX>$^{13}C$</TEX> NMR analysis. The CMCs of PGLE and PGLE3 surfactants were found to be <TEX>$3.59{\times}10^{-2}$</TEX> mol/L and <TEX>$8.80{\times}10^{-2}$</TEX> mol/L respectively and the surface tensions at their CMC conditions were 26.09 mN/m and 28.68 mN/m respectively. Dynamic surface tension measurement has shown that the adsorption rate of surfactant molecules at the interface between air and surfactant solution was found to be relatively fast in both surfactant systems, presumably due to high mobility of surfactant molecules. The contact angles of PGLE and PGLE3 nonionic surfactants were <TEX>$25.5^{\circ}$</TEX> and <TEX>$9.5^{\circ}$</TEX> respectively. Dynamic interfacial tension measurement showed that both surfactant systems reached equilibrium in 20 minutes and the interfacial tensions at equilibrium condition in both systems were 0.42 mN/m and 0.53 mN/m respectively. The PGLE surfactant system has indicated higher foam stability than the PGLE3 surfactant system, which is consistent with surface tension measurement. The phase behavior experiments performed at <TEX>$25{\sim}60^{\circ}C$</TEX> in systems containing nonionic surfactant, water, n-hydrocarbon oil and cosurfactant showed a lower phase or oil in water microemulsion in equilibrium with excess oil phase at all conditions investigated during this study.

Copper(II) sulphate solutions treatment by solvent extraction with Na-Cyanex 272

Cyanex 272 (bis(2,4,4-trimethylpentyl)phosphinic acid) and its sodium salt were used as extractants to treat copper(II) sulphate solutions. Solvent extraction of Cu(II) is more efficient with Na-Cyanex 272 than with its acidic form, and a significant change in pH of the raffinate is observed after extraction. Two mol/dm 3 H 2SO 4 is an effective stripping solution of Cu(II) from the loaded Na-Cyanex 272, and gives 75% of the stripped metal ions in one step. Adsorption parameters show that molecules of Na-Cyanex 272 are populated more densely at the saturated water/toluene interface than Cyanex 272 molecules. Equilibrium and dynamic interfacial tension measurements confirm the interfacial mechanism of Cu(II) extraction and high surface activity of Na-Cyanex 272. Diffusion coefficients estimated for Na-Cyanex 272 are two orders greater than with its acidic form. The differences between both FT-IR and UV/VIS spectra of metal–ligand complexes point out the presence of two types of complex structures depending on the metal–ligand molar ratio (probably planar for 1:1 and tetrahedral for 1:2). Finally, photostability studies show that both extractants are chemically resistant even after contact with H 2SO 4. The results presented in this paper confirm that conditioning of Cyanex 272 greatly improves the efficiency of Cu(II) extraction in comparison to its acidic form.

Influence of Cationic Composition and pH on the Formation of Metal Stearates at Oil–Water Interfaces

We study the formation of layers of metal stearates at the interface between a decane solution of stearic acid and aqueous salt solutions of variable composition and pH by monitoring the evolution of their mechanical, optical, and chemical properties as a function of time after formation of the interface. For values of the pH below the pK(a) of stearic acid hardly any interfacial activity is observed. For pH > pK(a), stearic acid deprotonates at the interface and forms metal stearates, eventually leading to the formation of macroscopic solid layers. Dynamic interfacial tension measurements reveal that the process takes place in several stages, which we attribute to the successive formation of dilute and dense monolayers followed by three-dimensional growth. In the presence of divalent ions, the solid layers display a significant increase in the dilatational storage modulus. Experiments performed with an aqueous phase containing multiple cation species (artificial seawater) give rise to particularly pronounced growth of solid layers, which preferentially incorporate Ca(2+) as revealed by X-ray photoelectron and infrared spectroscopy. Our results highlight in particular the importance of the complex synergistic effects of simultaneously present monovalent and divalent cation species on the interfacial adsorption.

Ultra-sound assisted formation of biodegradable double emulsion capsules from hen egg white

It was found that directed self-assembly of hen egg white proteins at oil–water interfaces in Pickering emulsions followed by acoustic denaturation of the proteins at the interface produces double emulsion capsules. The formation of capsules is dependent on the pH of the aqueous phase and on the concentration. At pH 5.5, the capsules are easily formed while at pH 8.5 the formation is difficult and only occurs at high concentration and in low quantity. The difference in capsule stability and interface stabilization at different pH was rationalized by competitive stabilization forces at the oil–water interface which was investigated by dynamic interfacial tension measurements. While the absolute interfacial stabilization does not differ much at different pH, when taking into account stabilization behavior of the pure buffer solutions without protein there is a significant difference. At neutral and alkaline conditions we observed that the pure buffer already stabilizes the interface which competes with the stabilization by the protein. This competitive stabilization resulted in less stable capsules formed at higher pH. Moreover, a lower pH enhances the ultra-sound assisted denaturation of the proteins, yielding an additional stabilization of the formed structures. In addition, the capsules are biodegradable. Upon digestion with trypsin, they were completely destroyed leaving behind small protein aggregates.

Measurement of Dynamic Interfacial Tension in a Carbohydrate Melt at High Temperature Using a Drop Volume Tensiometer

Many food and encapsulation products are dispersed systems with a highly viscous concentrated carbohydrate solution or melt as the continuous phase and for which interfacial properties are important at high temperatures. A drop volume interfacial tensiometer was utilized to provide objective characterization of surfactant behavior in such systems at temperatures typical of food production processes. Interfacial tension was measured over a range of flow rates within a maltodextrin–sucrose–water melt at 105 °C using limonene as the oil phase and a sucrose ester surfactant. The results showed that the kinetics of surfactant adsorption were faster when the surfactant was dissolved in the oil phase. However, the low critical micelle concentration in the oil phase resulted in a lower than expected diffusion coefficient.

Dynamic Interfacial Tension Measurements with Microfluidic Y-Junctions

Emulsification in microdevices (microfluidic emulsification) involves micrometer-sized droplets and fast interface expansion rates. In addition, droplets are formed in less than milliseconds, and therefore traditional tensiometric techniques cannot be used to quantify the actual interfacial tension. In this paper, monodisperse droplets formed at flat microfluidic Y-junctions were used to quantify the apparent dynamic interfacial tension during (microfluidic) emulsification. Hexadecane droplets were formed in ethanol-water solutions with a range of static interfacial tensions to derive a calibration curve, which was subsequently used to access the dynamic interfacial tension of hexadecane droplets formed in surfactant solutions. For SDS and Synperonic PEF108, various continuous- and disperse-phase (hexadecane) flow rates were studied, and these conditions were linked to interfacial tension effects, which also allowed convective transport of surfactants to be investiagted. On the basis of these findings, various strategies for the formation of emulsion droplets can be followed and are discussed.

Surfactant (TTAB) Role in the Preparation of 2,7-Poly(9,9-dialkylfluorene-co-fluorenone) Nanoparticles by Miniemulsion

The role of tetradecyltrimethylammonium bromide (TTAB) and its partition between water, chloroform, and the chloroform/water interface during the miniemulsification of a photoluminescent polymer was investigated by indirect interfacial tension/elasticity measurements. Dynamic interfacial tension and elasticity measurements showed the presence of a gas-liquid phase transition at the chloroform/water interface and the formation of a rigid interface, which was supposed to promote emulsion stability. The parameters of the adsorption isotherms and the TTAB partition coefficient were obtained from surface tension isotherms. Dynamic surface tension measurements performed after TTAB water/chloroform extraction were used to compute TTAB partition between water, chloroform, and the chloroform/water interface. Model calculations allowed identifying (for the tested conditions) the minimum size of emulsion droplets before the onset of instability and the segregation of a sizable amount of TTAB in the final polymer nanoparticles, which induced a shift in the 2,7-poly(9,9-dialkylfluorene-co-fluorenone) (PF) photoluminescence emission band. The size of the emulsion droplets of the final polymer particles and the amount of segregated TTAB were in good agreement with the corresponding experimental values.

Dynamic interfacial properties of drops relevant to W/O-emulsion-forming systems: A refined measurement apparatus

Detailed construction is presented for a refined instrument designed for the accurate measurement of dynamic interfacial tension, both in the frequency domain and in the time domain, in liquid/gas and in liquid/liquid systems. The instrument is an integrated combination of a profile-analysis tensiometer with a capillary-pressure tensiometer. The measurement process is firmly established on the independent acquisition in synchronism of two sets of observable physical properties (namely, the drop-profile coordinates and the differential pressure). The favourable comparison of the determined interfacial tension from the two data sets, in an overlapping range of experiment parameters, grants reliability of the measurement values. The requirements of the scientific metrology and the foundation principles of the measurement theory are included in the instrument presentation. Novel software features, for the instrument operation and for the subsequent data processing, characterise the described apparatus, as a distinct variant of similar tensiometers, based on axisymmetric liquid menisci methods. The example results show different measurement applications to water/model-hydrocarbon systems, illustrating the principal performances. The potentiality of the described instrumentation, in addition to the determination of the steady-state linear interfacial viscoelasticity, extends towards a quantitative assessment for the transient non-linear interfacial processes, which are essential phenomena inherent in drop–drop interactions.

Dynamic Interfacial Tension at the Oil/Surfactant−Water Interface

We have used dynamic interfacial tension measurements to understand the structure of the ordered monolayer at the hexadecane/water interface induced by the presence of surfactant molecules. No abrupt changes in the interfacial tension (gamma) are observed during the expansion and contraction cycle below the interfacial ordering temperature (Ti) as observed for alkanes in contact with air. The lack of an abrupt change in gamma and the magnitude of this change during the expansion process indicate that the ordered phase may not be crystalline. The change in the interfacial tension is due to an increase in contact between water and hexadecane molecules and the disordering of the interfacial ordered layer. At low surfactant concentrations, the recovery of the interfacial tension is slower below Ti, suggesting that there is a critical surfactant concentration necessary to nucleate an ordered phase at the interface.

Effect of nonionic surfactant partitioning on the dissolution kinetics of residual perchloroethylene in a model porous medium

At concentrations above the critical micelle concentration, surfactants can significantly enhance the solubilization of residual nonaqueous phase liquids (NAPL) and, for this reason, are the focus of research on surfactant-enhanced aquifer remediation (SEAR). As a consequence of their amphiphilic nature, surfactants may also partition to various extents between the organic and aqueous phases, thereby affecting SEAR performance. We report here on the observation and analysis of the effect of surfactant partitioning on the dissolution kinetics of residual perchloroethylene (PCE) by aqueous solutions (1000 mg/L) of the non-ionic surfactant Triton X-100 in a model porous medium. For this fluid system, batch equilibration experiments showed that the surfactant partitions strongly into the NAPL (NAPL–water partition coefficient equal to 12.5). Dynamic interfacial tension (IFT) measurements were employed to study surfactant diffusion and interfacial adsorption. The dynamic IFT measurements were consistent with partitioning of the surfactant between the two liquid phases. PCE dissolution experiments, conducted in a transparent glass micromodel using an aqueous surfactant solution, were contrasted to experiments using clean water. Surfactant partitioning was observed to delay significantly the onset of micellar solubilization of PCE, an observation reproduced by a numerical model. This effect is attributed to the reduction of surfactant concentration in the immediate vicinity of the NAPL–water interface, which accompanies transport of the surfactant into the NAPL. Accordingly, it is suggested that both the rate and the extent of diffusion of the surfactant into the NAPL affect the onset of and the driving force for micellar solubilization. While many surfactants do not readily partition in NAPL, this possibility must be considered when selecting non-ionic surfactants for the enhanced solubilization of residual chlorinated solvents in porous media.

Interfacial Behavior of Naphthenic Acids and Multivalent Cations in Systems with Oil and Water. I. A Pendant Drop Study of Interactions Between n‐Dodecyl Benzoic Acid and Divalent Cations

Dynamic interfacial tension (IFT) measurements were used to investigate the interactions between a dissociated model naphthenic acid (p‐(n‐dodecyl) benzoic acid) and various divalent metallic cations (Mg2+, Ca2+, Sr2+, and Ba2+) across a toluene/hexadecane–water interface. The measurements were performed by using the pendant drop technique. The results obtained, plotted as IFT vs. time gave curves with similar shapes but different slopes and levels of the equilibrium IFT, depending upon the acid and salt concentrations and the type of cation added. Due to differences in degree of hydration of the various cations, the products of the reaction between dissociated acid monomers and the cations showed differences in solubility, which, in turn, affected the IFT. Based on the shapes of the curves, the mechanisms of the reactions involved in the process are discussed.

Effects of aqueous-phase acidity and salinity on isotonic swelling of W/O/W emulsion liquid membranes under agitation conditions

The effects of aqueous-phase acidity and salinity on the isotonic swelling behavior of water-in-oil-in-water (W/O/W) emulsion liquid membranes (ELMs) under agitation conditions was studied. A non-ionic surfactant (EMSORB 2500) was used to stabilize the membranes. The acidity of the aqueous phase was controlled by sulfuric acid and the salinity was controlled by sodium sulfate. The swelling of the ELMs was monitored on a continuous basis using an on-line conductivity probe. The swelling rates of the ELMs were found to be strongly influenced by the aqueous-phase acidity and salinity. In order to elucidate the mechanism of swelling, the surfactant diffusivity was determined from dynamic interfacial tension measurements between the aqueous phase and the membrane phase (containing EMSORB 2500). The sedimentation rates of water droplets of a primary water-in-oil (W/O) were also measured to assess the stability of the water droplets. Based on the analysis of the results, it is concluded that the entrapment of external aqueous phase inside the membrane phase is due to droplet deformation and interfacial mixing mechanism. The net swelling rate of the ELM is equal to the difference between the entrapment rate and the escape rate of internal aqueous phase back to the external aqueous phase. The entrapment rate increases with the increase in surfactant diffusivity and the escape rate increases with the decrease in the stability of internal aqueous droplets. Both surfactant diffusivity and stability of internal aqueous droplets are strongly influenced by the acidity and salinity of the aqueous phase.

Equilibrium and Dynamic Interfacial Tension Measurements at Microscopic Interfaces Using a Micropipet Technique. 2. Dynamics of Phospholipid Monolayer Formation and Equilibrium Tensions at the Water−Air Interface

Phospholipid monolayers at the water−air interface have been used extensively as models approximating one-half of the biological bilayer membrane and are of particular interest as stabilizers of microbubbles and emulsions. Interfacial tension is an important measure of adsorption and monolayer formation, but for macroscopic interfaces, equilibrium conditions have been difficult if not impossible to reach, especially over the wide range of temperatures necessary to study homologous series of lipids such as the phosphatidylcholines (C12−C18). By use of a new micropipet technique, however, clean, freshly prepared interfaces on the scale of microns can be rapidly and repeatably produced and exposed to monolayer-forming materials (such as phospholipids and surfactants), the equilibrium condition can be quickly achieved in minutes, and changes in the surface tension by introducing new solutions to the interface (and hence, adsorption dynamics) can be accurately measured and tracked. We have used this technique to study the formation of monolayers of various insoluble surfactant systems (pure and mixed phospholipids including cholesterol and charged lipids) at the water−air interface by measuring equilibrium and dynamic surface tensions. We show that liquid-phase lipid systems spread rapidly (50 mN/m min-1) from vesicle suspensions to form monolayers and reach the same equilibrium surface tension of 25 mN/m. The incorporation of cholesterol to create a liquid ordered phase that dramatically changes the molecular packing, elastic modulus, and tensile strength of bilayers has no effect on the rate or equilibrium surface tension of the monolayers. From the limiting surface tension of 25 mN/m for all liquid lipid monolayers (including cholesterol rich), it appears that the liquid state of the lipid acyl chains is the major influence in determining a monolayer surface tension that is similar to the bulk liquid hydrocarbon/gas surface tension. For single-component lipids in their gel phase, the equilibrium surface tension and rate of monolayer formation decreases as the temperature is lowered below Tm, consistent with a reduced spreading pressure for gel-phase lipids. Finally, unscreened interfaces in deionized water were not readily coated by DOPC/DOPG lipids, while in salt solution, monolayers were readily formed, showing that even in the presence of large amounts of lipid as vesicles in suspension, the water−air interface can remain clean when electrostatically stabilized.

Segregation Dynamics of Block Copolymers to Immiscible Polymer Blend Interfaces

We report the first application of dynamic interfacial tension measurements to characterize segregation rates of diblock copolymers to a polymer−polymer interface. The system investigated is a poly(styrene-b-dimethylsiloxane) [P(S-b-DMS); Mn = 13 000] symmetric diblock copolymer added to a molten blend of the corresponding immiscible homopolymers. Interfacial tensions are measured by the pendant drop technique with poly(styrene) [PS; Mn = 4000] as the pendant drop phase and a mixture of poly(dimethylsiloxane) [PDMS; Mn = 1600] with silane-terminated P(S-b-DMS) as the surrounding phase. The pendant drop technique is found to suffer from two intrinsic limitations in this application: short time data are inaccessible due to the finite time required for the drop shape to equilibrate, and long time data may not be readily analyzed as a result of the finite drop size. Intermediate time data are compared to the predictions of diffusion-limited segregation models proposed by Budkowski, Losch, and Klein (BLK) and...