Coalescence Of Water Droplets Research Articles

Dynamic behavior of water droplets on wetted superhydrophobic surfaces under a high AC electric field

In this paper, the dynamic behavior of water droplets was investigated on wetted hydrophobic and surperhydrophobic surfaces under a high AC electric field. The droplets on superhydrophobic surface coalesced and slid off the inclined surface before the flashover occurred, while the droplets on hydrophobic surface stayed on the surface and could not roll off easily even under the breakdown field strength. Due to the behaviors of water droplets on the surfaces under the high AC electric field, the flashover voltage of the superhydrophobic surface was increased by 35.8 percent. A two-dimensional simulation model was developed to investigate the dynamic behavior of water droplets on surperhydrophobic surface. The results showed that, the adjacent water droplets were elongated and a liquid bridge was formed between the droplets under electric field. The coalescence of water droplets was impelled by the surface tension force and electric force.

Deformation and coalescence of water droplets in viscous fluid under a direct current electric field

Deformation and coalescence behaviors of water droplets in viscous fluid under a direct current electric field are investigated with experiments and numerical simulations. An electro-hydrodynamic model is developed based on fluid dynamics equations and electrostatic equations. A coupled level set and volume of fluid method is proposed to capture the dynamic interface. The numerical results are validated to be in good accordance with classic results in the literature. The effects of electric capillary number Ca and viscosity ratio λ on droplet deformation and coalescence are systematically analyzed. Two different droplet response modes are obtained, namely overdamping response and underdamping response. The conversion of response modes is discussed with the (Ca, λ) phase diagram. Moreover, the distributions of electric field and flow field before and after the droplets contact are compared and analyzed. Numerical results show the time evolution of liquid bridge is dominated by the interfacial tension and viscosity of continuous phase. The electric stress is extremely weak at the liquid bridge and has a slight influence on evolution of liquid bridge. The correlation formula between dimensionless liquid bridge radius and conical angle during the growth of liquid bridge is regressed. These results are helpful for the optimization of electro-coalescence process.

Water-in-oil emulsions separation using an ultrasonic standing wave coalescence chamber

The offshore extraction of crude oil produces stable water in oil emulsion. To separate this emulsion into oil and water phases, the oil/water interfacial film is commonly destroyed by the addition of chemical demulsifiers. The use of an ultrasonic standing wave force field could be an alternative to reduce the dosage of chemical demulsifiers in the coalescence process. In this work, an ultrasonic separator of water in crude oil emulsions is investigated through the use of a high frequency ultrasonic standing wave coalescence chamber. The coalescing chamber uses the acoustic radiation force to induce the coalescence of water droplets at the pressure nodes of a standing wave field. Due to temperature fluctuations, the excitation frequency is controlled to maintain the resonance in the coalescence chamber and the voltage amplitude is controlled to deliver a given acoustic power. Experimental tests using standardized emulsions of water in oil were carried out in a laboratory processing plant. The effects of ultrasound application, flow rate, initial water content, demulsifier dosage and chamber inlet temperature were analyzed. The results show that the use of the acoustic radiation force improves the emulsion separation in all the conditions analyzed, when compared with the gravitational separation technique.

Effect of pore size and wettability of multilayered coalescing filters on water-in-ULSD coalescence

Coalescing filters are generally used to increase the size of the small drops for their separation from gases and liquids. Many studies have been performed on coalescing filter media. The performances become much more complicated for multilayered media having different properties and systematic studies are needed. Wettability, pore size, face velocity and wetting properties are among the important factors controlling performance. In this work, microscopic studies were performed to visually observe coalescence of water droplets on hydrophilic Nylon and hydrophobic Polypropylene woven sheets in a water-in-diesel emulsion at different face velocities and spacings between the sheets. The combination of hydrophobic sheet followed by hydrophilic sheet had the best performance. Statistical analysis of variance showed that the variations of distance between sheets and the face velocity were not significant factors. To better understand the effects of multilayers in filter performance, pilot scale experiments of stacked layers of woven sheet media of different thicknesses and wetting properties were conducted. Thin media had poor performance, but the performance improved and reached a plateau as the thickness increased. Effects of fiber surface wettability and pore size variations in the layers were also evaluated. Similar to microscope results, the coalescence performance was better for dual layer media with different wetting properties of each layer compared to dual layer media having the same wetting properties of each layer. The pore size was a dominant parameter affecting the performance. When pore size was similar to the average drop size of the water in the upstream, the best performance was observed. For dual layer filters having one layer of small pores and the other of large pores, the observed performance was lower than that of dual layer filters having only large pores but higher than the filters having only small pores.

Numerical investigation of large bubble entrapment mechanism for micron droplet impact on deep pool

Water droplet impacting into a deep liquid pool is one of the most well-known flow phenomena in fluid mechanics. As a ubiquitous natural aeration process, the coalescence of water droplets in lakes and ponds and the subsequent bubble entrapment are one of the most notable ways of gas-liquid exchange in nature, and it is of great significance for underwater sound transmission, aquatic ecosystems and chemical process. The shape of an oscillating droplet in impact under different surrounding medium and initial condition is a key factor for the subsequent cavity formation and bubble entrapment. In this study, the adaptive mesh refinement technique and volume of fluid (VOF) method are applied to the study of the water droplet impact phenomena. Five kinds of deformed micron water droplets with different aspect ratios and impact velocities of 4 m/s and 6 m/s are selected to investigate the influences of drop deformation and impact velocity on the bubble entrapment, capillary wave propagation, and vortex ring evolution. The results show that at low impact velocities (<i>Fr</i> = 75, <i>We</i> = 64.4, <i>Re</i> = 1160, <i>V</i><sub>i</sub> = 4 m/s), the shape of water droplet does not cause the cavity formation and bubble entrapment to change significantly. However, under higher impact velocity (<i>Fr</i> = 112.5, <i>We</i> = 145, <i>Re</i> = 1740, <i>V</i><sub>i</sub> = 6 m/s), deformed droplet with an aspect ratio of 1.33 coalesces with the pool, and large bubble entrainment occurs. The large bubble entrapment is affected mainly by the vortex ring generated under the free surface at the neck between the droplet and the pool. The vortex ring penetrates more deeply before it pulls the free surface to generate a rolling jet at the upper interface of the cavity. The rolling jets then contact the center of the cavity and collapse to entrain a large bubble. At the end of the bubble entrapment phenomenon, the cyclone inside the cavity pushes the sidewall of the cavity continuously, and effectively increases the lateral volume of the bubble, which plays a vital role in the bubble entrainment process. In the initial stage of the impact, the flatter the shape of the droplet, the greater the curvature of the jet generated on the neck between the droplet and the pool, the greater the strength of the vortex ring generated under the free surface. However, the vortex ring formed by the oblate-shaped water droplet is generated too close to the free surface, and the early free surface pulling reduces the strength of the vortex ring, thus the vorticity maximum value decays relatively fast.

Bioinspired Dual-Tier Coalescence for Water-Collection Efficiency Enhancement.

Directly harvesting water from the atmosphere could aid in negating the issue of fresh water scarcity, garnering increased research interest in recent years. Typically, atmospheric water collection occurs via three main steps: accumulation, transportation, and collection. Although multiple studies have been published on bioinspired structures with enhanced directional fluid transportation, there is a significant lack of designs for enhancing water droplet coalescence. Long mean times before coalescence result in the re-evaporation of microdroplets, severely impeding the efficiency of atmospheric water collection. Herein, a water accumulator derived from a synergistic combination of inspiration from cacti spines and Tillandsia trichomes has been designed to encourage rapid coalescence. The drip-off volume measured in a fog chamber was found to be 220% that of a flat surface within 15 min, suggesting that improving the coalescence efficiency will be important in the future development of water-collection devices.

Free Energy and Dynamics of Water Droplet Coalescence

Droplet coalescence is a critical issue in atmospheric sciences. In this work, the coalescence of water nanodroplets was studied by performing equilibrium and nonequilibrium molecular dynamics simulations. To understand the intrinsic nature of the process, we obtained the free energy change as a function of droplet size and droplet–droplet distance. We decomposed the free energy change ΔF into energetic ΔU and entropic TΔS contributions to understand the molecular details. ΔU was dominated by the change in Coulomb interactions, which strongly correlated with the change in the number of hydrogen bonds. We found a strong positive correlation between the mobility of water molecules and TΔS. To analyze the dynamics, two colliding water droplets of the same size were given different initial speeds, impact parameters, and collision angles. We found when the collision is head-on, the time for thorough mixing between interfacial and bulk molecules decreases when the initial speed increases, whereas when the colli...

Coalescence of charged droplets in outer fluids

A controlled technique to produce a precise volume of fluid species, such as water droplets, has critical importance in a variety of industrial applications. Electric field provided a well-established method to produce charged water droplets with a controlled volume. The coalescence of produced charged water droplets, however, impedes the efficiency of electric field-assisted methods. Whereas the coalescence of stationary single droplets, often charged, is overwhelmingly studied in air or vacuum, the effects of surrounding medium and approaching velocity are neglected. Systematic series of experiments and simulations were designed to address the effect of viscosity as well as approaching velocity on the coalescence of charged water droplets in viscous surrounding mediums (μ = 100 & 1000 cSt). Results suggested that increasing the electrical conductivity of water droplets with lower approaching velocity diminishes the chance of coalescence between water droplets. The higher viscosity of surrounding medium resulted in a lower chance of coalescence between water droplets while droplets with stronger electrical conductivities underwent a lower deformation inside the dielectric medium. Finally, results suggested that water chain formation, which is reportedly a main retarding factor in electrocoalescers, took place for droplets with intermediate sizes in higher viscosities of surrounding medium.

Simulation on performance of a demulsification and dewatering device with coupling double fields: Swirl centrifugal field and high-voltage electric field

Abstract Demulsification and dewatering of emulsion oil are common in petroleum and chemical industries. For waste lubricating oil, as a common W/O emulsion, with high water content and complex construction, the multi-field coupling or integration technology can satisfactorily accomplish oil and water separation which cannot be realized by using a single technological method. A coupling device integrated swirl centrifugal and high-voltage electric fields are proposed. Water-droplet coalescence in emulsified oil under the high-voltage electric field can enlarge the droplet size, and the swirl centrifugal field can rapidly and efficiently achieve water-droplet separated from emulsified oil. In this study, the performance of the coupling device was investigated by using numerical simulation and experimental methods. The numerical results were in agreement with the values obtained by experimental methods. And the numerical results show that the dewatering and deoiling rates of overflow and outflow orifices increased by 4.5% and 6.2%, respectively, when voltage amplitude 10 kV increases to 11 kV. Moreover, when inlet velocity increased from 8 m s−1 to 12 m s−1, the separation efficiency gradually decreased. In conclusion, the results demonstrate that the double-field coupling device has a greatly performance at 11 kV and 8 m s−1.

Influence of electric field on water-droplet separated from emulsified oil in a double-field coupling device

Demulsification and dewatering of emulsion are widely used in the petroleum and chemical industries. However, demulsification and dewatering of emulsion by using a single physical technique are difficult to achieve efficiently. Therefore, swirl centrifugal and high voltage electric fields are integrated in a coupling device. Water-droplet coalescence in emulsified oil under the high-voltage electric field can enlarge the droplet size, and the swirl centrifugal field can rapidly and efficiently achieve water-droplet separated from emulsified oil. For this device, the influence of high voltage electric field on flow characteristics and separation efficiency was investigated by numerical simulation and experimental methods. Meanwhile, five voltage amplitudes (0, 10 kV, 11 kV, 12 kV and 13 kV) were applied in the study. The numerical results show that high voltage electric field has a minimal effect on flow characteristics. Specifically, the tangential velocity increased slightly due to the effect of high voltage electric field is beneficial to improve the separation efficiency. In addition, high voltage electric field can promote oil-water separation effectively by affecting the droplet size. And the separation efficiencies at 10 kV, 11 kV, 12 kV, and 13 kV are increased by 11.7%, 17.4%, 17.5%, and 18.3%, respectively, comparing with that under the action of single swirl centrifugal field. Furthermore, the separation efficiencies simulated by numerical methods are in agreement with the experimental results.

The roles of contact time and contact pressure on the coalescence of water droplets suspended in concentrated bitumen solutions

The coalescence of water drops in diluted bitumen solutions has been previously studied via micropipette experiments in which water drops are brought to contact, and the onset of drop coalescence is evaluated as a function of bitumen dilution, type of solvent, etc. Due to the opacity of concentrated bitumen solutions, most of these studies have been conducted in highly diluted systems. Under those conditions (typically solvent/bitumen or S/B mass ratios greater than 4), asphaltenes tend to form skins at the oil/water interface that prevent coalescence altogether. In ‘naphthenic’ froth treatment (NFT) process, however, S/B ratios of 0.5–2 are often used. To study S/B ratios relevant to NFT operations, we developed a Hele-Shaw version of the micropipette aspiration experiment, in which drops held on micropipettes are sandwiched to disks by two hydrophobically-treated plates spaced less than a few hundred microns apart. The resulting thin layer of bitumen is optically transparent in brightfield microscopy, allowing us to execute micropipette contacting experiments at industrially relevant bitumen dilutions. Another departure from almost all the literature in the past is that we have measured the time, tc, required to observe coalescence from the moment two disk-shaped drops are contacted under an imposed contact pressure, P. tc was observed to decrease when P, pH or bitumen dilution was increased. Deasphalting bitumen also led to a significant decrease in tc, while increasing the aging time of the interface from 15 to 60 min dramatically increased tc. The coalescence time was found to obey an exponential relationship with P, and was explained to be a result of an activated process related to the formation of a bridge of water across the thin film of bitumen in the contact zone between the drops. The tc-P correlations measured in this work provide the foundation for designing coalescers that coarsen the drop size distribution in water-in-bitumen emulsions under given conditions of bitumen concentration and water pH.

Electrohydrodynamic coalescence of droplets using an embedded potential flow model.

The coalescence, and subsequent satellite formation, of two inviscid droplets is studied numerically. The initial drops are taken to be of equal and different sizes, and simulations have been carried out with and without the presence of an electrical field. The main computational challenge is the tracking of a free surface that changes topology. Coupling level set and boundary integral methods with an embedded potential flow model, we seamlessly compute through these singular events. As a consequence, the various coalescence modes that appear depending upon the relative ratio of the parent droplets can be studied. Computations of first stage pinch-off, second stage pinch-off, and complete engulfment are analyzed and compared to recent numerical studies and laboratory experiments. Specifically, we study the evolution of bridge radii and the related scaling laws, the minimum drop radii evolution from coalescence to satellite pinch-off, satellite sizes, and the upward stretching of the near cylindrical protrusion at the droplet top. Clear evidence of partial coalescence self-similarity is presented for parent droplet ratios between 1.66 and 4. This has been possible due to the fact that computational initial conditions only depend upon the mother droplet size, in contrast with laboratory experiments where the difficulty in establishing the same initial physical configuration is well known. The presence of electric forces changes the coalescence patterns, and it is possible to control the satellite droplet size by tuning the electrical field intensity. All of the numerical results are in very good agreement with recent laboratory experiments for water droplet coalescence.

Rheological study of crude oil/water interface – The effect of temperature and brine on interfacial film

It is of fundamental importance for the petroleum industry to promote the coalescence of water droplets that may be dispersed in oil. Therefore, it is necessary to understand the rheological properties of the water/oil interface, which is straightly related to emulsion stability. This work aimed to present a practical approach about water/oil interface in realistic systems, i.e., using a crude oil and water or brine solution, the last one with the same composition of the water produced along with the oil production. Thus, it was investigated the water/crude oil and brine/crude oil interfaces in terms of their rheological properties at temperatures ranging from 30 °C to 60 °C. The interfacial rheological measurements were performed by the oscillatory pendant drop technique. The results indicated that temperature has a negligible effect on the interface properties; however, the salt content in the water phase resulted in an increase in the viscous modulus between 30 °C and 60 °C. The interfacial viscous and elastic moduli were shown to be dependent on the droplet oscillation period. Despite the elastic modulus being almost the same for water and brine, for the brine system the viscous modulus was significantly higher than that observed for water.

Mechanistic Study of Water Droplet Coalescence and Flocculation in Diluted Bitumen Emulsions with Additives Using Microfluidics

Synthetic crude oils derived from mined oil sands processed via the Clark hot water extraction process do not meet current specifications for pipeline transport and are corrosive to upgrader equipment by virtue of the high residual water content (2–5%) and salts. Formulated chemical additives used in this process can improve the oil quality by accelerating and enhancing the separation of water from oil. The identification and selection of these formulated additives is typically based on performance data collected in field testing for each component or blend. Herein, two methods are reported to study the effect of chemical additives on the phase separation behavior of water in diluted bitumen emulsions prepared in microfluidic devices. First, water droplets in diluted bitumen were created in the presence of chemical additives and the kinetics of droplet coalescence were compared for various additives and concentrations. Second, using a custom-made device geometry, water droplets in diluted bitumen were for...

Fabrication of thermally stable porous films from a cured epoxy resin via the Breath Figures process

In the current work, porous films based on epoxy resin have been obtained via the Breath Figures method. It was shown that the use of a low-temperature curing agent and fluoro-containing organosilicon copolymer, compatible with epoxy resin, makes it possible to obtain porous, thermostable, highly hydrophobic coatings with a pore diameter of 3–4 µm. When the epoxy resin/copolymer mixture is homogenous, the modifier prevents water droplet coalescence; otherwise, the mixture becomes heterogeneous, and the positive influence of the modifier is lost. The obtained modified porous films are highly hydrophobic and maintain their porous structure until polymer devitrification occurs. The simplicity of the Breath Figures method shows great potential for the manufacture of water-repellent paint coatings based on epoxy resin for use in a variety of applications.

Fast Transport of Water Droplets over a Thermo-Switchable Surface Using Rewritable Wettability Gradient.

In spite of the reported temperature dependent tunability in wettability of poly(N-isopropylacrylamide) (PNIPAAm) surfaces for below and above lower critical solution temperature (32 °C), the transport of water droplets is inhibited by the large contact angle hysteresis. Herein, for the first time, we report on-demand, fast, and reconfigurable droplet manipulation over a PNIPAAm grafted structured polymer surface using temperature-induced wettability gradient. Our study reveals that the PNIPAAm grafted on intrinsically superhydrophobic surfaces exhibit hydrophilic nature with high contact angle hysteresis below 30 °C and superhydrophobic nature with ultralow contact angle hysteresis above 36 °C. The transition region between 30 and 36 °C is characterized by a large change in water contact angle (∼100°) with a concomitant change in contact angle hysteresis. By utilizing this "transport zone" wherein driving forces overcome the frictional forces, we demonstrate macroscopic transport of water drops with a maximum transport velocity of approximately 40 cm/s. The theoretical calculations on the force measurements concur with dominating behavior of driving forces across the transport zone. The tunability in transport velocity by varying the temperature gradient along the surface or the inclination angle of the surface (maximum angle of 15° with a reduced velocity 0.4 mm/s) is also elucidated. In addition, as a practical application, coalescence of water droplets is demonstrated by using the temperature controlled wettability gradient. The presented results are expected to provide new insights on the design and fabrication of smart multifunctional surfaces for applications such as biochemical analysis, self-cleaning, and microfluidics.

Dynamics of a Water Droplet over a Sessile Oil Droplet: Compound Droplets Satisfying a Neumann Condition.

We report the dynamics of compound droplets with a denser liquid (water) droplet over a less dense sessile droplet (mineral oil) that satisfies the Neumann condition. For a fixed size of an oil droplet, depending on the size of the water droplet, either it attains the axisymmetric position or tends to migrate toward the edge of the oil droplet. For a water droplet-to-oil droplet at volume ratio Vw/Vo ≥ 0.05, stable axisymmetric configuration is achieved; for Vw/Vo < 0.05, migration of water droplet is observed. The stability and migration of water droplets of size above and below critical size, respectively, are explained using the force balance at the three-phase contact line and film tension. The larger and smaller droplets that initially attain the axisymmetric position or some radial position, respectively, evaporate continuously and thus migrate toward the edge of the oil droplet. The radial location and migration of the water droplets of different initial sizes with respect to time are studied. Experiments with water droplets on a flat oil-air interface did not show migration, which signified the role of the curved oil-air interface for droplet migration. Finally, coalescence of water droplets of size above the critical size at the axisymmetric position is demonstrated. Our compound droplet studies could be beneficial for applications involving droplet transport where contamination due to direct contact and pinning of droplets on solid surfaces is of concern. Migration and coalescence of water droplets on curved oil-air interfaces could open new frontiers in chemical and biological applications including multiphase processing and biological interaction of cells and atmospheric chemistry.

CHEMICAL DEMULSIFICATION OF MODEL WATER-IN-OIL EMULSIONS WITH LOW WATER CONTENT BY MEANS OF IONIC LIQUIDS

The demulsification of model water-in-oil (w/o) emulsions containing 1% wt. water by [Omim][PF6] and Aliquat® 336 ionic liquids (IL) as demulsifiers was investigated in batch mode at different temperatures (30, 45 and 60 °C) and demulsifier concentrations (2.5×10‒3, 1.2×10‒2 and 2.9×10‒2 mol L‒1). The model oil is a mixture n-heptane/toluene (70/30% wt.) with 1% wt. of Span® 83 as a surfactant. Experimental results showed that the main differences in demulsification dynamics between systems containing IL and blank (i.e., in the absence of demulsifier) are detected at 30 °C and for short demulsification times (t≤4 h). In particular, the demulsification efficiency is 8, 21 and 74% for the blank sample, [Omim][PF6] and Aliquat® 336 tested under the more concentrated IL condition, respectively. The superior demulsification performances of Aliquat® 336 with respect to [Omim][PF6] were related to the greater molecular weight and more hydrophobic character of its cation, likely able to induce a faster desorption of the surfactant at the w/o interface and consequently promoting water droplet coalescence. Moreover, the kinetic demulsification data were successfully interpreted by an empirical pseudo-first order model. In general, the obtained outcomes encourage future research efforts in the use of ionic liquids for the removal of low water fractions from w/o emulsions.

Numerical and experimental study on electric field driven coalescence of binary falling droplets in oil

Abstract The movement and coalescence of binary water droplets falling in stagnant oil exposed to an external electric field is studied. The electrostatic forces in competition with the viscous forces effect on electrocoalescence of the droplets. An electrohydrodynamic (EHD) model is developed based on Computational Fluid Dynamics technique in conjunction with an electrostatic model. Volume of Fluid (VOF) approach is applied in the hydrodynamic model which able to consider circulation streams inside and outside of drops, surface tension effects and interface tracking. The effect of applied voltage amplitude, initial distance between the drops, and skew angle of the electric field are investigated. Both the simulation and experimental results verified an improvement in electrocoalescence rate could be achieved using either a stronger electric field or closer drops. It was also revealed that a larger skew angle of the electric field slows down the approaching movement of the drops until they become relatively aligned with the field. The numerical results are compared and validated against the experimental observations. The velocity vectors and the pressure profiles are analyzed to improve understanding of the microscale phenomena in electrocoalescence.

Improvement of rheological properties of invert drilling fluids by enhancing interactions of water droplets using hydrogen bonding linker

The conventional method to improve rheological properties of oil based muds (OBMs) by dispersing organoclays in continuous oil phase has generally been considered inefficient and deficient. This paper presented a new method to improve the rheological properties by using a hydrogen bonding linker (HBL) that can replace organoclay to enhance the interactions of water droplets through hydrogen bonding. Hydrogen bonding linker was synthesized by reaction of dimer acid and diethanolamine in the presence of TiCl4 as catalyst at 130°C, and characterized by FTIR. Emulsions with different concentrations of HBL were then characterized by bottle tests, focused beam reflectance measurements (FBRM) and rheological measurements. Experimental results show that optimum stability is achieved when the concentration of HBL is 2g/L. But when concentration of HBL exceeds 2g/L, the trend of instability is increasingly stronger because HBL leads to coalescence of water droplets. Emulsions with HBL demonstrate a more fragile gel structure and a stronger thixotropic behavior than emulsion without HBL. The application of HBL in an organoclay-free OBM shows that a higher yield point (YP), higher Gel strengths and a higher electrical stability (ES) were achieved in comparison with the conventional OBM.