The effect of temperature on coalescence of liquid drops at liquid/liquid interfaces

Drop/interface coalescence times measured at 293–343K are reported for three oil/water systems (benzene, paraffin oil and 1,1,2,2-tetrabromoethane) with different interface ages. The anomalous coalescence behavior of water droplets is explained by considering the static electrical double-layer residing at the interface which influences the film thinning and the film rupture processes. Analyses using simplified coalescence models reveal that the incorporation of temperature dependence on the physical properties such as density difference between phases, viscosity of the continuous medium and interfacial tension, does not produce satisfactory agreement with the measured coalescence times. The effect of mutual saturation in contrast to unsaturated systems on coalescence times is illustrated. The reproducibility of the drop/interface coalescence times is examined and explanations are offered, relating the method and the conditions of the experimentation. Finally the importance of both the coalescence time and the film thickness to drop stability analysis is demonstrated.     

The role of organically modified layered silicate in the breakup and coalescence of droplets in PBT/PE blends

In this study, we investigated the effect of organically modified nanoclay (organoclay) on the morphology of immiscible polymer blends (PBT/PE) with various compositions of PBT ranging from 1 to 90 wt%. When a small amount of organoclay between 1 and 3 phr is added to the blend, the thin clay tactoids of the thickness of the order of 10 nm are located at the interface between PBT and PE phase. As its content is increased, the additional organoclay positions in a specific component depending on its affinity with the component. The addition of a small amount of organoclay results in the effective size reduction for PBT/PE blend. The organoclay located at the interface forms the interfacial phase with a non-homogeneous distribution of clay along the interface and changes the interfacial tension, which result in the coalescence suppression of the droplets. Rigid organoclay with a high aspect ratio allows the blend morphology with long-term thermal stability by suppressing the Brownian motion. This ability of the organoclay to suppress the coalescence of the droplets effectively reduces the droplet size. On the other hand, additional organoclay results in the rheological properties of particular component being increased, which means the change in the viscosity ratio. The change in the viscosity ratio, together with the coalescence suppression effect, affects the determination of the droplet size, depending on the location of the organoclay. Therefore, the organoclay suppresses the coalescence of the droplets at the interface, while simultaneously influencing the breakup of the droplets due to the change of viscosity ratio.     

Coalescence dynamics of two droplets of different viscosities in T-junction microchannel with a funnel-typed expansion chamber

The coalescence behavior of two droplets with different viscosities in the funnel-typed expansion cham-ber in T-junction microchannel was investigated experimentally and compared with droplet coalescence of the same viscosity.Four types of coalescence regimes were observed:contact non-coalescence,squeeze non-coalescence,two-droplet coalescence and pinch-off coalescence.For droplet coalescence of different viscosities,the operating range of non-coalescence becomes narrowed compared to the droplet coalescence of same viscosity,and it shrinks with increasing viscosity ratio η of two droplets,indicating that the difference in the viscosity of two droplets is conducive to coalescence,especially when 1 ＜ η＜ 6.Furthermore,the influences of viscosity ratio and droplet size on the film drainage time (Tdr) and critical capillary number (Cac) were studied systematically.It was found that the film drainage time declined with the increase of average droplet size,which abided by power-law relation with the size dif-ference and viscosity ratio of the two droplets:Tdr ～ (ld)0.25±0.04 and Tdr ～ (η)-0.1±002.For droplet coales-cence of same viscosity,the relation of critical capillary number with two-phase viscosity ratio and dimensionless droplet size is Cac =0.48λ0.26l-2.64,while for droplet coalescence of different viscosities,the scaling of critical capillary number with dimensionless average droplet size,dimensionless droplet size difference and viscosity ratio of two droplets is Cac =0.11 η-0.07ls-2.23ld0.16.     

Drop coalescence in planar extensional flow and gravity

A film drainage model, which allowed for the effect of the ratio of the viscosities of the drop and liquid film on the interfacial mobility, is found to represent reasonably well the published experimental data on the coalescence times of drops in two different flow fields at a constant force. The data of Yang et al. [2001. The coalescence of two equal-sized drops in a two-dimensional linear flow. Physics of Fluids 13, 1087-1106] on the head-on collision of two equisized drops at the center of the inflow axis under planar extensional hyberbolic flow in a four-roll mill, and the data of several authors on the coalescence time of a single drop with its bulk homophase at a deformable interface under gravity are considered. The scaled coalescence time increased linearly with the product of the viscosity ratio and the applied external to interfacial tension forces ratio over three orders of magnitude. Higher applied external forces resulted in larger film areas while more viscous drops reduced the interfacial mobility thereby increasing the coalescence times.     

Mesophase/isotropic phase interfacial energy: Determination from coalescence kinetics of mesophase pitch

According to the theory of Frenkel, surface energy, viscosity and droplet size are the factors which determine the coalescence kinetics of two identical spherical droplets (spherules) which are in contact. From the Frenkel relationship and observations of mesophase spherule coalescence kinetics, we have evaluated the mesophase/isotropic interfacial energy for a mesophase-containing pitch. The interfacial energy is quite small (<1 dyne/cm) in agreement with a prediction of White.     

COALESCENCE OF SINGLE DROPS AT A LIQUID-LIQUID INTERFACE IN THE PRESENCE OF SURFACTANTS/POLYMERS

The stability of single oil droplets at a liquid-liquid interface in the presence of surfactants/polymers was investigated and the coalescence time of oil droplets at an oil-aqueous surfactant solution interface was measured in a specially constructed coalescence cell. A unique feature of the experimental set-up was that the drop size could be controlled quite accurately and was independent of the interfacial tension and the difference in the densities of the two phases used. The coalescence time of oil droplets correlated favorably with interfacial viscosity of the system. However, no correlation was found between interfacial tension and coalescence time or the rate of coalescence. Both the coalescence time and interfacial viscosity were found to be strong functions of the surfactant concentration.     

Fat Crystals Influence Methylcellulose Stabilization of Lipid Emulsions

Oil-in-water emulsions stabilized with methylcellulose (MC) varied in stability depending on the composition of the fat phase. When droplets were composed entirely of liquid oil, MC was able to form a continuous, protective film around the droplets. Therefore, when two liquid oil droplets were brought into contact, they underwent extreme shape deformation but did not coalesce, even when excess force was used. Subsequently, interfacial crystals extending into the aqueous phase from palm kernel oil droplets were aimed into an entirely liquid oil droplet. The MC-coated droplet would deform wherever the crystal contacted; however, the protruding crystals could not penetrate into the liquid oil droplet. Conversely, when the target droplet was composed of a small amount of solid fat that resulted in localized crystalline regions and the interfacial crystals of the second droplet were aimed at this region, they then easily pierced the droplet. This demonstrates that MC is an excellent stabilizer for liquid oil droplets but internal lipid crystals within fat globules can alter MC surface conformation to allow for crystal penetration and arrested coalescence.     

Morphology evolution and dynamics of droplet coalescence on superhydrophobic surfaces

In this work, the coalescence of two equal‐sized water droplets on superhydrophobic surfaces (SHSs) is experimentally investigated. The morphologies of droplet coalescence are observed from side‐view and bottom‐view using high‐speed camera system. The related morphology evolution and dynamics of droplet coalescence are explored. The dynamic behaviors of droplet coalescence on SHSs can be decomposed into liquid bridge growth, contact line evolution, and droplet jumping. The liquid bridge radius is proportional to the square root of time, whereas the dimensionless prefactor is decreased from 1.18 to 0.83 due to the transition of interface curvature. The retraction velocity of the contact line shows limited dependence on initial droplet radii as the retraction dynamics considered here are governed by the capillary–inertial effect. The coalesced droplet finally departs the substrate with a dimensionless jumping velocity of around 0.2. A heuristic argument is made to account for the nearly constant dimensionless jumping velocity. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2913–2921, 2018     

Interfacial tension measurement between immiscible polymers: improved deformed drop retraction method

This paper describes an improvement of the technique to measure interfacial tension in immiscible polymer blends. Our method is based on the droplet retraction method, in which one relates the kinetics of relaxation of a deformed droplet to the interfacial tension between the matrix and droplet. Previously, the problem with this technique has been the difficulty in preparing axisymmetric ellipsoidal droplets. In our work, we demonstrate that perfect axisymmetric ellipsoidal droplets are produced at a later stage of relaxation of short imbedded fibers. With this technique, we utilize the strengths of both the deformed droplet method and the imbedded fiber retraction method while overcoming their shortcomings. The interfacial tension value thus obtained was compared to that by conventional methods. Additionally, the effect of confinement by external walls on the interfacial tension measurement was studied. Confinement affects interfacial tension measurement when the gap between the walls is less than two times the equilibrium drop size.     

Coalescence Behavior of Pure and Natural Fat Droplets Characterized via Micromanipulation

For two approaching oil droplets, a region of arrested coalescence lies between full coalescence and total stability. Here the fusion of two droplets begins, but they are stopped from fully relaxing into one spherical droplet. The internal rigidity of the solid fat network within each droplet can provide the resistance necessary to arrest the shape change driven by Laplace pressure. These intermediate doublet structures lead to the partially‐coalesced fat networks important for the desired physical properties of ice cream and whipped topping. The use of micromanipulation techniques allows coalescence events between two oil droplets to be microscopically observed. In this study, oil droplets composed of different fats were manipulated at varying elastic moduli, interfacial tension, and radii. It was seen that increasing the elastic moduli of the droplets or increasing droplet radii resulted in coalescence being arrested earlier. Under these experimental conditions, different interfacial tensions did not change the coalescence behavior between two oil droplets.     

Description and application of an interfacial viscometer

This paper describes an interfacial viscometer. Rheological data of polymeric surfactants were obtained with the use of the instrument for liquid–liquid and liquid–gas interfaces. The surfactants studied were methylcellulose, poly(vinyl alcohol), polymer of diethylene glycol n-butyl ether acrylate, and sulfonated poly(vinyltoluene). Investigations were conducted over a wide range of temperatures and concentrations of the surfactants in the liquid phase. The instrument was used to obtain surface area coverage data for the surfactants studied. The experimental data were compared with theoretical calculations. The data indicated that the absorbed film at the interface was an irregular film, i.e., not an ordered packing of the molecules at the interface. Significance of interfacial viscosity on droplet break-up in a simple shear field was investigated. Droplet breakup studies substantiate the hypothesis that an absorbed polymeric film at an interface forms a complex viscous film.     

EFFECTS OF SURFACTANT ADSORPTION—DESORPTION KINETICS AND INTERFACIALRHEOLOGICAL PROPERTIES ON THE RATE OFDRAINAGE OF FOAM AND EMULSION FILMS

An analysis has been developed to describe the rate of drainage of an axi-symmetric plane-parallel foam and emulsion films associated with bubble-bubble and drop-drop coalescence phenomena. The present analysis extends the earlier work of Zapryanov et al(1983) and of others which was limited to the diffusion controlled case of the surfactant transport from the bulk phase onto the interface. Here the surfactant transport from the bulk phase to the interface is accounted for by a two step mechanism: (i) diffusion of the surfactant from the bulk to the sublayer, followed by (ii) the adsorption of the surfactant from the sublayer onto the interface. The results of a parametric study indicate that the rate of drainage of surfactant stabilized films is strongly dependent upon the surfactant adsorption-desorption kinetics, selective surfactant solubility, and interfacial Theological properties such as elasticity and interfacial viscosity. Predictions of the film drainage time are compared with our experimental data to establish the range of the applicability of the present model with respect to film radius.     

小液滴在水平液液界面上的聚并

对纯净系统中的小液滴在水平液液界面上聚并时的薄液膜排液过程进行了理论分析，研究两相物理性质、范德华力、界面切向运动对聚并速率的影响，得到了计算聚并时间的公式．理论预测与实验结果符合良好．     

Electro-coalescence of an aqueous droplet at an oil-water interface

The coalescence of an aqueous droplet at an oil-water interface under an electric field has been investigated, with a view to quantify conditions that give rise to secondary droplet formation. Two patterns of drop-interface coalescence may occur: complete coalescence and partial coalescence. The former is obviously the desirable pattern for industrial coalescers. However in practice, the process of coalescence could actually produce smaller droplets, which become more difficult to remove, and hence undesirable. This is caused by either necking, due to extensive elongation of the droplet, or reaction to a fast and energetic coalescence and is referred to as partial coalescence. The volume of the droplets formed in this way has been analyzed as a function of the initial droplet size, electric field strength and the distance between the droplet and the interface. The expansion speed of the neck connecting the droplet and interface at the beginning of the pumping process has also been quantified. These results are useful in optimizing the electro-coalescence process.     

INFLUENCE OF INTERFACIAL TENSION AND VISCOSITY ON THE BEHAVIOR OF A PACKED COLUMN IN NEAR-CRITICAL FLUID EXTRACTION

Density and viscosity of the coexisting phases and interfacial tension of the following binary systems were measured: pelargonic acid, linoleic acid, oleic acid or stearic acid as subcritical and carbon dioxide as superecritical components. Also the corresponding phase equilibria were investigated at pressures ranging from 2 to 20 MPa and temperatures from 313 K to 393 K. With increasing pressure the concentration of the supercritical component in the liquid phase increases and viscosity and liquid interfacial tension decreases. At greater activities of the dense gases the interfacial tension decreases dramatically to values less than 2 mN/m. If the pressure exceeds a certain limit, a falling film disintegrates into small droplets. The surface excess passes through a maximum at these conditions. At first appearance of instabilities on a falling film, the logarithm of the Reynolds number is a linear function of the logarithm of the film number. Independent of the type of the investigated packings, the number of theoretical stages per meter versus Hodenstein number fall all on the same curve when an extraction is carried out in the droplet regime. Also the logarithm of the capacity of a column al the flooding point versus the logarithm of the density difference between the coexisting phases is a linear function when an extraction is carried out in the droplet regime.     

Interactions between fat crystal networks and sodium caseinate at the sunflower oil-water interface

A Couette-type torsion wire surface shear viscometer was used to measure the apparent interfacial shear viscosity of pH 7 (I=0.05 M) buffered solutions of sodium caseinate in contact with sunflower oil. The sunflower oil contained 1% fat crystals in either the β or β′ polymorphic form, or was crystal free. In all cases, the fat crystals increased the interfacial shear viscosity synergistically, with the β′ crystals causing the biggest increase. Substituting the protein for a small-molecule surfactant (Tween-40) showed that this was not simply due to the protein lowering the interfacial tension. Sedimentation studies of the different fat crystal slurries suggested that the extent of the interfacial shear viscosity increase was related to the strength of crystal-crystal interactions in the oil phase. It seems likely that when protein is present at the interface, it fixes the adsorbed layer of fat crystals to the cross-linked protein film at the interface. When this film was sheared, the strength of the crystal-crystal interactions in the oil phase became important. However, when Tween-40 was in the aqueous phase instead of the protein, the crystal-crystal interactions were not relevant, presumably because the Tween-40 interfacial film simply flowed around the adsorbed crystals     

ACOUSTIC STUDIES OF INTERFACIAL EFFECTS IN AIRLIFT REACTORS

Rational design of airlift reactors for the culture of plant and animal cells is impeded by a lack of understanding of the causes of loss of viability. Some recent speculations in the literature suggest that gas-liquid interfacial phenomena have played prominent roles in observed instances of loss of viability in both insect and animal cell cultures. The important interfacial events may include bubble formation and detachment at the sparger, bubble coalescence and breakage, and bubble disengagement at the free surface-entailing film thinning, film rupture, and film droplet ejection.

We have carried out an experimental investigation of interfacial phenomena using macrovideography and acoustic signals recorded at both the sparger and the free surface. Data have been collected for several liquid media including distilled water, distilled water with electrolyte (NaCl), and aqueous solutions of glycerol (μ = 3 to 9.5 cP). The studies were conducted in a 3-liter, acrylic plastic, split-column airlift reactor, and air was introduced through interchangeable sieve plates (with hole diameters of 1, 2, and 3 mm). Time-series data collected from the microphone were processed for mean and root-mean-square values, and Fourier transforms were computed to identify important signal energies. A sequence of experiments was also conducted that was designed to relate specific interfacial phenomena to the frequencies of the noises produced.     

Adsorption Behavior of Nonionic Surfactants Studied by Drop Volume Technique

This study shows that the drop volume technique can be used to determine the adsorption behavior and interfacial adsorption kinetics of surfactants at fluid interfaces. Using this tensiometric method, one can determine not only the interfacial tension of the pure phases, but also the critical concentration for the formation of micelles (CMC) in a surfactant system, the quasi‐static (equilibrium) interfacial tension, the diffusion coefficient as a function of surfactant concentration, and the maximum adsorption density at the interface. The determination of the dynamic interfacial tension allows to indirectly characterize the kinetics of surfactant adsorption. The time dependence of the interfacial coverage resulting from this adsorption process is well described by two approximation solutions (for short and long adsorption times), with the result that the diffusion coefficients calculated as a function of surfactant concentration using these two methods show good agreement. The droplet formation and dripping process of a surfactant solution in a capillary was found to be quite different depending on whether the process occurred in gaseous or fluid surroundings. In particular, the formation of satellite droplets was different for the two different media, in terms of both the volume and shape of the satellite droplets.     

Effect of marangoni stresses on the deformation and coalescence in compatibilized immiscible polymer blends

The effect of physical compatibilization on the deformation and coalescence of droplets in immiscible polymer blends is discussed. Evidence is provided for the existence of concentration gradients in block copolymers along the interface during deformation. This causes complex changes in droplet shapes during deformation and relaxation. These concentration gradients also result in Marangoni stresses, which stabilize the droplets against deformation and breakup. Coalescence experiments have been performed, varying both the compatibilizer concentration and the shear rate. Existing coalescence models have been evaluated. An empirical extension of Chesters' partially mobile interface model is presented, that treats the effects of Marangoni stresses on the coalescence process as a higher effective viscosity ratio.