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Coalescence characteristics of silica nanoparticle-laden droplets with a planar interface under direct current electric field
Accepted Manuscript Title: Coalescence characteristics of silica nanoparticle-laden droplets with a planar interface under direct current electric field Authors: Donghai Yang, Yongxiang Sun, Limin He, Xiaoming Luo, Yuling Lu, ¨ Haoran Yin, Xue Xia, Huihui Zhang PII: DOI: Reference:
S0263-8762(18)30532-X https://doi.org/10.1016/j.cherd.2018.10.010 CHERD 3380
To appear in: Received date: Revised date: Accepted date:
4-3-2018 6-10-2018 9-10-2018
Please cite this article as: Yang , Donghai, Xiaoming, Lu, ¨ Yuling, Yin, Haoran, Xia, characteristics of silica nanoparticle-laden under direct current electric field.Chemical https://doi.org/10.1016/j.cherd.2018.10.010
Sun, Yongxiang, He, Limin, Luo, Xue, Zhang, Huihui, Coalescence droplets with a planar interface Engineering Research and Design
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Coalescence characteristics of silica nanoparticle-laden droplets with a planar interface under direct current electric field
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Donghai YANGa, b, *, Yongxiang SUNa, b, Limin HEa, b, Xiaoming LUOa, b, Yuling LÜa, b, Haoran YIN a, b, Xue XIA a, b, Huihui ZHANG a, b (a College of Pipeline and Civil Engineering, China University of Petroleum, Qingdao 266580, Shandong, P R China b Shandong Provincial Key Laboratory of Oil & Gas Storage and Transportation Safety, China University of Petroleum, Qingdao 266580, P R China)
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Corresponding Author is Donghai Yang Tel: +86 532 86981224-86 Fax: +86 532 86981222 E-mail: [email protected]
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Graphical abstract
Highlights
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Addition of silica nanoparticles affects droplet–interface electrocoalescence. Reduced interfacial tension, increased water conductivity are responsible for effect. Effect of field strength is pronounced compared to that of initial droplet size. Formation of a ring by the separation of vortices is visualized by nanoparticles. A group (electric We × Oh) is not suitable for coalescence containing nanoparticles.
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Abstract
Electric dehydration method is widely used in oil industry to separate water from oil. Droplets resting on the oil–water interface discharge their liquids into the bulk phase,
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and the process can be altered by the change of electric field strength (E) or properties of water. Phenomenon of silica (SiO2) nanoparticle-laden droplets coalescing with a planar interface under direct current electric field was observed using a high-speed digital video camera. The effects of initial droplet diameter (D), E, and weight percent of nanoparticles in deionized water (C) were studied experimentally. The results showed that increasing D or E contributed to the formation of secondary droplets. Addition of SiO2 nanoparticles to water produced two competing effects: reduction in oil–water interfacial tension and increase in water conductivity, and a shift in the dominant effect occurred with the increase of E. Furthermore, formation of a ring by the separation of vortices was visualized by movement of nanoparticles in experiments, and its downward moving velocity decreased with time, which is small compared to that reported in literature. Finally, it was confirmed that the dimensionless WO number is not suitable for describing the volume fraction of secondary droplets for droplet– interface coalescence containing nanoparticles. Keywords: Electrocoalescence; Interface; Nanoparticle; Droplet; Interfacial tension; Conductivity
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1. Introduction
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With the increase in the worldwide energy demand and depletion of recoverable reserves, enhanced oil recovery (EOR) has become progressively more important (Ragab and Hannora, 2015). For most of the oil fields in China, more than 60% of crude oil could not be extracted by conventional secondary oil extraction (Kong and Ohadi, 2010). EOR methods, including chemical flooding technique (surfactant, polymer, and alkaline), thermal flooding technique, and microbial flooding technique, are widely used to further extract the remaining oil (Ragab and Hannora, 2015). As a new technique, nano-fluid flooding has been proven to be effective in enhancing oil recovery in laboratory researches and field applications. Nano-fluids are defined as nanoparticles suspended in traditional heat transfer fluids such as water. Silica (SiO2) nanoparticles are commonly used for preparing nano-fluids because of their stable properties and cost effectiveness, and the weight-percent of nanoparticles in water is usually 0.1–2% (Ogolo et al., 2012; Parvazdavani et al., 2012; Ragab, 2015; Tarek, 2015). 1.1. Silica nanoparticles-stabilized emulsions In crude oil extraction, the injected SiO2 nano-fluids are re-extracted to the ground along with oils, forming oil–water mixtures containing nanoparticles. Several depressurizations in the extraction and transportation processes further lead to the conversion of these mixtures into emulsions (Berg et al., 2010; Eow et al., 2001; Sjöblom, 2001), which are then stabilized by the SiO2 nanoparticles.
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Fig. 1 - Microscopy images of colloid-stabilized droplets (And et al., 2003)
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The investigation of mechanism of SiO2 nanoparticles stabilizing emulsions indicates that SiO2 nanoparticles can hinder collisions between droplets by adsorbing at the oil– water interface to form a single or multi-layer film (Fig.1) (And et al., 2003). The bending resistance of the granular film and the interfacial viscoelasticity ensure that the dispersed droplets do not easily coalesce when in motion (Huang et al., 2016; Hunter et al., 2008). Moreover, particles get partially flocculated to form three-dimensional network structures according to interaction forces in water (Huang et al., 2016). These network structures can increase the viscosity of emulsions, thus slowing down droplet motion. 1.2. Droplet–interface electro-coalescence Oil–water emulsions should be separated into their constituent phases before subsequent operations, as required by quality in refinement process, environmental regulations, and customer specifications (Mousavichoubeh et al., 2011a, b; Mousavi et al., 2014). Electrostatic dehydration is an effective strategy for separating water-in-oil emulsions, enabling the oil having less than 0.5% water content. In micro-scale, electrostatic dehydration promotes oil–water separation by promoting droplet–droplet coalescence and droplet–interface coalescence in an electro-coalescer.
Fig. 2 - (a) A droplet resting on the interface (being separated by a thin oil film) and (b) Forces acting on droplet under an electric field during the droplet–interface coalescence process
With the rapid development of high-speed camera technology, droplet–interface electro-coalescence has been widely studied (Aryafar and Kavehpour, 2009). Fig. 2
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shows two stages in droplet–interface coalescence. Fig. 2(a) demonstrates that a droplet rests on the interface, separated by a thin oil film. Under an electric field, the droplet and interface are polarized and the induced charges unevenly distribute at its surface. Then, a strong electrical attractive force is generated between the bottom of the droplet and the interface, promoting the formation of ‘liquid bridge’ to connect the droplet and interface (Aryafar and Kavehpour, 2009; Atten and Aitken, 2010; Lundgaard et al., 2006). Fig. 2(b) exhibits that the ‘liquid bridge’ acts as a channel for both the charge and water transfer (Lundgaard et al., 2006). The droplet is in the same potential as the interface due to the redistribution of charges through the ‘bridge’. Therefore, the attractive force between the droplet and interface disappears, and an upward electric force FE applied by the electrode stretches the droplet (Taylor, 1964). The force acting on the droplet also includes the drag force FD, buoyancy force FB, additional pressure ΔP, and gravity force G (Chiesa et al., 2006; Yang et al., 2018). All these forces together determine the droplet–interface coalescence results. Under these forces, the ‘liquid bridge’ expands rapidly, almost reaching the same width as the water droplet, and then decreases (Blanchette and Bigioni, 2006). The height of droplet also gradually decreases. When the coalescence process comes to an end and no small droplets are left over the interface, the coalescence is named as complete coalescence. In contrast, when partial coalescence occurs, small droplets are left behind (Blanchette and Bigioni, 2006; Charles and Mason, 1960; Chen et al., 2006; Paulsen et al., 2014). The most extreme case is that the droplet bounces from the interface, and the phenomenon is named as non-coalescence (Allan and Mason, 1962; Chabert et al., 2005; Hamlin et al., 2012; Mousavi et al., 2014; Ristenpart et al., 2012). In these three patterns of droplet–interface coalescence, partial coalescence is detrimental to dehydration because of the challenge of removing fine secondary droplets. Therefore, study of partial coalescence is significantly important. Charles and Mason (1960) first observed droplet–interface partial coalescence in 1960s. Since then, many scholars have carried out a series of studies on coalescence mechanisms. J.A.F. Plateau (1873) ascribed the partial coalescence to Rayleigh–Plateau instability generated by capillary waves. However, Blanchette and Bigioni (2006) denied Plateau’s viewpoint of Rayleigh–Plateau instability by numerical simulation, and suggested the determination of partial coalescence by a competition between the vertical and horizontal collapses of the coalescing droplet, both of which are driven by the interfacial tension. Mousavichoubeh et al. (2011b) argued that the competing processes of necking and pumping determine whether secondary droplets are formed. 1.3. Factors influencing droplet–interface coalescence The influencing factors of droplet–interface coalescence can be divided into physical factors (for e.g., the initial droplet size, interfacial tension, and conductivity) and external factors (for e.g., the release height, and electric field strength). Allan and Mason (1961) found that electric fields could effectively accelerate the coalescence process; however, higher field strength led to partial coalescence. Mousavichoubeh et al. (2011b) reported that larger initial diameter of droplet and higher release height resulted in the formation of larger secondary droplets. Moreover, hydrophilic surfactants can significantly reduce the oil–water interfacial tension, promoting the
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formation of secondary droplets. Vivacqua et al. (2016) reported that increased water conductivity within a certain range could reduce the volume of secondary droplets. Hamlin et al. (2012) proposed the existence of a critical conductivity beyond which the coalescence transforms from partial to non-coalescence. The appropriate dimensionless numbers, including the Weber Number We (describing droplet deformation and necking due to the electric field) , the Ohnesorge Number Oh (describing the pumping of water into the continuous phase in the process of coalescence), the Bond Number Bo (reflecting the relative influence of gravity and interfacial tension factors), and the density and viscosity ratios between the droplet and matrix phases well govern the droplet–interface coalescence patterns (Aryafar and Kavehpour, 2006; Aryafar and Kavehpour, 2009; Chen et al., 2006; Eow and Ghadiri, 2002; Williams, 1989). 2𝜀2 𝑅𝐸 2 We = (1) 𝜎12 𝜇 Oh = (2) (𝜌𝜎12 𝑅)0.5
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(ρ1 -ρ2 )gD2 Bo = (3) 𝜎12 ρ ρ* = 1 (4) ρ2 μ μ* = 1 (5) μ2 where subscripts 1 and 2 represent the droplet and oil, respectively. R, D are the initial droplet radius and diameter, respectively. E is the applied electric field strength, σ12 is
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the oil–water interfacial tension, and is the dynamic viscosity. ε represents the
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permittivity, ρ denotes the density, and g is gravitational acceleration. The critical Oh value for partial coalescence is usually around 1, above which coalescence would ideally become complete coalescence. Nevertheless, Blanchette and Bigioni (2006) obtained a lower critical Oh value, which was about 0.026±0.001. Chen et al. (2006) observed complete coalescence for a much lower Bo·Oh = 3.19 × 10−6. Furthermore, Mousavichoubeh et al. (2011b) coupled We and Oh, giving the WO number as (WO=We×Oh =
2r0.5 𝜀2 E2 μ ρ0.5 σ1.5
) to describe the volume fraction of secondary
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droplets, which had a good unification with the experimental data. 1.4. Coalescence of droplets containing nanoparticles Researches on the coalescence of droplets containing nanoparticles have rarely been reported yet. Harbottle et al. (2011) ever studied the coalescence between the SiO2 nanoparticle-laden water droplets and interface without applying electric fields. They found that the concentration and interaction energy of particles aid in determination of coalescence pattern, and the transition from partial coalescence to complete coalescence is caused by the formation of a film structure of nanoparticles. Simultaneously, this transition is always accompanied by a change in rheology, i.e. from Newtonian to non-
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2. Experimental methodology
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Newtonian fluids. Nevertheless, the concentration of particle-containing solutions used by them was far more than that used in nano-fluid flooding. Chen et al. (2013) investigated the coalescence between two NaCl droplets (12.5 wt%) containing nanoscale polymer particles (PMMA) under a high voltage electric field. They proposed that a thin film of nanoparticles formed on the droplet surface could decrease the droplet deformation under electric fields. At low electric field strengths, non-coalescence occurs due to the steric hindrance of the film. With the increase in the electric field strength, some tapered bulges are formed at the defective position of the film, providing an opportunity for droplets to contact and coalesce, and the specific coalescence pattern is determined by the size of these defects. Unfortunately, they studied droplet–interface coalescence without using nanoparticles in experiments. The nanoparticles adsorbed on the oil–water interface can move, but not stay stationary. Prior to the coalescence of two droplets, nanoparticles are observed to gather near the contact area forming a mono layer (And et al., 2003). This could keep the droplets at a finite distance, hindering interdroplet film drainage. Furthermore, nanoparticles at the planar interface are swept from the contact region by draining oil film (Stancik et al., 2004). All in all, nanoparticles may impact electrostatic dehydration by stabilizing the produced emulsions in macro-scale. In micro-scale, they may affect droplet–interface electro-coalescence, which has not been investigated in detail. In this study, we investigated the effects of initial droplet size, strength of direct current (DC) electric field, and the weight percent of nanoparticles in deionized water on droplet–interface coalescence using a high-speed camera. This study should be useful for the development of electric dehydration facilities.
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2.1. Experimental set-up and procedure
Fig. 3 - Schematic illustration of the experimental setup for observation of droplet–interface coalescence under DC electric field
The basic experimental set-up and process of the experiment are shown in Fig. 3. Realtime observation and recording of the coalescence process between droplet and interface could be realized by using a high-speed camera (NAC Hotshot 1280) equipped with a 100× lens (Mitutoyo 5× objective with a 20× tube made by Pomeas), using a frame rate of 1000 fps. A halogen lamp with four flexible fiber optic heads was used
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for lighting, and the intensity of the lighting could be accurately adjusted to facilitate focusing. Moreover, the data were obtained by image processing using the professional image processing software Image J.
Fig. 4 - Schematic illustration of the test cell
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Fig. 4 shows the structure of the micro-experimental cell, which is similar to that used by Mousavichoubeh et al. (2011b). The cell was made of Perspex to facilitate visualization of the phenomenon. A high voltage electrode was fixed at the bottom of the slider, and the grounded electrode was fixed at the insulating base. The electrodes were polished brass plates with dimensions of 90 mm × 25 mm. The slider could be moved to adjust the distance between the electrodes. In our experiments the distance was fixed at 64 mm. To generate an electric field, a high voltage electrode was connected to the output of the high voltage power amplifier (TREK 20/20C), and the grounding electrode was well grounded. A small hole with a diameter of 1 mm was made in the center of the movable slider and the high voltage electrode, so that a microsyringe could be penetrated therefrom to inject droplets into the cell. The droplet radius ranged from 914 to 1240 μm and the standard deviations were within the range of 4 to 8 μm. The released droplets would fall down slowly and then rest on the interface. Finally, the coalescence occurred and the droplet merged into the bulk phase. The experiments were performed at 22±1C. 2.2. Preparation of experimental liquids In our experiments, two different liquids were added in the experimental cell: the upper layer consisted of dimethyl silicone oil, and the lower one consisted of nano-fluids. The droplet injected in the cell was the same liquid as the lower medium. To prepare nano-fluids, SiO2 nanoparticles were purchased from Aladdin Industrial Corporation (www.aladdin-e.com) and they were characterized by Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD). The results are shown in Figs. 5 (a) and (b), respectively.
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(a) (b) Fig. 5 - (a) The SEM image of the SiO2 nanoparticles used in our experiments and (b) The infrared absorption spectrum of the SiO2 nanoparticles
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Fig. 5 (a) exhibits SiO2 nanoparticles with size of about 30nm (±10nm). XRD could detect the type of chemical groups present on the SiO2 nanoparticles. Comparison with the Standard Infrared Spectrum Atlas confirmed that the wide absorption peak at 3439.88 cm1 corresponds to the antisymmetric and symmetrical stretching vibrations of the –OH group. The strong absorption band at 1100 cm1 is attributed to Si–O–Si antisymmetric stretching vibration. The peak at 958 cm1 belongs to the bending vibration absorption peak of Si–OH, and the peaks at 800 and 469 cm1 correspond to symmetrical stretching and bending vibrations of Si–O bond. Therefore, the surface of the SiO2 nanoparticles did not contain any other impurity groups except the hydroxyl, and the particles exhibited hydrophilicity. SiO2 nanoparticles were well dispersed in deionized water by ultrasonic vibration (ultrasonic processor, Shanghai Sheng-xi Co., Ltd.) for about 20 min. Deionized water used in experiments was produced by Millipore ultrapure water systems. The weight percent of nanoparticles in deionized water (C) (i.e., the concentration of nano-fluids mentioned in the latter part of this paper) was 0.5, 1, and 2 wt%. Conductivity is a dependent variable to reflect the stability of nano-fluid. The conductivity of nano-fluids (κ) was measured using a Rex Conductivity meter, and the results were shown in Fig. 6. A good linear relationship was observed between κ and C, and the fitting formula was κ = 49.505C + 6.285. Furthermore, Figs. 7 (a)–(c) show the change of the conductivity over time for different concentrations of nano-fluids. The conductivity slightly decreased at the beginning and then remained stable. We believe that this decline is due to the reunion between the nanoparticles, thus the stable conductivity should be used for the later analysis.
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Fig. 6 - The change of conductivity over the concentration of nano-fluids
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(b) (c) Fig. 7 - The change of conductivity of nano-fluids over time Table 1. Properties of liquids used in experiments Liquids Deionized water
Conductivity (μS·cm-1) 1.57 -7
Viscosity (mPa·s)
Density (kg·m-3)
1.046
1000
Dimethyl silicone oil
2.3×10
51.408
937
0.5wt% nano-fluid
31.2
1.089
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1wt% nano-fluid
58.3
1.124
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2wt% nano-fluid
108.6
1.212
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Table 2. Interfacial tension between dimethyl silicone oil and nanoparticle-containing water
The
weight
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(mN·m-1)
0 wt%
35.68
0.5wt%
4.56
1wt%
9.57
2wt%
34.14
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The properties of liquids used in experiments are listed in Table 1, and the interfacial tensions between different concentrations of nano-fluids and dimethyl silicone oil are presented in Table 2. The viscosities of liquids were measured using an Anton Paar Physica MCR102. The density was measured using a volumetric flask. The permittivity of oil is about 2.7 and that of water is about 80, which were obtained from literature report and the supplier. The interfacial tensions were measured using a full Automatic Surface & Interface tensiometer (USA KINO Industry).
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Fig. 8 - Plot of the interfacial tension between nano-fluids and dimethyl silicone oil versus the concentration of nano-fluids (The red circle indicates minimum value of the interfacial tension within the measured range, and the concentration of nano-fluids equals to 0.5% at this point).
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Noteworthy, variation of the interfacial tension exhibited a strange behavior. The interfacial tension first decreased with the introduction of SiO2 nanoparticles, and then increased with the increase in the concentration of nano-fluids, until it became close to the deionized water–dimethyl silicone oil interfacial tension. This phenomenon was also discovered by Dong and Johnson (2003), and they explained it from the perspective of entropy. Besides, And et al. (2003) found similar irregularities in the change of interfacial tension between oil and water by adding silanized SiO2 nanoparticles, but the decrease in interfacial tension was much smaller in their experiment.
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3. Results and discussion
Fig. 9 - Sequence of a typical partial coalescence pattern for a droplet with diameter of 1240 μm under electric field strength of 192.63 kV·m1, and the concentration of nano-fluids is 1%
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Partial coalescence was observed in our experiments. A typical partial coalescence of a droplet at the interface under electric fields is shown in Fig. 9. Owing to the low viscosity ratio between the oil and nano-fluids, secondary droplets were formed even in the absence of electric fields. To normalize the volume of formed secondary droplets in partial coalescence, the volume ratio of secondary droplets to the initial droplet (Vnor) is defined as follows: Vnor=Vsecondary/Vinitial (6) Vnor could be influenced by the diameter of initial droplet, electric field strength, or concentration of nano-fluids, which were investigated in this study. To reflect the regularity of experiments, multiple experiments on each coalescence condition were performed and values of Vnor were calculated. The error bars in the figures show the standard deviation of Vnor obtained from multiple experiments under the same conditions. Owing to the discontinuous permittivity between oil and aqueous phase, the strength of DC electric field in oil should be calculated according to the following equation: 2 ⁄ε2 +H1 ⁄ε1 )ε2
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where U is the applied voltage (V); H2 is the height of the oil layer (mm)-our case: 30±0.5mm; H1 is the height of the aqueous phase layer (m)-our case: 34±0.5mm; and ε1, ε2 are the permittivity of aqueous phase and oil, respectively. According to the uncertainty analysis, the uncertainty of calculated electric field strength derived from the instrumental error is within 0 – 0.0276 kV·m1.
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4.1. Effect of initial droplet diameter and electric field strength on Vnor
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(c) (d) Fig. 10 - The effect of diameter of initial droplet on Vnor under different electric field strengths, for (a) deionized water, (b) 0.5wt% nano-fluid, (c) 1wt% nano-fluid, and (d) 2wt% nano-fluid
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Fig. 10 shows the influence of initial droplet diameter on Vnor, and the initial droplet diameters used in experiments were 914, 1046, 1152, and 1240 μm. For different concentrations of nano-fluids, Vnor increased as the initial droplet diameter increased. In particular, when the electric field strength was low, Vnor changed less with the increase in initial droplet diameter, indicating Vnor was relatively independent of the droplet size. However, some dependency on droplet size was observed when the field strength was high. These results were consistent with the conclusions by Mousavichoubeh et al. (2011b).
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(c) (d) Fig. 11 - The effect of electric field strength on Vnor at different initial droplet diameters, for (a) deionized water, (b) 0.5wt% nano-fluid, (c) 1wt% nano-fluid, and (d) 2wt% nano-fluid
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The data shown in Fig. 10 were replotted in terms of the electric field strength and presented in Fig. 11. Increased electric field strength promoted the formation of secondary droplets. It could also be seen that the effect of electric field strength was weak at low strengths; nonetheless, it was strong at high strengths. In droplet–interface coalescence, the downward flow of liquids in the droplet is mainly driven by the difference of additional pressure between the droplet and underlying bulk phase, leading to a vertical collapse of the droplet (Blanchette and Bigioni, 2006). The additional pressure (ΔP) is given by: 2 (8) ΔP R
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The external electric force (FE) can pull the droplet upward, thus hindering the vertical collapse of the droplet. Furthermore, an electrostatic repulsion between the droplet and interface promotes the horizontal collapse of the droplet (Mousavi et al., 2014; Techaumnat and Matsusaka, 2016), helping the droplet be pinched off. It is difficult to calculate the value of FE because the shape of the droplet changes constantly during coalescence process. To analyse the influence of applied electric field strength, FE acting on the droplet at the very beginning of the coalescence, i.e. when the ‘liquid bridge’ just established, is given by Eq. (9) (Disna Jayampathi Karunanayake and
Hoshino, 2010; Félici, 1966; Yang et al., 2018). At this point, charges are redistributed through the ‘liquid bridge’ and the droplet remains nearly spherical (Fig. 2(b)). FE =6.52πε2 R2 E2 (9) Therefore, large initial droplet size leads to a small difference of additional pressure between the droplet and bulk phase, and also leads to an increase in FE. Both of them act synergistically, resulting in the formation of larger secondary droplets. Moreover, increased electric field could increase FE in quadratic, which significantly
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promotes the generation of larger secondary droplets. The results of Vnor is shown in Figs. 10 and 11, concluded that the effect of electric field strength was more
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pronounced compared to that of initial droplet diameter. Vnor showed strong
dependency on droplet size and electric field strength at high field strengths.
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4.2. Effect of the concentration of nano-fluids on Vnor
(c) (d) Fig. 12 - The effect of electric field strength on Vnor at different concentrations of nano-fluids, for the initial droplet diameter of (a) 914, (b) 1046, (c) 1152, and (d) 1240 μm
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Fig. 12 shows the effect of the concentration of nano-fluids on Vnor under DC electric fields. The experimental results showed that for different initial droplet diameters, the order of Vnor was Vnor0.5wt% > Vnor1wt% > Vnor2wt% > Vnorwater at low electric field strengths. However, at high strengths it became Vnorwater > Vnor0.5wt%> Vnor1wt%> Vnor2wt%. In these two different results, the order of three different concentrations of nano-fluids was fixed at Vnor0.5wt% > Vnor1wt% > Vnor2wt%; however, Vnorwater changed from the minimum to the maximum with the increase in electric field strength. Addition of SiO2 nanoparticles to deionized water could change the properties of water by reducing the oil–water interfacial tension and increasing the water conductivity. The reduction of oil–water interfacial tension is favorable for droplet deformation, promoting the formation of secondary droplets (Mousavichoubeh et al., 2011b). The increase in the conductivity of water could also influence the formation of secondary droplets. With the increase in the conductivity of water, the charge transfer is rapid due to the reduction of the relaxation time and the amount of charge on the summit of the droplet may reduce (Vivacqua et al., 2016). According to Vivacqua et al.’s research (2016), Vnor gradually decreased to 0 as the conductivity increased to a certain range. Yang et al. (2018) also reported that increased conductivity of water could raise the critical electric field of droplet–interface partial coalescence. Therefore, high conductivity may suppress the formation of secondary droplets. Furthermore, the formation of secondary droplets may also be influenced by the solid film formed by nanoparticles at the oil–water interface (Chen et al., 2013; Harbottle et al., 2011; Huang et al., 2016; Hunter et al., 2008), and the flocculated structures (Huang et al., 2016) inside the droplet. The physical properties listed in Table. 2 exhibited that the order of the interfacial tension between the different concentrations of nano-fluids and oil was σwater > σ2wt% > σ1wt% > σ0.5wt%, which was consistent with the order of Vnor at low electric field strengths. However, the order of conductivity was κ2wt% > κ1wt% > κ0.5wt% > κwater, which was contrary to the order of Vnor at high field strengths. These indicate that the decreased interfacial tension should be dominant for promoting the formation of secondary droplets when the electric field strength is low; nonetheless, the increased conductivity due to added nanoparticles plays dominant role for suppressing the formation of secondary droplets when the strength is high. In particular, when the concentration of nano-fluids increases from 0.5 to 2 wt%, both the interfacial tension and conductivity increase, synergistically inhibiting the formation of secondary droplets. Therefore, Vnor0.5wt% > Vnor1wt% > Vnor2wt% within all tested electric field strengths. Moreover, Vnorwater is the maximum at low strengths because the interfacial tension between the deionized water and oil is the largest, and it is minimum at high strengths because the conductivity of deionized water is the least.
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(a) (b) Fig. 13 - (a) Distribution of nanoparticles without electric fields and (b) Rearrangement of nanoparticles under electric fields
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SiO2 nanoparticles used in experiments have certain hydrophilicity; therefore, not all nanoparticles can be adsorbed on the droplet surface and portion of nanoparticles stay inside the droplets. The nanoparticles adsorbed on the droplet surface form a particle film, which leads to an increase in surface strength (Chen et al., 2013; Harbottle et al., 2011; Huang et al., 2016; Hunter et al., 2008). Other particles inside the droplet are prone to flocculation (Huang et al., 2016), forming spatial structures out of order. Moreover, it has been reported that particles would be aggregated near the contact area of coalescence (And et al., 2003) and the region of ‘liquid bridge’ (Connington et al., 2015) (Fig. 13(a)). During coalescence, particles remaining near the ‘liquid bridge’ may occupy the space for the flow of liquids, resulting in flow resistance that hinders the eruption of the liquids in the coalescing droplet into the bulk phase. The suppression of flow around neck of the fluid column was found by Yue et al. (2006) when a polymer drop drains its fluid into the bulk, whose mechanism may also be instructive for that of nanoparticles. However, the existence of electric fields may rearrange the nanoparticles (Fig. 13 (b)) (Cui et al., 2014; Flatté et al., 2008; Musinski et al., 2006) and make them away from regions near the liquid bridge. This may reduce the flow resistance, thus inhibiting the formation of secondary droplets. The above mentioned descriptions stayed in relying on relevant literature for reasonable analysis, because we did not observe the direct movement of nanoparticles between the droplets and interface. Therefore, more experimental methods are required to be conducted to study the role of nanoparticles in mass transfer and breakup of ‘liquid bridge’ in droplet–interface coalescence in further study.
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4.3. Effect of nanoparticles absorbed at the planar interface on coalescence The above mentioned analysis mainly focuses on the effect of SiO2 nanoparticles in the coalescing droplet on the droplet–interface coalescence; however, lacks the study of the effect of nanoparticles adsorbed on the planar interface on coalescence. According to the previous experimental process, we studied the electro-coalescence between a droplet containing nanoparticles and a pure planar interface (i.e. without nanoparticles). Compared to previous experimental results, the differences are shown in Fig.14.
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(c) (d) Fig. 14 - (a) The effect of electric field strengths on Vnor at different concentrations of nano-fluids while the nanoparticle-containing droplet coalesces with a pure planar interface without containing nanoparticles. The initial droplet diameter is 1240 μm; (b) to (d) Comparison of the coalescence results between a nanoparticle-containing droplet and a planar interface containing nanoparticles or a pure interface (the concentration of nano-fluids is 0.5% for (b), 1% for (c), and 2% for (d))
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It could be seen that the coalescence with pure interface gives higher Vnor at high electric field strength (Fig. 14(b) and (c)), indicating that the absence of nanoparticles at the interface promotes the formation of secondary droplets. The conductivity of the conducting plane is considered to have an effect on the charge distribution between the droplet and interface (Techaumnat and Matsusaka, 2016). When the bulk phase consists of deionized water without nanoparticles, the interface is defined as a pure interface. Compared to the interface containing nanoparticles (i.e. the lower bulk phase was nanofluids), the conductivity of the bulk phase reduces. This should not be conducive to the leakage of charges from the droplet to the bulk, resulting in more charges remain in the droplet. Therefore, under electric fields FE would be more effective to pull up the droplet, leaving bigger secondary droplets after coalescence. Furthermore, the presence of nanoparticles on the contact surface between the droplet and interface could promote the occurrence of coalescence due to bridging mechanism (Malmazet et al., 2015; Wang et al., 2016). Then the absence of nanoparticles on the interface should be unfavorable for coalescence. However, compared to the coalescence with pure interface, the flow resistance due to the nanoparticles may be larger for the coalescence with the interface
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containing nanoparticles. When the concentration of nano-fluids is high, the effect of the flow resistance on promoting the formation of secondary droplets becomes relatively large, which may instead result in the coalescence with the interface containing nanoparticles gives higher Vnor instead (Fig. 14(d)).
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Fig. 15 - Time lapse images of the evolution process of the “ring” formed by the separation of vortices (the diameter of the coalescing droplet is 1240 μm, the electric field strength is 192.63
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kV·m1, and the concentration of nano-fluid droplet is 2%)
Fig. 16 - The average downward velocity of vortex ring when a nanoparticles-containing droplet coalesces with a pure planar interface
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Droplet–interface coalescence is connected with the formation of a vortex ring that propagates into the receiving liquid (Ray et al., 2013). In these experiments the vortex ring was visualized by the nanoparticles, as shown in the shaded area in Fig. 15. As the droplet is connected with the interface through the ‘liquid bridge’ and the coalescence occurs above the interface, the coalescing droplet discharges its liquid into the pool through a laminar jet. The erupting liquid quickly rolls up, giving rise to the vortexes (Peña-Polo et al., 2014). As the coalescence process proceeds, the growing vortexes fed by the drainage of liquids inside the droplet, continue to move downwards and form a ring-shape. The ‘liquid bridge’ gradually shrinks until it is pinched off, resulting in less
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amount of liquids discharged by the droplet. Therefore, the jet contracts and decays into a thin thread, eventually diffusing away. Moreover, the leading edge of the jet is unstable and rolls up into the nascent vortex, while only a small amount of mass at its center flows down undisturbed, crossing the plane of the ring and bulging the lower cap (Peña-Polo et al., 2014). Fig. 15 exhibits that the diameter of the vortex ring increased with the increase in its penetration depth and time. Moreover, Fig. 16 shows that the velocity of the vortex ring decreased with time. These results were consistent with the findings of Shankar et al. (1995) and Ray et al. (2013). Shankar and Kumar (1995) suggested that it is the drop surface energy which generates the vortex ring. Hamlin et al. (2012) accepted this viewpoint and they estimated that the vortex velocity should be equal to 0.17 m·s1, but the real average velocity they measured was approximately 0.1 m·s1. In our experiments, the average velocity was much smaller than 0.1 m·s1 and we believe that it is due to the presence of nanoparticles which dissipate a lot of kinetic energy required for the eruption.
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4.5. Relationship between the Vnor and criteria dimensionless number WO
(c) (d) Fig. 17 - Unification of the data for the (a) pure, (b) 0.5 wt%, (c) 1 wt%, and (d) 2wt% coalescence system by WO number
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Fig. 18 - Unification of all data for secondary droplet formation for all concentrations of nano-fluids used in our experiments by WO number
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Fig. 17 demonstrates that for different concentrations of nano-fluids, Vnor increased with elevated WO number for different initial droplet diameters and a certain degree of unification of data was obtained. Fig.18 shows the replotting of data presented in Fig. 17 in the same picture and the results were interesting. The result showed that when the aqueous phase was deionized water or 2% nano-fluids, the values of (WO×103) were approximately between 0 and 2, and Vnor increased rapidly within this range, which was nearly linear with (WO×103). For 1 wt% and 0.5 wt% nano-fluids, the range of (WO×103) was about 2–6 and 6–20, respectively. Vnor increased relatively slowly with the increase in (WO×103).
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According to the definition of the WO number (WO =
2r0.5 𝜀2 E2 μ ρ0.5 σ1.5
), as the ranges of initial
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droplet diameter and applied electric field strength are same in our experiments, the main reason for different ranges of (WO×103) for different aqueous phases is the irregular change in interfacial tension. Table 2 summarizes that the interfacial tension of 2% nano-fluids–oil system is close to that of deionized water-oil system and they are maximum, correspondingly their calculated values of (WO×103) are close and also the least. The interfacial tension of 0.5 wt% nano-fluids–oil is minimum, thus the (WO×103) is the largest. The interfacial tension of 1 wt% nano-fluids–oil is in between, and the range of (WO×103) also lies in the middle. Mousavichoubeh et al. (2011b) conducted droplet–interface coalescence experiments using deionized water and surfactant solutions. Surfactants can also significantly reduce oil–water interfacial tension, and the corresponding calculated WO values are large. However, the secondary droplets generated by surfactant droplets are also large, resulting in a large Vnor. Therefore, a good unification of data by WO number was obtained for their experiments. However, for droplet–interface coalescence in the presence of nanoparticles, except for reducing interfacial tension nanoparticles also increase water conductivity and form surface films. Both of these effects could suppress the formation of secondary droplets, resulting in Vnor of nanoparticle-containing coalescence system being only slightly larger or smaller than that of pure water coalescence system. Therefore, no unification of all data by WO number was obtained in our experiments. This unexpected result reflects the complexity of the electro-
coalescence system containing nanoparticles. In a subsequent study, a lot more systematic explorations will be carried out to seek the new quantitative formula to evaluate electro-coalescence systems containing nanoparticles.
Conclusions
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In this study, effects of the initial droplet diameter, electric field strength, and concentration of nano-fluids on the SiO2 nanoparticle-laden droplet–interface coalescence were studied experimentally using a high-speed digital video camera. The results showed that larger initial droplet diameter and larger electric field strength could facilitate the formation of larger secondary droplets. The effect of electric field strength was more pronounced compared to that of initial droplet diameter, and the normalized volume of secondary droplets showed strong dependency on droplet diameter and electric field strength at high field strengths. Nanoparticles may affect coalescence by: (1) reducing interfacial tension, (2) increasing water conductivity, and (3) forming particle film at droplet surface and spatial structures inside the droplet. The draining of the droplet fluid into the bulk may be suppressed due to the flow resistance, caused by particle spatial structures and particles accumulated near the ‘liquid bridge’; however, these structures would be rearranged under electric fields. At low applied electric field strengths, the decreased interfacial tension played a dominant role in promoting the formation of secondary droplets. In contrast, at high strengths the increased conductivity became dominant for suppressing the formation of secondary droplets. Apart from the effect of nanoparticles in the coalescing droplet on the coalescence, the absence of nanoparticles at the planar interface was verified to have influence on the formation of secondary droplets through comparative tests. Droplet–interface coalescence is connected with the formation of a vortex ring that propagates into the bulk. The ring was visualized by nanoparticles this time, and its diameter increased with the increase in its penetration depth and time. Furthermore, the velocity of the vortex ring was small compared to that reported in literature and it decreased with time, because of the dissipation of kinetic energy by nanoparticles. Finally, it was confirmed that good unification of experimental data was not obtained by using the dimensionless WO number for nanoparticle-containing coalescence system, because it cannot reflect the complex effects of nanoparticles on the coalescence. The complexity of nanoparticle-containing electrocoalescence is still rarely studied and further investigations are definitely required to be conducted by focusing on the fundamental mechanism. The interaction behaviors of nanoparticles are hard to verify experimentally; therefore, numerical simulation methods or molecular dynamics method can be considered in the future study. Acknowledgements This work is supported by grants from the National Natural Science Foundation of China (Grant No. 51704318), the Natural Science Foundation of Shandong Province, China (Grant No. ZR2017BEE008), and the Fundamental Research Funds for the Central Universities of China (Grant No. 16CX02001A, 15CX08008A). Introduction
of Talent Research Start-up Fund (Grant No. YJ201601016).
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S0263-8762(18)30532-X https://doi.org/10.1016/j.cherd.2018.10.010 CHERD 3380
To appear in: Received date: Revised date: Accepted date:
4-3-2018 6-10-2018 9-10-2018
Please cite this article as: Yang , Donghai, Xiaoming, Lu, ¨ Yuling, Yin, Haoran, Xia, characteristics of silica nanoparticle-laden under direct current electric field.Chemical https://doi.org/10.1016/j.cherd.2018.10.010
Sun, Yongxiang, He, Limin, Luo, Xue, Zhang, Huihui, Coalescence droplets with a planar interface Engineering Research and Design
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Coalescence characteristics of silica nanoparticle-laden droplets with a planar interface under direct current electric field
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Donghai YANGa, b, *, Yongxiang SUNa, b, Limin HEa, b, Xiaoming LUOa, b, Yuling LÜa, b, Haoran YIN a, b, Xue XIA a, b, Huihui ZHANG a, b (a College of Pipeline and Civil Engineering, China University of Petroleum, Qingdao 266580, Shandong, P R China b Shandong Provincial Key Laboratory of Oil & Gas Storage and Transportation Safety, China University of Petroleum, Qingdao 266580, P R China)
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Corresponding Author is Donghai Yang Tel: +86 532 86981224-86 Fax: +86 532 86981222 E-mail: [email protected]
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Graphical abstract
Highlights
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Addition of silica nanoparticles affects droplet–interface electrocoalescence. Reduced interfacial tension, increased water conductivity are responsible for effect. Effect of field strength is pronounced compared to that of initial droplet size. Formation of a ring by the separation of vortices is visualized by nanoparticles. A group (electric We × Oh) is not suitable for coalescence containing nanoparticles.
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Abstract
Electric dehydration method is widely used in oil industry to separate water from oil. Droplets resting on the oil–water interface discharge their liquids into the bulk phase,
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and the process can be altered by the change of electric field strength (E) or properties of water. Phenomenon of silica (SiO2) nanoparticle-laden droplets coalescing with a planar interface under direct current electric field was observed using a high-speed digital video camera. The effects of initial droplet diameter (D), E, and weight percent of nanoparticles in deionized water (C) were studied experimentally. The results showed that increasing D or E contributed to the formation of secondary droplets. Addition of SiO2 nanoparticles to water produced two competing effects: reduction in oil–water interfacial tension and increase in water conductivity, and a shift in the dominant effect occurred with the increase of E. Furthermore, formation of a ring by the separation of vortices was visualized by movement of nanoparticles in experiments, and its downward moving velocity decreased with time, which is small compared to that reported in literature. Finally, it was confirmed that the dimensionless WO number is not suitable for describing the volume fraction of secondary droplets for droplet– interface coalescence containing nanoparticles. Keywords: Electrocoalescence; Interface; Nanoparticle; Droplet; Interfacial tension; Conductivity
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1. Introduction
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With the increase in the worldwide energy demand and depletion of recoverable reserves, enhanced oil recovery (EOR) has become progressively more important (Ragab and Hannora, 2015). For most of the oil fields in China, more than 60% of crude oil could not be extracted by conventional secondary oil extraction (Kong and Ohadi, 2010). EOR methods, including chemical flooding technique (surfactant, polymer, and alkaline), thermal flooding technique, and microbial flooding technique, are widely used to further extract the remaining oil (Ragab and Hannora, 2015). As a new technique, nano-fluid flooding has been proven to be effective in enhancing oil recovery in laboratory researches and field applications. Nano-fluids are defined as nanoparticles suspended in traditional heat transfer fluids such as water. Silica (SiO2) nanoparticles are commonly used for preparing nano-fluids because of their stable properties and cost effectiveness, and the weight-percent of nanoparticles in water is usually 0.1–2% (Ogolo et al., 2012; Parvazdavani et al., 2012; Ragab, 2015; Tarek, 2015). 1.1. Silica nanoparticles-stabilized emulsions In crude oil extraction, the injected SiO2 nano-fluids are re-extracted to the ground along with oils, forming oil–water mixtures containing nanoparticles. Several depressurizations in the extraction and transportation processes further lead to the conversion of these mixtures into emulsions (Berg et al., 2010; Eow et al., 2001; Sjöblom, 2001), which are then stabilized by the SiO2 nanoparticles.
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Fig. 1 - Microscopy images of colloid-stabilized droplets (And et al., 2003)
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The investigation of mechanism of SiO2 nanoparticles stabilizing emulsions indicates that SiO2 nanoparticles can hinder collisions between droplets by adsorbing at the oil– water interface to form a single or multi-layer film (Fig.1) (And et al., 2003). The bending resistance of the granular film and the interfacial viscoelasticity ensure that the dispersed droplets do not easily coalesce when in motion (Huang et al., 2016; Hunter et al., 2008). Moreover, particles get partially flocculated to form three-dimensional network structures according to interaction forces in water (Huang et al., 2016). These network structures can increase the viscosity of emulsions, thus slowing down droplet motion. 1.2. Droplet–interface electro-coalescence Oil–water emulsions should be separated into their constituent phases before subsequent operations, as required by quality in refinement process, environmental regulations, and customer specifications (Mousavichoubeh et al., 2011a, b; Mousavi et al., 2014). Electrostatic dehydration is an effective strategy for separating water-in-oil emulsions, enabling the oil having less than 0.5% water content. In micro-scale, electrostatic dehydration promotes oil–water separation by promoting droplet–droplet coalescence and droplet–interface coalescence in an electro-coalescer.
Fig. 2 - (a) A droplet resting on the interface (being separated by a thin oil film) and (b) Forces acting on droplet under an electric field during the droplet–interface coalescence process
With the rapid development of high-speed camera technology, droplet–interface electro-coalescence has been widely studied (Aryafar and Kavehpour, 2009). Fig. 2
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shows two stages in droplet–interface coalescence. Fig. 2(a) demonstrates that a droplet rests on the interface, separated by a thin oil film. Under an electric field, the droplet and interface are polarized and the induced charges unevenly distribute at its surface. Then, a strong electrical attractive force is generated between the bottom of the droplet and the interface, promoting the formation of ‘liquid bridge’ to connect the droplet and interface (Aryafar and Kavehpour, 2009; Atten and Aitken, 2010; Lundgaard et al., 2006). Fig. 2(b) exhibits that the ‘liquid bridge’ acts as a channel for both the charge and water transfer (Lundgaard et al., 2006). The droplet is in the same potential as the interface due to the redistribution of charges through the ‘bridge’. Therefore, the attractive force between the droplet and interface disappears, and an upward electric force FE applied by the electrode stretches the droplet (Taylor, 1964). The force acting on the droplet also includes the drag force FD, buoyancy force FB, additional pressure ΔP, and gravity force G (Chiesa et al., 2006; Yang et al., 2018). All these forces together determine the droplet–interface coalescence results. Under these forces, the ‘liquid bridge’ expands rapidly, almost reaching the same width as the water droplet, and then decreases (Blanchette and Bigioni, 2006). The height of droplet also gradually decreases. When the coalescence process comes to an end and no small droplets are left over the interface, the coalescence is named as complete coalescence. In contrast, when partial coalescence occurs, small droplets are left behind (Blanchette and Bigioni, 2006; Charles and Mason, 1960; Chen et al., 2006; Paulsen et al., 2014). The most extreme case is that the droplet bounces from the interface, and the phenomenon is named as non-coalescence (Allan and Mason, 1962; Chabert et al., 2005; Hamlin et al., 2012; Mousavi et al., 2014; Ristenpart et al., 2012). In these three patterns of droplet–interface coalescence, partial coalescence is detrimental to dehydration because of the challenge of removing fine secondary droplets. Therefore, study of partial coalescence is significantly important. Charles and Mason (1960) first observed droplet–interface partial coalescence in 1960s. Since then, many scholars have carried out a series of studies on coalescence mechanisms. J.A.F. Plateau (1873) ascribed the partial coalescence to Rayleigh–Plateau instability generated by capillary waves. However, Blanchette and Bigioni (2006) denied Plateau’s viewpoint of Rayleigh–Plateau instability by numerical simulation, and suggested the determination of partial coalescence by a competition between the vertical and horizontal collapses of the coalescing droplet, both of which are driven by the interfacial tension. Mousavichoubeh et al. (2011b) argued that the competing processes of necking and pumping determine whether secondary droplets are formed. 1.3. Factors influencing droplet–interface coalescence The influencing factors of droplet–interface coalescence can be divided into physical factors (for e.g., the initial droplet size, interfacial tension, and conductivity) and external factors (for e.g., the release height, and electric field strength). Allan and Mason (1961) found that electric fields could effectively accelerate the coalescence process; however, higher field strength led to partial coalescence. Mousavichoubeh et al. (2011b) reported that larger initial diameter of droplet and higher release height resulted in the formation of larger secondary droplets. Moreover, hydrophilic surfactants can significantly reduce the oil–water interfacial tension, promoting the
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formation of secondary droplets. Vivacqua et al. (2016) reported that increased water conductivity within a certain range could reduce the volume of secondary droplets. Hamlin et al. (2012) proposed the existence of a critical conductivity beyond which the coalescence transforms from partial to non-coalescence. The appropriate dimensionless numbers, including the Weber Number We (describing droplet deformation and necking due to the electric field) , the Ohnesorge Number Oh (describing the pumping of water into the continuous phase in the process of coalescence), the Bond Number Bo (reflecting the relative influence of gravity and interfacial tension factors), and the density and viscosity ratios between the droplet and matrix phases well govern the droplet–interface coalescence patterns (Aryafar and Kavehpour, 2006; Aryafar and Kavehpour, 2009; Chen et al., 2006; Eow and Ghadiri, 2002; Williams, 1989). 2𝜀2 𝑅𝐸 2 We = (1) 𝜎12 𝜇 Oh = (2) (𝜌𝜎12 𝑅)0.5
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(ρ1 -ρ2 )gD2 Bo = (3) 𝜎12 ρ ρ* = 1 (4) ρ2 μ μ* = 1 (5) μ2 where subscripts 1 and 2 represent the droplet and oil, respectively. R, D are the initial droplet radius and diameter, respectively. E is the applied electric field strength, σ12 is
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the oil–water interfacial tension, and is the dynamic viscosity. ε represents the
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permittivity, ρ denotes the density, and g is gravitational acceleration. The critical Oh value for partial coalescence is usually around 1, above which coalescence would ideally become complete coalescence. Nevertheless, Blanchette and Bigioni (2006) obtained a lower critical Oh value, which was about 0.026±0.001. Chen et al. (2006) observed complete coalescence for a much lower Bo·Oh = 3.19 × 10−6. Furthermore, Mousavichoubeh et al. (2011b) coupled We and Oh, giving the WO number as (WO=We×Oh =
2r0.5 𝜀2 E2 μ ρ0.5 σ1.5
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droplets, which had a good unification with the experimental data. 1.4. Coalescence of droplets containing nanoparticles Researches on the coalescence of droplets containing nanoparticles have rarely been reported yet. Harbottle et al. (2011) ever studied the coalescence between the SiO2 nanoparticle-laden water droplets and interface without applying electric fields. They found that the concentration and interaction energy of particles aid in determination of coalescence pattern, and the transition from partial coalescence to complete coalescence is caused by the formation of a film structure of nanoparticles. Simultaneously, this transition is always accompanied by a change in rheology, i.e. from Newtonian to non-
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Newtonian fluids. Nevertheless, the concentration of particle-containing solutions used by them was far more than that used in nano-fluid flooding. Chen et al. (2013) investigated the coalescence between two NaCl droplets (12.5 wt%) containing nanoscale polymer particles (PMMA) under a high voltage electric field. They proposed that a thin film of nanoparticles formed on the droplet surface could decrease the droplet deformation under electric fields. At low electric field strengths, non-coalescence occurs due to the steric hindrance of the film. With the increase in the electric field strength, some tapered bulges are formed at the defective position of the film, providing an opportunity for droplets to contact and coalesce, and the specific coalescence pattern is determined by the size of these defects. Unfortunately, they studied droplet–interface coalescence without using nanoparticles in experiments. The nanoparticles adsorbed on the oil–water interface can move, but not stay stationary. Prior to the coalescence of two droplets, nanoparticles are observed to gather near the contact area forming a mono layer (And et al., 2003). This could keep the droplets at a finite distance, hindering interdroplet film drainage. Furthermore, nanoparticles at the planar interface are swept from the contact region by draining oil film (Stancik et al., 2004). All in all, nanoparticles may impact electrostatic dehydration by stabilizing the produced emulsions in macro-scale. In micro-scale, they may affect droplet–interface electro-coalescence, which has not been investigated in detail. In this study, we investigated the effects of initial droplet size, strength of direct current (DC) electric field, and the weight percent of nanoparticles in deionized water on droplet–interface coalescence using a high-speed camera. This study should be useful for the development of electric dehydration facilities.
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2.1. Experimental set-up and procedure
Fig. 3 - Schematic illustration of the experimental setup for observation of droplet–interface coalescence under DC electric field
The basic experimental set-up and process of the experiment are shown in Fig. 3. Realtime observation and recording of the coalescence process between droplet and interface could be realized by using a high-speed camera (NAC Hotshot 1280) equipped with a 100× lens (Mitutoyo 5× objective with a 20× tube made by Pomeas), using a frame rate of 1000 fps. A halogen lamp with four flexible fiber optic heads was used
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for lighting, and the intensity of the lighting could be accurately adjusted to facilitate focusing. Moreover, the data were obtained by image processing using the professional image processing software Image J.
Fig. 4 - Schematic illustration of the test cell
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Fig. 4 shows the structure of the micro-experimental cell, which is similar to that used by Mousavichoubeh et al. (2011b). The cell was made of Perspex to facilitate visualization of the phenomenon. A high voltage electrode was fixed at the bottom of the slider, and the grounded electrode was fixed at the insulating base. The electrodes were polished brass plates with dimensions of 90 mm × 25 mm. The slider could be moved to adjust the distance between the electrodes. In our experiments the distance was fixed at 64 mm. To generate an electric field, a high voltage electrode was connected to the output of the high voltage power amplifier (TREK 20/20C), and the grounding electrode was well grounded. A small hole with a diameter of 1 mm was made in the center of the movable slider and the high voltage electrode, so that a microsyringe could be penetrated therefrom to inject droplets into the cell. The droplet radius ranged from 914 to 1240 μm and the standard deviations were within the range of 4 to 8 μm. The released droplets would fall down slowly and then rest on the interface. Finally, the coalescence occurred and the droplet merged into the bulk phase. The experiments were performed at 22±1C. 2.2. Preparation of experimental liquids In our experiments, two different liquids were added in the experimental cell: the upper layer consisted of dimethyl silicone oil, and the lower one consisted of nano-fluids. The droplet injected in the cell was the same liquid as the lower medium. To prepare nano-fluids, SiO2 nanoparticles were purchased from Aladdin Industrial Corporation (www.aladdin-e.com) and they were characterized by Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD). The results are shown in Figs. 5 (a) and (b), respectively.
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(a) (b) Fig. 5 - (a) The SEM image of the SiO2 nanoparticles used in our experiments and (b) The infrared absorption spectrum of the SiO2 nanoparticles
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Fig. 5 (a) exhibits SiO2 nanoparticles with size of about 30nm (±10nm). XRD could detect the type of chemical groups present on the SiO2 nanoparticles. Comparison with the Standard Infrared Spectrum Atlas confirmed that the wide absorption peak at 3439.88 cm1 corresponds to the antisymmetric and symmetrical stretching vibrations of the –OH group. The strong absorption band at 1100 cm1 is attributed to Si–O–Si antisymmetric stretching vibration. The peak at 958 cm1 belongs to the bending vibration absorption peak of Si–OH, and the peaks at 800 and 469 cm1 correspond to symmetrical stretching and bending vibrations of Si–O bond. Therefore, the surface of the SiO2 nanoparticles did not contain any other impurity groups except the hydroxyl, and the particles exhibited hydrophilicity. SiO2 nanoparticles were well dispersed in deionized water by ultrasonic vibration (ultrasonic processor, Shanghai Sheng-xi Co., Ltd.) for about 20 min. Deionized water used in experiments was produced by Millipore ultrapure water systems. The weight percent of nanoparticles in deionized water (C) (i.e., the concentration of nano-fluids mentioned in the latter part of this paper) was 0.5, 1, and 2 wt%. Conductivity is a dependent variable to reflect the stability of nano-fluid. The conductivity of nano-fluids (κ) was measured using a Rex Conductivity meter, and the results were shown in Fig. 6. A good linear relationship was observed between κ and C, and the fitting formula was κ = 49.505C + 6.285. Furthermore, Figs. 7 (a)–(c) show the change of the conductivity over time for different concentrations of nano-fluids. The conductivity slightly decreased at the beginning and then remained stable. We believe that this decline is due to the reunion between the nanoparticles, thus the stable conductivity should be used for the later analysis.
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Fig. 6 - The change of conductivity over the concentration of nano-fluids
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(b) (c) Fig. 7 - The change of conductivity of nano-fluids over time Table 1. Properties of liquids used in experiments Liquids Deionized water
Conductivity (μS·cm-1) 1.57 -7
Viscosity (mPa·s)
Density (kg·m-3)
1.046
1000
Dimethyl silicone oil
2.3×10
51.408
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0.5wt% nano-fluid
31.2
1.089
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1wt% nano-fluid
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1.124
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2wt% nano-fluid
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1.212
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Table 2. Interfacial tension between dimethyl silicone oil and nanoparticle-containing water
The
weight
percent
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0 wt%
35.68
0.5wt%
4.56
1wt%
9.57
2wt%
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The properties of liquids used in experiments are listed in Table 1, and the interfacial tensions between different concentrations of nano-fluids and dimethyl silicone oil are presented in Table 2. The viscosities of liquids were measured using an Anton Paar Physica MCR102. The density was measured using a volumetric flask. The permittivity of oil is about 2.7 and that of water is about 80, which were obtained from literature report and the supplier. The interfacial tensions were measured using a full Automatic Surface & Interface tensiometer (USA KINO Industry).
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Fig. 8 - Plot of the interfacial tension between nano-fluids and dimethyl silicone oil versus the concentration of nano-fluids (The red circle indicates minimum value of the interfacial tension within the measured range, and the concentration of nano-fluids equals to 0.5% at this point).
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Noteworthy, variation of the interfacial tension exhibited a strange behavior. The interfacial tension first decreased with the introduction of SiO2 nanoparticles, and then increased with the increase in the concentration of nano-fluids, until it became close to the deionized water–dimethyl silicone oil interfacial tension. This phenomenon was also discovered by Dong and Johnson (2003), and they explained it from the perspective of entropy. Besides, And et al. (2003) found similar irregularities in the change of interfacial tension between oil and water by adding silanized SiO2 nanoparticles, but the decrease in interfacial tension was much smaller in their experiment.
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3. Results and discussion
Fig. 9 - Sequence of a typical partial coalescence pattern for a droplet with diameter of 1240 μm under electric field strength of 192.63 kV·m1, and the concentration of nano-fluids is 1%
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Partial coalescence was observed in our experiments. A typical partial coalescence of a droplet at the interface under electric fields is shown in Fig. 9. Owing to the low viscosity ratio between the oil and nano-fluids, secondary droplets were formed even in the absence of electric fields. To normalize the volume of formed secondary droplets in partial coalescence, the volume ratio of secondary droplets to the initial droplet (Vnor) is defined as follows: Vnor=Vsecondary/Vinitial (6) Vnor could be influenced by the diameter of initial droplet, electric field strength, or concentration of nano-fluids, which were investigated in this study. To reflect the regularity of experiments, multiple experiments on each coalescence condition were performed and values of Vnor were calculated. The error bars in the figures show the standard deviation of Vnor obtained from multiple experiments under the same conditions. Owing to the discontinuous permittivity between oil and aqueous phase, the strength of DC electric field in oil should be calculated according to the following equation: 2 ⁄ε2 +H1 ⁄ε1 )ε2
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4.1. Effect of initial droplet diameter and electric field strength on Vnor
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(c) (d) Fig. 10 - The effect of diameter of initial droplet on Vnor under different electric field strengths, for (a) deionized water, (b) 0.5wt% nano-fluid, (c) 1wt% nano-fluid, and (d) 2wt% nano-fluid
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Fig. 10 shows the influence of initial droplet diameter on Vnor, and the initial droplet diameters used in experiments were 914, 1046, 1152, and 1240 μm. For different concentrations of nano-fluids, Vnor increased as the initial droplet diameter increased. In particular, when the electric field strength was low, Vnor changed less with the increase in initial droplet diameter, indicating Vnor was relatively independent of the droplet size. However, some dependency on droplet size was observed when the field strength was high. These results were consistent with the conclusions by Mousavichoubeh et al. (2011b).
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(c) (d) Fig. 11 - The effect of electric field strength on Vnor at different initial droplet diameters, for (a) deionized water, (b) 0.5wt% nano-fluid, (c) 1wt% nano-fluid, and (d) 2wt% nano-fluid
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The data shown in Fig. 10 were replotted in terms of the electric field strength and presented in Fig. 11. Increased electric field strength promoted the formation of secondary droplets. It could also be seen that the effect of electric field strength was weak at low strengths; nonetheless, it was strong at high strengths. In droplet–interface coalescence, the downward flow of liquids in the droplet is mainly driven by the difference of additional pressure between the droplet and underlying bulk phase, leading to a vertical collapse of the droplet (Blanchette and Bigioni, 2006). The additional pressure (ΔP) is given by: 2 (8) ΔP R
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Hoshino, 2010; Félici, 1966; Yang et al., 2018). At this point, charges are redistributed through the ‘liquid bridge’ and the droplet remains nearly spherical (Fig. 2(b)). FE =6.52πε2 R2 E2 (9) Therefore, large initial droplet size leads to a small difference of additional pressure between the droplet and bulk phase, and also leads to an increase in FE. Both of them act synergistically, resulting in the formation of larger secondary droplets. Moreover, increased electric field could increase FE in quadratic, which significantly
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dependency on droplet size and electric field strength at high field strengths.
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4.2. Effect of the concentration of nano-fluids on Vnor
(c) (d) Fig. 12 - The effect of electric field strength on Vnor at different concentrations of nano-fluids, for the initial droplet diameter of (a) 914, (b) 1046, (c) 1152, and (d) 1240 μm
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Fig. 12 shows the effect of the concentration of nano-fluids on Vnor under DC electric fields. The experimental results showed that for different initial droplet diameters, the order of Vnor was Vnor0.5wt% > Vnor1wt% > Vnor2wt% > Vnorwater at low electric field strengths. However, at high strengths it became Vnorwater > Vnor0.5wt%> Vnor1wt%> Vnor2wt%. In these two different results, the order of three different concentrations of nano-fluids was fixed at Vnor0.5wt% > Vnor1wt% > Vnor2wt%; however, Vnorwater changed from the minimum to the maximum with the increase in electric field strength. Addition of SiO2 nanoparticles to deionized water could change the properties of water by reducing the oil–water interfacial tension and increasing the water conductivity. The reduction of oil–water interfacial tension is favorable for droplet deformation, promoting the formation of secondary droplets (Mousavichoubeh et al., 2011b). The increase in the conductivity of water could also influence the formation of secondary droplets. With the increase in the conductivity of water, the charge transfer is rapid due to the reduction of the relaxation time and the amount of charge on the summit of the droplet may reduce (Vivacqua et al., 2016). According to Vivacqua et al.’s research (2016), Vnor gradually decreased to 0 as the conductivity increased to a certain range. Yang et al. (2018) also reported that increased conductivity of water could raise the critical electric field of droplet–interface partial coalescence. Therefore, high conductivity may suppress the formation of secondary droplets. Furthermore, the formation of secondary droplets may also be influenced by the solid film formed by nanoparticles at the oil–water interface (Chen et al., 2013; Harbottle et al., 2011; Huang et al., 2016; Hunter et al., 2008), and the flocculated structures (Huang et al., 2016) inside the droplet. The physical properties listed in Table. 2 exhibited that the order of the interfacial tension between the different concentrations of nano-fluids and oil was σwater > σ2wt% > σ1wt% > σ0.5wt%, which was consistent with the order of Vnor at low electric field strengths. However, the order of conductivity was κ2wt% > κ1wt% > κ0.5wt% > κwater, which was contrary to the order of Vnor at high field strengths. These indicate that the decreased interfacial tension should be dominant for promoting the formation of secondary droplets when the electric field strength is low; nonetheless, the increased conductivity due to added nanoparticles plays dominant role for suppressing the formation of secondary droplets when the strength is high. In particular, when the concentration of nano-fluids increases from 0.5 to 2 wt%, both the interfacial tension and conductivity increase, synergistically inhibiting the formation of secondary droplets. Therefore, Vnor0.5wt% > Vnor1wt% > Vnor2wt% within all tested electric field strengths. Moreover, Vnorwater is the maximum at low strengths because the interfacial tension between the deionized water and oil is the largest, and it is minimum at high strengths because the conductivity of deionized water is the least.
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(a) (b) Fig. 13 - (a) Distribution of nanoparticles without electric fields and (b) Rearrangement of nanoparticles under electric fields
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SiO2 nanoparticles used in experiments have certain hydrophilicity; therefore, not all nanoparticles can be adsorbed on the droplet surface and portion of nanoparticles stay inside the droplets. The nanoparticles adsorbed on the droplet surface form a particle film, which leads to an increase in surface strength (Chen et al., 2013; Harbottle et al., 2011; Huang et al., 2016; Hunter et al., 2008). Other particles inside the droplet are prone to flocculation (Huang et al., 2016), forming spatial structures out of order. Moreover, it has been reported that particles would be aggregated near the contact area of coalescence (And et al., 2003) and the region of ‘liquid bridge’ (Connington et al., 2015) (Fig. 13(a)). During coalescence, particles remaining near the ‘liquid bridge’ may occupy the space for the flow of liquids, resulting in flow resistance that hinders the eruption of the liquids in the coalescing droplet into the bulk phase. The suppression of flow around neck of the fluid column was found by Yue et al. (2006) when a polymer drop drains its fluid into the bulk, whose mechanism may also be instructive for that of nanoparticles. However, the existence of electric fields may rearrange the nanoparticles (Fig. 13 (b)) (Cui et al., 2014; Flatté et al., 2008; Musinski et al., 2006) and make them away from regions near the liquid bridge. This may reduce the flow resistance, thus inhibiting the formation of secondary droplets. The above mentioned descriptions stayed in relying on relevant literature for reasonable analysis, because we did not observe the direct movement of nanoparticles between the droplets and interface. Therefore, more experimental methods are required to be conducted to study the role of nanoparticles in mass transfer and breakup of ‘liquid bridge’ in droplet–interface coalescence in further study.
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4.3. Effect of nanoparticles absorbed at the planar interface on coalescence The above mentioned analysis mainly focuses on the effect of SiO2 nanoparticles in the coalescing droplet on the droplet–interface coalescence; however, lacks the study of the effect of nanoparticles adsorbed on the planar interface on coalescence. According to the previous experimental process, we studied the electro-coalescence between a droplet containing nanoparticles and a pure planar interface (i.e. without nanoparticles). Compared to previous experimental results, the differences are shown in Fig.14.
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(c) (d) Fig. 14 - (a) The effect of electric field strengths on Vnor at different concentrations of nano-fluids while the nanoparticle-containing droplet coalesces with a pure planar interface without containing nanoparticles. The initial droplet diameter is 1240 μm; (b) to (d) Comparison of the coalescence results between a nanoparticle-containing droplet and a planar interface containing nanoparticles or a pure interface (the concentration of nano-fluids is 0.5% for (b), 1% for (c), and 2% for (d))
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It could be seen that the coalescence with pure interface gives higher Vnor at high electric field strength (Fig. 14(b) and (c)), indicating that the absence of nanoparticles at the interface promotes the formation of secondary droplets. The conductivity of the conducting plane is considered to have an effect on the charge distribution between the droplet and interface (Techaumnat and Matsusaka, 2016). When the bulk phase consists of deionized water without nanoparticles, the interface is defined as a pure interface. Compared to the interface containing nanoparticles (i.e. the lower bulk phase was nanofluids), the conductivity of the bulk phase reduces. This should not be conducive to the leakage of charges from the droplet to the bulk, resulting in more charges remain in the droplet. Therefore, under electric fields FE would be more effective to pull up the droplet, leaving bigger secondary droplets after coalescence. Furthermore, the presence of nanoparticles on the contact surface between the droplet and interface could promote the occurrence of coalescence due to bridging mechanism (Malmazet et al., 2015; Wang et al., 2016). Then the absence of nanoparticles on the interface should be unfavorable for coalescence. However, compared to the coalescence with pure interface, the flow resistance due to the nanoparticles may be larger for the coalescence with the interface
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containing nanoparticles. When the concentration of nano-fluids is high, the effect of the flow resistance on promoting the formation of secondary droplets becomes relatively large, which may instead result in the coalescence with the interface containing nanoparticles gives higher Vnor instead (Fig. 14(d)).
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Fig. 15 - Time lapse images of the evolution process of the “ring” formed by the separation of vortices (the diameter of the coalescing droplet is 1240 μm, the electric field strength is 192.63
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Fig. 16 - The average downward velocity of vortex ring when a nanoparticles-containing droplet coalesces with a pure planar interface
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amount of liquids discharged by the droplet. Therefore, the jet contracts and decays into a thin thread, eventually diffusing away. Moreover, the leading edge of the jet is unstable and rolls up into the nascent vortex, while only a small amount of mass at its center flows down undisturbed, crossing the plane of the ring and bulging the lower cap (Peña-Polo et al., 2014). Fig. 15 exhibits that the diameter of the vortex ring increased with the increase in its penetration depth and time. Moreover, Fig. 16 shows that the velocity of the vortex ring decreased with time. These results were consistent with the findings of Shankar et al. (1995) and Ray et al. (2013). Shankar and Kumar (1995) suggested that it is the drop surface energy which generates the vortex ring. Hamlin et al. (2012) accepted this viewpoint and they estimated that the vortex velocity should be equal to 0.17 m·s1, but the real average velocity they measured was approximately 0.1 m·s1. In our experiments, the average velocity was much smaller than 0.1 m·s1 and we believe that it is due to the presence of nanoparticles which dissipate a lot of kinetic energy required for the eruption.
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4.5. Relationship between the Vnor and criteria dimensionless number WO
(c) (d) Fig. 17 - Unification of the data for the (a) pure, (b) 0.5 wt%, (c) 1 wt%, and (d) 2wt% coalescence system by WO number
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Fig. 18 - Unification of all data for secondary droplet formation for all concentrations of nano-fluids used in our experiments by WO number
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Fig. 17 demonstrates that for different concentrations of nano-fluids, Vnor increased with elevated WO number for different initial droplet diameters and a certain degree of unification of data was obtained. Fig.18 shows the replotting of data presented in Fig. 17 in the same picture and the results were interesting. The result showed that when the aqueous phase was deionized water or 2% nano-fluids, the values of (WO×103) were approximately between 0 and 2, and Vnor increased rapidly within this range, which was nearly linear with (WO×103). For 1 wt% and 0.5 wt% nano-fluids, the range of (WO×103) was about 2–6 and 6–20, respectively. Vnor increased relatively slowly with the increase in (WO×103).
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droplet diameter and applied electric field strength are same in our experiments, the main reason for different ranges of (WO×103) for different aqueous phases is the irregular change in interfacial tension. Table 2 summarizes that the interfacial tension of 2% nano-fluids–oil system is close to that of deionized water-oil system and they are maximum, correspondingly their calculated values of (WO×103) are close and also the least. The interfacial tension of 0.5 wt% nano-fluids–oil is minimum, thus the (WO×103) is the largest. The interfacial tension of 1 wt% nano-fluids–oil is in between, and the range of (WO×103) also lies in the middle. Mousavichoubeh et al. (2011b) conducted droplet–interface coalescence experiments using deionized water and surfactant solutions. Surfactants can also significantly reduce oil–water interfacial tension, and the corresponding calculated WO values are large. However, the secondary droplets generated by surfactant droplets are also large, resulting in a large Vnor. Therefore, a good unification of data by WO number was obtained for their experiments. However, for droplet–interface coalescence in the presence of nanoparticles, except for reducing interfacial tension nanoparticles also increase water conductivity and form surface films. Both of these effects could suppress the formation of secondary droplets, resulting in Vnor of nanoparticle-containing coalescence system being only slightly larger or smaller than that of pure water coalescence system. Therefore, no unification of all data by WO number was obtained in our experiments. This unexpected result reflects the complexity of the electro-
coalescence system containing nanoparticles. In a subsequent study, a lot more systematic explorations will be carried out to seek the new quantitative formula to evaluate electro-coalescence systems containing nanoparticles.
Conclusions
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In this study, effects of the initial droplet diameter, electric field strength, and concentration of nano-fluids on the SiO2 nanoparticle-laden droplet–interface coalescence were studied experimentally using a high-speed digital video camera. The results showed that larger initial droplet diameter and larger electric field strength could facilitate the formation of larger secondary droplets. The effect of electric field strength was more pronounced compared to that of initial droplet diameter, and the normalized volume of secondary droplets showed strong dependency on droplet diameter and electric field strength at high field strengths. Nanoparticles may affect coalescence by: (1) reducing interfacial tension, (2) increasing water conductivity, and (3) forming particle film at droplet surface and spatial structures inside the droplet. The draining of the droplet fluid into the bulk may be suppressed due to the flow resistance, caused by particle spatial structures and particles accumulated near the ‘liquid bridge’; however, these structures would be rearranged under electric fields. At low applied electric field strengths, the decreased interfacial tension played a dominant role in promoting the formation of secondary droplets. In contrast, at high strengths the increased conductivity became dominant for suppressing the formation of secondary droplets. Apart from the effect of nanoparticles in the coalescing droplet on the coalescence, the absence of nanoparticles at the planar interface was verified to have influence on the formation of secondary droplets through comparative tests. Droplet–interface coalescence is connected with the formation of a vortex ring that propagates into the bulk. The ring was visualized by nanoparticles this time, and its diameter increased with the increase in its penetration depth and time. Furthermore, the velocity of the vortex ring was small compared to that reported in literature and it decreased with time, because of the dissipation of kinetic energy by nanoparticles. Finally, it was confirmed that good unification of experimental data was not obtained by using the dimensionless WO number for nanoparticle-containing coalescence system, because it cannot reflect the complex effects of nanoparticles on the coalescence. The complexity of nanoparticle-containing electrocoalescence is still rarely studied and further investigations are definitely required to be conducted by focusing on the fundamental mechanism. The interaction behaviors of nanoparticles are hard to verify experimentally; therefore, numerical simulation methods or molecular dynamics method can be considered in the future study. Acknowledgements This work is supported by grants from the National Natural Science Foundation of China (Grant No. 51704318), the Natural Science Foundation of Shandong Province, China (Grant No. ZR2017BEE008), and the Fundamental Research Funds for the Central Universities of China (Grant No. 16CX02001A, 15CX08008A). Introduction
of Talent Research Start-up Fund (Grant No. YJ201601016).
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