Oil Drops In Water Research Articles

High flow, low shear impellers versus high shear impellers; dispersion of oil drops in water and other examples

High flow, low shear impellers versus high shear impellers; dispersion of oil drops in water and other examples

Anomalous Wetting of Underliquid Systems: Oil Drops in Water and Water Drops in Oil.

We have investigated the wetting phenomena of two underliquid systems, i.e., oil (drop) in water medium and water (drop) in oil medium for two different substrates, poly(methyl methacrylate) (PMMA) and glass. We have conducted detailed static (equilibrium) and dynamic contact angle measurements of drops on substrates kept in air, water, and oils of varying densities, viscosities, and surface tensions. We compared the experimentally observed contact angles with those predicted by the conventional wetting theories, namely, Young's equation and the Owens and Wendt approach. The results reported herein showed that experimental values vary in the range of 8-20% with the conventional theoretical model for water (drop) in oil (viscous surrounding medium) on PMMA substrate. However, oil (drop) in water medium on PMMA does not show such an anomaly. By taking into consideration a thin oil film between a water drop and PMMA originating from the surrounding oil medium, the modified Young's equation is proposed here. We found that the percentage difference between experimentally observed contact angles with modified Young's equation is in the range of 0.88-5.88%, which is very less compared to percentage difference with classic Young's equation. For glass substrates, the standard Young's equation does not translate to the underliquid systems whereas the Owens and Wendt theory could not correctly predict the underliquid contact angles. However, the modified Young's equation with thin-film consideration agrees very well with the experimental values and thereby demonstrated the presence of a thin film between a drop and glass substrate originating from the surrounding viscous medium. This present experimental study coupled with detailed theoretical analyses demonstrates the anomalous wetting signature of drops on substrates submerged in surrounding viscous medium, which is very different from the reported studies for drops on substrates kept in air (inviscid medium).

Detection and characterization of elongated bubbles and drops in two-phase flow using magnetic fields

We report a method to detect and to measure the size and velocity of elongated bubbles or drops in a dispersed two-phase flow. The difference of the magnetic susceptibilities between two phases causes a force on the interface between both phases when it is exposed to an external magnetic field. The force is measured with a state-of-the-art electromagnetic compensation balance. While the front and the back of the bubble pass the magnetic field, two peaks in the force signal appear, which can be used to calculate the velocity and geometry parameters of the bubble. We achieve a substantial advantage over other bubble detection techniques because this technique is contactless, non-invasive, independent of the electrical conductivity and can be applied to opaque or aggressive fluids. The measurements are performed in an inclined channel with air bubbles and paraffin oil drops in water. The bubble length is in the range of 0.1–0.25 m and the bubble velocity lies between 0.02–0.22 m s−1. Furthermore we show that it is possible to apply this measurement principle for nondestructive testing (NDT) of diamagnetic and paramagnetic materials like metal, plastics or glass, provided that defects are in the range of 10‒2 m. This technique opens up new possibilities in industrial applications to measure two-phase flow parameters and in material testing.

Rise of oil drops in water and fall of water drops in oil

Rise and fall velocities of oil drops in water and water drops in oil, respectively, are measured in a liquid column. The main perspective is on the similarity of the flows in these largely different cases. When the experimental data are plotted as the normalized rise velocity, which is the Reynolds number, versus the Archimedes number a common curve appears. This proves that similarity exists at least with respect to the velocity. Larger drops deviate considerably from the spherical shape. Nevertheless the data set can be represented by a formula derived from data on hard spheres. In particular the Stokes limit for Re→0 is nicely confirmed by measuring down to 10− 4.

Big Cells Cleave as Fast as Small Ones: the Physics of Cytokinesis

Big Cells Cleave as Fast as Small Ones: the Physics of Cytokinesis

The electrophoretic mobility of an uncharged particle

HypothesisIt has been claimed that uncharged particles can have negative electrophoretic mobilities, and so a negative mobility need not imply a negative particle charge. We show that although a steady electrophoresis may be possible for the uncharged infinite slabs studied in Molecular Dynamics simulations, it is not possible for a finite particle. Experiments and theoryAn uncharged particle may initially move when the field is turned on, but our analysis shows that this motion ceases as charges of opposite sign build up on the front and back of the particle. Uncharged particles may move in alternating electric fields, but their mobility is predicted to increase with electrolyte conductivity. Experimentally, however, the mobility of hexadecane oil drops in water at pH 9 decreases with increasing NaCl concentrations. FindingsOur results are consistent with the usual compression of the double layer with added salt, and with the traditional interpretation, that hydrophobic particles have negative mobilities because they are negatively charged. Uncharged particles may have a transient mobility but it is quickly quenched by polarisation of the double layer.

Measurement of the Hydrophobic Force in a Soft Matter System.

The hydrophobic attraction describes the well-known tendency for nonpolar molecules and surfaces to agglomerate in water, controlled by the reorganization of intervening water molecules to minimize disruption to their hydrogen bonding network. Measurements of the attraction between chemically hydrophobised solid surfaces have reported ranges varying from tens to hundreds of nanometers, all attributed to hydrophobic forces. Here, by studying the interaction between two hydrophobic oil drops in water under well-controlled conditions where all known surface forces are suppressed, we observe only a strong, short-ranged attraction with an exponential decay length of 0.30 ± 0.03 nm-comparable to molecular correlations of water molecules. This attraction is implicated in a range of fundamental phenomena from self-assembled monolayer formation to the action of membrane proteins and nonstick surface coatings.

Nanodispersions of bubbles and oil drops in water

Explanations are suggested for the experimental observations that nanobubbles and oil nanodrops dispersions in water have a long life. The increased stability of nanobubbles occurs because their ascensional velocity, provided by the Hadamard–Rybczynski equation, is proportional to the square of their radius. As a consequence, for radii in the nanorange, the ascensional velocity is extremely low. In addition, the repulsive electrostatic interactions among them during their ascensional motion oppose coalescence. The presence of air in nanoemulsions increases the ascensional velocity of the nanodrops because it generates bubbles which being hydrophobic become attached to the hydrophobic drops, thus decreasing both the average density of the drop–bubble complex and the friction between the oil drop and water. As a result, the ascensional velocity is increased above the value provided by the Hadamard–Rybczynski equation. Consequently, by degassing the nanoemulsions one can increase their stability. In addition, evidence is brought for the existence of a hydrodynamic attractive force which generates coalescence.

Application of the multicomponent lattice Boltzmann simulation method to oil/water dispersions

We study the propagation of acoustic fields in bounded, two-dimensional, mono-disperse oil/water emulsions using a carefully modified and appropriately calibrated single relaxation time multicomponent lattice Boltzmann equation simulation. Our model is a variant of an algorithm applying both interface forces based on macroscopic surface tensions and a kinematic condition for phase separation, adapted to allow sonic speed variations between its oil and water components. Appropriate second-order accurate acoustic boundary conditions are obtained from a node-based lattice closure with local mass conservation and applicability for varying fluid viscosities. Data from an example simulation of a single oil drop in water interacting with a generated standing acoustic wave are presented and, where appropriate, compared with empirical theories and analogous calculations for a solid object.

Onset of entrainment and degree of dispersion in dual continuous horizontal oil–water flows

The transition from stratified to dual continuous oil–water flow (where each phase retains its continuity but there is dispersion of one phase into the other) as well as the dispersed phase fractions in the layers of the dual continuous pattern, were studied experimentally. Transition to this pattern from stratified flow occurs when drops of one phase appear into the other (onset of entrainment). The studies were carried out in a 38 mm ID horizontal stainless steel test section using two different inlet geometries, a T- and a Y-junction. The patterns were visualized through a transparent acrylic section located at 7 m from the inlet using a high speed video camera. Phase distribution measurements in a pipe cross section were obtained just before the acrylic section with a local impedance probe and the results were used to calculate the volume fraction of each phase entrained into the other. The onset of entrainment was found to occur at lower superficial water velocities as the oil superficial velocities increased. However, the inlet geometry did not affect significantly the transition line. During dual continuous flow, the dispersion of one phase into the opposite was found to extend further away from the interface with increasing water superficial velocity for a certain oil superficial velocity. An increase in the superficial water velocity increased the entrained fraction of water in oil ( E w/o ) but there was no trend with the oil velocity. Similarly, an increase in the superficial oil velocity increased the fraction of oil drops in water ( E o/w ) but the water velocity had no clear effect. The entrainment fractions were affected by the inlet geometry, with the T-inlet resulting in higher entrainment than the Y-inlet, perhaps because of the increased mixing induced by the T-inlet. The difference between the two inlets increased as the oil and water velocities increased.

The surface of neat water is basic

Theoretical studies which conclude that the surface of neat water is acidic (with a pH < or = 4.8), due to the preferential adsorption of hydronium ions, are contrary to the available experimental evidence. Air bubbles in water have a negative charge, as do hydrophobic oil drops in water, and streaming potential measurements on inert surfaces such as Teflon in water show a similar negative surface charge. In each case the pH dependence of the zeta potential has an isoelectric point between pH 2-4. An isoelectric point of pH 4 implies a preference for hydroxide over protons of 10(6), the opposite of what was inferred from the theoretical simulations. Water behaves similarly at all inert hydrophobic interfaces with the preferential adsorption of hydroxide ions to give a negatively charged surface at neutral pH. The surface-charge density at the oil/water interface in mM salt solutions is -5 to -7 microC cm(-2), which corresponds to one hydroxide ion on every 3 nm2 of the surface. The homogenisation of an inert oil such as hexadecane in water in the absence of any salt or base still leads to formation of an emulsion. The hydroxide adsorbed on the large surface area of the emulsion greatly exceeds that present at 10(-7) M in neutral water; it is created by the increased autolysis of water, driven by the strong adsorption of hydroxide ions at the oil/water interface. These surfactant-free, salt-free emulsions are stable for some hours, with protons as the only counterions to the negative hydroxide surface.

Investigation of the effect of a simple salt on the kinetics of gravity induced coalescence for a viscosity matched emulsion system

In this work, we study the effect of a simple salt such as sodium chloride on the kinetics of gravity driven coalescence for a viscosity matched oil and water system. The time it takes for a drop to coalesce with a macroscopic interface is experimentally determined for oil drops in water and vice-versa. For a given set of fluids, the viscosities of the phases are matched and the drop sizes of the oil and water are controlled such that they are close. This ensures a similarity of buoyancy and gravitational forces for both cases. In the absence of interfacial effects, the coalescence of both water and oil drops at the interface should have been equal, due to the similarity of hydrodynamic conditions. It is observed that the presence of salt in the continuous phase retards the flow at the interface, thereby increasing the coalescence time. Such a phenomena was hitherto unobserved in liquid–liquid systems and will significantly influence the coalescence rates in emulsions. The experimental data are compared with existing theory. The results are explained on the basis of the creation of an interfacial tension gradient due to the presence of the salt molecules at the interface. The adsorption coefficient of the salt at the interface has been back calculated by including the force opposing the drainage in the film due to the interfacial tension gradients. The phenomenon is also discussed in relation to some of the observations that have been already reported in the literature.

Studies on the Binary Coalescence Model: II. Effects of Drops Size and Interfacial Tension on Binary Coalescence Time

A binary coalescence model of a two-component system of mixed oil drops in water is established. The original coalescence driving force directly connected to the chemical potential difference of the coalesced oil drops has been investigated under a mimetic “weightlessness” condition. Gravitation is minimized nearly to zero to overcome drop deformation by mixing hydrocarbon compounds and carbon tetrachloride. Any external force out of the drops is reduced nearly to zero, when one liquid drop goes near enough to be tangential to another. The coalescence time, defined as the binary coalescence time (tbi-coal), can be expressed as lnkdσ(1r1−1r2)=kbtbi-coal+b′ (1). The coalescence phenomenon was first proposed as initiated by chemical potential differences between liquid drops (B. Y. Yu, Ph.D. dissertation, Swiss Fed. Inst. Technol., Zurich, 1995). The greater the chemical potential difference between two liquid drops, the larger the factor σ(1/r1−1/r2); as a result, the coalescence tendency will increase (i.e., tbi-coal decreases). Equation [1], including interfacial tension, drop size difference, and thin-film coefficient supported by the experimental data, is quantitatively established.

Interferometric oil-spill detection system

An automatic, oil-spill-detection system, which employs thin-film and wavefront-splitting interference techniques to determines the existence of surface oil or oil drops in water is disclosed. Two independent automatic, decision-making systems are disclosed, which provide a reliable means for oil spillage detection. A computer code and a image extraction system are employed to discern and monitor the interference fringes generated from oil slicks. The computer-based imaging system can be further replaced by a compact and low-cost imaging circuit that functions reliably in a buoy with relatively low power consumption.

Using Acoustics to Study and Stimulate the Coalescence of Oil Drops Surrounded by Water

The coalescence of oil drops in water is studied using acoustic levitation and stimulated with acoustic cavitation. Unlike most earlier studies, which investigate the coalescence of a single drop with an initially plannar interface, the use of acoustic radiation forces allows two drops to be brought into contact and allowed to coalesce. This acoustic technique has the advantage over other drop-drop coalescence systems in that the drops remain in contact until they coalesce without the use of solid supports to control them. Additionally, acoustic cavitation is observed to deposit sufficient energy in the oil-water interface to trigger the coalescence of a pair of 2-mm-diameter drops. This stimulation mechanism could have application to emulsion breaking. Some of the factors that affect spontaneous and stimulated coalescence are investigated.

The use of acoustical levitation to study the coalescence of oil drops in water

The behavior of emulsions composed of immiscible liquids has been the object of many theoretical and experimental studies [for review, see Chen et al., AIChE J. 30, 622–630 (1984)]. The coalescence of a drop separated by a film from its homophase has been studied to explain the stability of emulsions; there have not been experiments, however, on free drops contacting other free drops over long periods of time due to the difficulty encountered in containing the drops. Acoustical radiation pressure provides a means of controlling free fluid drops and was considered for application to the study of coalescence. Hexane drops were trapped by a 45-kHz standing wave in a resonant water column. The time two drops remained in contact before coalescing was measured for both pure and surfactant stabilized systems. This time was then compared to theoretical predictions and nonacoustical coalescence experiments. [Work supported by the Heyl Foundation.]

Linear Oil Displacement by the Emulsion Entrapment Process

Abstract Lack of mobility control is a major impediment to successful EOR, especially for high-viscosity oils. This paper presents experimental and theoretical results for continuous, linear, secondary oil displacement using dilute, stable suspensions of oil drops. The major hypothesis is that the oil/water (O/W) emulsion provides microscopic mobility control through entrapment or local permeability reduction not through viscosity-ratio improvement. To describe the displacement process, previous emulsion filtration theory is extended to longer cores and to two-phase flow. Agreement between theory and experiment is satisfactory for continuous secondary oil displacement with 1- to 2-μm [1- to 2-micron] diameter drops of volume concentrations up to 5 % in unconsolidated sandpacks with permeabilities ranging from 1 to 3 μm2 [1 to 3 darcies]. Dilute suspensions of stable oil drops in water also are successful in diverting flow in parallel-coreflooding to the lower-permeability core; therefore, they provide macroscopic mobility control.

The flocculation of aqueous sols of a graphitized carbon black by a potential-determining electrolyte

Recent work on the film thinning behavior during the coalescence of oil drops in water is summarized. In the experimental work, color movies were taken of the light interference patterns produced by the thin film of water trapped between the rising drop and the bulk oil phase. The drops were relatively small (0.2–0.32 cm diam.); sufficient KCl was added to minimize double layer repulsion. Concentrations of 10 −6 –10 −2 g l −1 of sodium lauryl sulfate were used. The oils used were toluene, anisole, cyclohexanol and a mixture of anisole and cyclohexane. Five different patterns of film behavior were observed. The mechanisms to explain these patterns are based on concepts of dynamic immobility (when the distribution of surfactant along the interface gives an interfacial tension gradient that every-where balances the interfacial shear stress) and of steric immobility. The mechanisms include rapid approach (when both interfaces are mobile), dimple formation (when the interfacial tension gradient exceeds the interfacial shear stress), even drainage (when the interfaces are dynamically or sterically immobile) and uneven drainage (when there is local mobility). Reasons for uneven drainage are discussed. The different theoretical models developed to describe film thinning are reviewed. The emphasis is on the assumptions and the relationship between the equations describing flow in the film and those describing surface behavior. The predictions of the different models are compared but the emphasis is on the success of more recent models based on the above mentioned mechanisms to describe symmetrical film drainage.

The stability of emulsions

Recent work on the film thinning behavior during the coalescence of oil drops in water is summarized. In the experimental work, color movies were taken of the light interference patterns produced by the thin film of water trapped between the rising drop and the bulk oil phase. The drops were relatively small (0.2–0.32 cm diam.); sufficient KCl was added to minimize double layer repulsion. Concentrations of 10−6–10−2 g l−1 of sodium lauryl sulfate were used. The oils used were toluene, anisole, cyclohexanol and a mixture of anisole and cyclohexane.

Studies at phase interfaces: II. The stabilization of water-in-oil emulsions using oil-soluble emulsifiers

A range of oil-soluble emulsifying agents, representing various chemical classes, has been examined for the stabilization of water-in-oil (w/o) emulsions of high water/oil ratio. Some of the physical properties of oil solutions of these emulsifiers have been measured, including their interfacial tensions against water, and the relative rates of coalescence of oil drops in water with bulk oil phase and of water drops in oil with bulk water phase. These properties have provided some information on the type of interfacial film required for w/o emulsification. It is concluded that a strong interfacial film should be formed which prevents the coalescence of water droplets while permitting the coalescence of oil droplets. In almost all systems where stable w/o emulsions are formed, it is possible to postulate some form of molecular interaction, either electrostatic attraction or hydrogen bonding, in the interface. The emulsifier should be hydrophobic in character and have the correct hydrophilic-lipophilic balance (HLB) for the particular solvent concerned. However, the published methods for expressing the HLB value quantitatively do not appear to be suitable for selecting emulsifiers for w/o emulsions. A low interfacial tension between the two phases is desirable, but is not essential if a sufficiently strong interfacial film is formed. Interfacial tension may be important, however, in cases where very rapid emulsification is required, and in these cases the emulsifier should also be adsorbed rapidly at the interface.