Electroemulsification in a Uniform Electric Field - ACS Publications

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Electroemulsification in a uniform electric field Rahul Bapusaheb Karyappa, Ankita Naik, and Rochish Thaokar Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b03188 • Publication Date (Web): 30 Nov 2015 Downloaded from http://pubs.acs.org on December 9, 2015

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Electroemulsification in a uniform electric field Rahul B. Karyappa,† Ankita V. Naik,‡ and Rochish M. Thaokar∗,† Department of Chemical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai - 400 076, India., and Department of Chemical Engineering, Institute of Chemical Technology, Nathalal M. Parekh Marg, Matunga (East), Mumbai 400 019, Maharashtra, India. E-mail: [email protected] Phone: +91 (22) 2576 7241. Fax: +91 (22) 2572 6895

Abstract Emulsification using electric fields is an easy alternative to flow-induced drop breakup and the former is reported to be more effective and economical than the latter, especially when the medium phase is poorly conducting and highly viscous. The emulsification of a coarse water-in-oil emulsion in a uniform electric field is studied. We perform a detailed experimental analysis of the effect of applied electric field strength and the duration of applied electric field on the drop size distribution. The average diameter as well as the time for emulsification decreases with an increase in the intensity of the electric field. Moreover a narrow size distribution is observed. An average size of a few microns of the dispersed phase could be achieved. New breakup mechanisms at play in the emulsification process are discussed. Identified mechanisms involve charged lobe disintegration, charged drop breakup, chain formation in which several water droplets are interconnected by thin water bridges, electrospraying and charge transfer ∗

To whom correspondence should be addressed Indian Institute of Technology Bombay, Mumbai, India ‡ Institute of Chemical Technology, Mumbai, India †

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and coalescence. The study shows that charged drop disintegration could be the key mechanism of fine emulsification of an initially electrically neutral coarse emulsion.

Introduction Emulsions find many applications in almost every industry like food, dairy, pharmaceutical, paint, petroleum, road, agriculture etc. 1,2 Depending upon the dispersed phase drop size distribution, the emulsions can be scaled as macro-emulsions (typically in the range of 2–20 µm) or micro-emulsions (also called as mini-emulsions or nano-emulsions or ultrafine emulsions, typically in the range of 20–200 nm). 2 Simple emulsions, especially nano-emulsions, have potential applications as antiviral agents, drug delivery, advanced materials and even in the cosmetics industry. 3–6 On the contrary macro-emulsions have applications in beauty, cleaning, fabric care as well as in automotive products. 7 For preparing emulsions, energy can be provided by either trituration, homogenization, agitation or heat. 2 The classical methods of emulsification are typically multistep processes and thereby time consuming. The resulting drop size distribution in such emulsions is wide. Mechanical agitation, which is inefficient in terms of energy consumption, is widely used in the chemical industry because of its simplicity and hence used in applications in a variety of operations. The input energy is given directly to the bulk liquids and hence only part of this energy is actually transferred to the liquid interface. Alternative approaches include, smooth stretching of the liquid interface by the action of capillary or hydrodynamic forces. 8–12

These forces stretch the interface until either they overcome the restoring surface tension

forces or induce capillary instabilities. Although these methods permit good control over the droplet size distributions, the sizes of the drops thus obtained are limited to a few tens of microns. An alternative approach to mechanical emulsification is the use of electric forces. 13–15

The electric-field-assisted emulsification is reported to be more efficient and economical

than conventional mechanically agitated systems especially when the suspending phase is


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poorly conducting 16 and highly viscous. In such systems the energy required per unit volume of the dispersed phase to create an emulsion by application of electric field can be less by two orders of magnitude compared to that of mechanically agitated systems. 17,18 Apart from efficient formation of emulsions, the droplets undergo oscillations, in pulsed electric fields, that greatly enhance mass transfer. 19–21 Electrostatic methods for production of ultrafine powders, printing, paint spraying and crop spraying are important applications that have been developed over the years. 22 Emulsion phase contactor for liquid-liquid extraction 23 and dispersion reactors for multiphase reactive systems 24–27 are other applications of electric field assisted emulsification in chemical processing. The deformation and breakup of a liquid drop in an electric field to produce a dispersion of liquid in any liquid has been studied for a few decades for applications discussed earlier. Zeleny 28 showed that when drops were held at the end of the capillary tubes and electric field was applied (such that the destabilizing electric stress overcomes the interfacial tension), they disintegrated by formation of a pointed end from which a narrow jet was emitted. Similar kind of breakup was observed when a water drop passes between two parallel plates in an electric field. 29,30 The pointed end or a conical shape formed at the poles of a drop, from which a jet emanates is known as Taylor cone which has a semi-angle of 49.3o . 31 The emanated jet undergoes a varicose instability leading to formation of tiny charged droplets. Electrosprays have been studied in both direct current (DC) 14,15,32 as well as alternating current (AC) 33,34 electric fields. The droplets generated using DC electric field have charges of the same polarity as the dispensing electrode while the AC electric fields produce neutral or oppositely charged droplets. Electrospraying of both liquid-in-air and liquid-liquid systems have been studied in the literature. Kim et. al. 35 and He et. al. 36 produced emulsion drops of varying sizes using an electric field in a flow-focusing microfluidic device. Almost all the electric-field-assisted emulsification systems, till now, have been studied by the application of non-uniform electric field. On the contrary most studies on breakup of a single drop are conducted using uniform electric fields. It is therefore useful to see if uniform electric fields


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can be directly employed for emulsification and is the focus of the present work. When a neutral conducting drop is suspended in a dielectric fluid in a uniform electric field, it undergoes axisymmetric prolate spheroid deformation (which is a result of a balance between the electric stress, 0 E02 , and the capillary stress, γ/a) due to contrast in the dielectric properties. The axis of symmetry always lies parallel to the direction of the applied electric field. 37 The deformation depends upon the capillary number Ca = (a0 E02 )/γ (a, , 0 , E0 and γ are the drop radius, the dielectric constant of the medium, the permittivity of the free space, the magnitude of the uniform electric field and the surface tension, respectively). As the capillary number is increased beyond a certain critical value (Cac ), the electric stress overcomes the interfacial tension resulting in drop breakup. The steady-state deformation of the drop is independent of the viscosity ratio (λ) of the drop to that of medium phase. But the breakup is dependent upon λ and Ca. 38 The three axisymmetric shapes prior to breakup (ASPB) observed are lobes, non-pointed ends and pointed ends. It is observed that the lobes, non-pointed ends and pointed ends in ASPB give way to non-axisymmetric shapes at breakup (NASB) modes of charged lobe disintegration, open jets and regular jets, respectively. 38 There have been few experimental studies available on the water-in-oil emulsification in uniform electric field. Raut et al. 39 reported novel observations of catastrophic breakup of water drops containing surfactant molecules, but did not study the drop size distribution or the effect of electric field or time duration of application of electric field on the drop size distribution. Motivated by the need to understand emulsification in uniform electric field, the specific objectives of the current work are as follows: (i) To understand the effect of intensity of uniform electric field and the duration of application of electric field on the drop size distribution, which has not hitherto reported in the literature. (ii) To understand the mechanism behind the drop breakup in such a multiple drop system.


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Experimental setup and methods Materials We consider the emulsification of a coarse water-in-oil emulsion when subjected to a uniform electric field. The objective here is to examine the effect of electric field strength and time of application of electric field on the droplet size distribution. The experimental realization was achieved using castor oil (780 cSt, Sigma Aldrich) as the medium phase and deionised (DI) water as the conducting drop phase. Deionised water (mili-Q conductivity, σ = 0.0016 S/m) was used to prepare an aqueous solution for the drop phase. Very high electric field strengths were realized by increasing the electric field. Table 1 presents relevant physical parameters for an experimental system used in this work. The ratio R of the conductivity of the medium to the drop phase is very small, validating the assumption of a perfectly conducting drop. 40 Table 1: The measured material properties and the experimental system used to study the water-in-oil emulsification in a dielectric fluid. Notation: W, Milli-Q water; C, castor oil. R is the ratio of conductivities of oil to aqueous phase. Notation

µ Pa.s

σ S.m−1

ρ Kg.m−3

W C

0.001 0.79

0.0016 4 × 10−11

996 970

Continuous Phase

Drop Phase

λ

R

C

W

0.00126

∼ 10−8

Experimental set-up and procedure A coarse water-in-oil emulsion is prepared by magnetic stirring of water in oil as shown schematically in figure 1(a). The microscopic image of the coarse emulsion and average droplet size distribution are also shown. A schematic diagram of the experimental set-up for studies of the deformation and breakup of a drop, drop pair or emulsion is shown in figure 1(b). The set-up consisted of a rectangular plastic cuvette with a high-voltage generator 5 ACS Paragon Plus Environment

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Number fraction average size disribution

Oil

Water Water-in-Oil Emulsion

Water-in-Oil Emulsion (Coarse)

Volume fraction average size disribution

(a)

(b)

Figure 1: (a) Schematic of coarse water-in-oil emulsion preparation by magnetic stirring of water and oil. Also shown are the microscopic image of the wmulsion prepared and number average as well as volume average drop size distribution. Scale bar 300 µm. (b) Schematic of the experimental setup: (1, High-voltage supply; 2, High speed camera; 3, Stereo zoom; 4, Cuvette with copper electrodes placed parallel to each other; 5, Fibre optic illuminator; 6, Computer control. connected to two parallel copper plate electrodes of size 10 mm (length) × 3 mm (breadth) × 45 mm (height). The distance between the two electrodes was kept at 4 mm. A DC electric field was generated by applying a high voltage across the electrodes. The high-voltage 6 ACS Paragon Plus Environment

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source (ES60P-5W, Gamma High Voltage Research Inc., Ormond Beach, FL) was capable of generating DC voltages up to 60 kV in steps of 120 V. The coarse emulsion formed by magnetic stirring was placed inside the space between the two copper electrodes, and electric field was applied. The deformation and breakup of the coarse emulsion droplets were recorded with a high-speed camera (Phantom V 12, Vision Research, USA) and observations were made using a stereo zoom microscope (SMZ1000, Nikon Instruments Inc., 20X objective) with a frame speed of 3000 frames per second. The drop distribution sizes were measured using the ImageJ software. The densities of the drop phase and the surrounding medium phase were quite similar and therefore buoyancy effects were discounted. A drop with an average size of 200 µm had a very low terminal settling velocity in the high-viscosity medium and negligible displacement in the gravity direction was observed during the course of the experiments. The time required for the drop to move its own distance was of the order of 30–40 seconds as compared to the breakup time scale which was about 60 ms. The drop size distribution of the emulsions were represented using the Log-Normal distribution and is calculated as 

y = y0 + √

A e 2πwx

x

2 

 − ln   xc    2w2  

       

where y0 , xc , w and A are offset, mean, standard deviation and area respectively.


(1)

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Results and discussions In-situ emulsification of a single drop and two drops suspended in a dielectric fluid: λ = 0.00126 In a preliminary series of experiments three systems were considered (i) breakup of a single drop, (ii) breakup of two drops placed collinear in the direction of the applied electric field and (iii) line joining their centers being perpendicular to the field or one below other and were studied using high-speed videography. The drop size distributions after application of electric field were estimated (See Supplementary Information for further details). 0.8

0.8

Single

Two 0.6

Log-Normal

a

P ( )

0.6

0.4

0.4

Distribution

0.2

0.2

0

0 0

10

20

30

40

50

0

10

20

30

40

50

0.8 E

Two

0

Drop/s

0.6

a

avg

Single

a

P ( )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(

m)

7.40 ± 0.16

Two

0.4

(110

m)

m, 90

Two

0.2 (98

m, 80

m)

8.24 ± 0.78

7.71 ± 1.15

0 0

10

20

a(

30

40

50

m)

Figure 2: Droplet size and Log-Normal size distributions after complete breakup of a single drop, two drops placed side by side and two drops placed one below another. Figure 2 shows the final drop size distribution of the fine emulsion that was formed from the breakup of a single drop as well as two drops placed side by side and two drops placed one below another. A log-normal distribution fit yielded a mean emulsion droplet size of


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about 8 µm for all the three cases. The standard deviation though indicating increasing polydispersity as one goes from single drop to parallel to perpendicular configuration. Thus, although the mechanics of breakup is different the mean drop size is fairly insensitive to the mode of breakup. It is interesting to note that the mean size of the drops (∼ 8 µm) is much smaller than the size obtained from the critical capillary number. This indicated that the secondary breakup could be predominantly by the breakup of charged drops formed due to contact with electrodes.

Emulsification of a coarse water-in-oil emulsion A uniform electric field was applied to a coarse emulsion for two different times, t = 4 s and 10 s. The field was varied from 7.5 kV/cm to 20 kV/cm. The breakup occurs by several mechanisms which are discussed later in this section. Figure 3(a) shows the breakup of a coarse emulsion when E0 = (7.5 kV/cm) and t = 4 s. On application of field, most droplets with Ca < 0.21 deformed into prolate shapes whereas the ones with Ca > Cac disintegrated into smaller droplets by forming lobes at the ends (figure 3, t = 0.33 s). A bead-like chain of water droplets was formed connecting the two electrodes (figure 3(a), t = 0.79 s) which breaks into smaller droplets at the thin cylindrical connections between the droplets (the biggest drop in figure 3(a), has Ca = 0.28). Formation and breaking of the bead-like chain of droplets continued till all the droplets in the bead-like chain disintegrate into smaller droplets (figure 3(a), t = 4.00 s). The droplets which were below Cac were deformed into steady-state prolate spheroids and kept migrating electrophoretically from one electrode to other by the charging discharging mechanism. Thus, the electroemulsification is effective only for the drops which satisfy Ca > Cac in the size distribution of the initial emulsion. When the electric field was increased to E0 = (17.5 kV/cm) and for total time of application of electric field t = 4 s, the breakup sequence of a coarse emulsion is as shown in figure 3(b). Almost all the droplets were above Cac after application of the electric field of E0 = 9 ACS Paragon Plus Environment

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̃t ( s) 0

0.79

0.33

4.00

(a)

0

0.0330

0.0480

0.3723

0.6560

1.5413

(b)

Figure 3: Emulsification of coarse water-in-oil emulsion suspended in castor oil. (a) (E0 , t) = (7.5 kV/cm, 4 s) and (b) (E0 , t) = (17.5 kV/cm, 4 s). The black shading at the edges of the images shows the electrodes (Scale bar 200 µm). (17.5 kV/cm). The drops deformed into prolate shape and became unstable by forming lobes at both the ends(figure 3(b), t = 0.0330 s). Breakup of lobes was observed as discussed in the breakup of two drops placed side by side in parallel configuration. After breakup of the lobes the droplets migrated towards opposite electrodes and break by charging discharging mechanism (figure 3(b), from t = 0.3723 s to 0.6560 s). After about t = 0.7760 s (not shown in the figure) the cuvette was filled with a dense cloud of small droplets. The droplet cloud became so dense indicating a large increase in the number density of the drops at the same volume fraction, that the illumination was inadequate for further observation (figure 3(b), 10 ACS Paragon Plus Environment

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t = 1.5413 s). The dynamics was rapid and the breakup of all the drops was observed in about t = 1.5 s. The drop size distributions for t = 4 s and varying electric fields are plotted in figure 4. The error bars on the columns are deviations from the mean number fraction, which are obtained for the investigation of 3 different sets of experiments. The drop size distribution from a single experiment was based on measurement of diameters of about 600 drops measured before application of electric field and about 1000 drops measured after application of electric field. For measuring the drop sizes, the photographs were taken at 10 different locations as well as planes in the cuvette. The graphs of number fraction P (d) versus drop diameter d and volume fraction φ(d) versus diameter d are plotted for comparison purposes. The initial drop size distribution (before application of electric field) (figure 4(a)) was found to be very wide, varying between 10 µm – 550 µm. The error bars on the columns are the deviations from the mean number fraction, which are obtained for the investigation of 18 different sets of experiments. When electric field of (7.5 kV/cm) was applied, the drops having sizes between 300 – 550 µm were above the critical capillary number and break into smaller droplets mainly in the size range of 100 – 300 µm (figure 4(b)). As electric field was increased further (from 10 kV/cm – 20 kV/cm), more and more drops, being above critical capillary number, disintegrated into smaller and smaller droplets. This can also be observed in the shift of columns towards smaller drop sizes. When an electric field of 20 kV/cm was applied, an almost uniform size distribution was observed (figure 4(c)). It should be noted that we were able to perform only one experiment at (20 kV/cm) due to discharge observed at the sharp edges at the top of the electrodes. Figure 5 (a-b) shows the average cumulative frequencies for different applied electric fields (t = 4 s and 10 s). The cumulative curves became gradually steeper for successive increase in the electric field, confirming the dependence of emulsification on electric field and indicating decreasing polydispersity. In case of t = 10 s, almost similar estimates for drop size and polydispersity were observed as for t = 4 s indicating that the chosen t in these set


0.21

Coarse Emulsion 0.18 0.15

davg = 95

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m

1 0.12 2 0.09 3 4 0.06 dc Coarse Emulsion 5 dc Eo dmax dc 0.03 6 (kV /cm) (µm) (µm) 0 7 0 100 200 300 400 500 600 7.5 365 346 dc < dmax 8 d ( m) 10 198 194 dc < dmax 9 10 12.5 150 124 dc < dmax (a) 11 15 88 86 dc < dmax 12 17.5 84 63 dc < dmax 13 d ( m) 20 22 48 dc > dmax 14 15 (b) (a) (b) 16 17 18 19 0.16 0.16 7.5 kV/cm 20 7.5 kV/cm 21 0.12 22 0.12 23 0.08 24 t = 4 s 0.08 0.7 25 0.04 26 0.6 t = 4 s 0.7 27 0.04 0.00 0.5 0 100 200 300 400 500 28 0.6 0.4 d ( m) 29 0.00 0.5 0.3 30 0 100 200 300 400 500 0.2 31 0.4 d ( m) 0.1 32 Before 0.3 33 0 7.5 kV/cm 10 kV/cm 0.2 0 100 200 300 400 500 600 34 12.5 kV/cm 15 kV/cm 35 0.1 17.5 kV/cm 20 kV/cm 0.6 36 0 0.4 37 0 100 200 300 400 500 600 38 0.2 39 0.6 0 40 0 10 20 30 40 50 60 70 80 90 100 41 0.4 42 d, ( m) 0.2 43 44 0 (c) 45 0 10 20 30 40 50 60 70 80 90 100 46 Figure 4: Effect of electric field (time of application of electric field, t = 4 s) on the drop size 47 , ( m) 48 distributions. (a) The normal size distribution of the corase emulsion. The number fractions 49 are plotted versus volume average diameter d. The error bars represent the deviations from 50 the mean number fraction, which were obtained for the investigation of 3 different sets of (c) 51 experiments. (b) The table shows comparison of the drop sizes before and after application 52 53 of electric field. The was varied from 7.5 kV/cm to t20=kV/cm. Figure 4: Effect of electric fieldelectric (timefield of application of electric field, 4 s) on dthe drop size c ; critical 54 size of a drop, calculated using Ca = 0.22, above which all drops can undergo breakup, c distributions. (a) The normal size distribution of the corase emulsion. The number fractions 55 d ; maximum drop size in a given drop size distribution and dσ2 ; variance in the drop size max 56 plotted versus are volume average diameter d. The error bars represent the deviations from distribution. (c) Log-Normal drop size distribution after varying the electric field from 7.5 57 the mean number wereshows obtained the investigation 3 different sets of 58 kV/cm tofraction, 20 kV/cm.which The inset fitting for of log-normal distributionofcurve over normal 59 experiments. (b) Thecurve. table shows comparison of the drop sizes before and after application distibution 60 of electric field. The electric field was varied from 7.5 kV/cm to 20 kV/cm. dc ; critical 12 ACS Paragon Plus Environment size of a drop, calculated using Cac = 0.22, above which all drops can undergo breakup, = 5303

d

P ( )

d

dmax = 520

0.21

= 194

m

m for 4 kV/0.4 cm

0.18

= 346

m for 3 kV/0.4 cm

0.15

davg = 95

0.12

d

d

P ( )

2

m

m

2

= 5303

m

0.09

dmax = 520

0.06

dc

= 194

m

m for 4 kV/0.4 cm

dc

0.03

= 346

m for 3 kV/0.4 cm

0

0

100

200

300

400

500

600

Normal distribution

Log-Normal distribution

P ( )

Normal distribution

Before

7.5 kV/cm

10 kV/cm

12.5 kV/cm

15 kV/cm

17.5 kV/cm

20 kV/cm

d

P ( )

d

P ( )

d

P ( )

d

P ( )

d

P ( )

d

Log-Normal distribution

d

dmax ; maximum drop size in a given drop size distribution and dσ2 ; variance in the drop size

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1.2

1.2

t = 4 s

Cumulative Frequency

Cumulative Frequency

1.0

0.8

0.6

Before 3 kV

0.4

4 kV 5 kV 6 kV

0.2

7 kV

t = 10 s 1.0

0.8

0.6

0.4

Before 3 kV 4 kV

0.2

5 kV

8 kV

6 kV

0

0 0

100

200

300

d(

400

500

600

0

100

200

m)

(a)

300

d(

400

500

600

m)

(b)

350

d(

m)

300

Average

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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250

200

V

Before application of electric field

t = 4 s

S

t = 10 s

150

100

N

50

0 Before

3 kV

4 kV

5 kV

6 kV

7 kV

8 kV

Electric Field

(c) E0 (kV /cm) 7.5 10 12.5 15 17.5 20

dmean dσ2 dstdev (µm) (µm2 ) (µm) 86.45 2703.03 51.99 64.78 1132.49 33.65 52.28 479.39 21.89 34.38 157.71 12.56 22.41 119.18 10.92 8.72 11.25 3.35

Figure 5: Cumulative droplet size distribution based on number fraction. (a) t = 4 s. (b) t = 10 s. Only lines are for drop size cumulative frequency before application of electric field and symbols are for drop size cumulative frequency after application of electric field and (c) Average diameter of the water-in-oil emulsion as a function of applied electric field E0 and time t of application of electric field. V , S and N denotes volume average diameter, Sauter diameter and number average diameter respectively. The table shows comparison of mean, variance and standard deviation in the drop size distribution after application of electric field (when varied from 7.5 kV/cm to 20 kV/cm. 13 ACS Paragon Plus Environment

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of experiments did not play any significant role on the final drop size distribution for the corresponding applied electric field. For a volume fraction of water 0.5% (V/V) in castor oil, the variations of number average diameter (d10 ), volume average diameter (d30 ) and Sauter diameter (d32 ), as a function of the applied electric field E0 and time (t) of application of electric field are presented in figure 5 (c). The average diameters before application of electric field are denoted by V for volume average diameter, S for Sauter diameter and N for number average diameter. In all the cases, the average diameter decreases with the applied electric field as well as time of application of electric fields.

Droplet breakup mechanisms Figure 6 shows different breakup mechanisms involved when a very high electric field is applied to a coarse emulsion. There are broadly three breakup conditions discussed in the literature, (1) a neutral drop breakup placed in a uniform electric field (breakup occurs above Taylor limit of field), 31 (2) charged drop breakup when no electric field is applied (breakup occurs above Rayleigh limit of charge), 41 and (3) a charged drop breakup placed in a uniform electric field (a general case where the Maxwell’s stress results from both the net charge as well as the applied electric field and breakup occurs between Taylor limit of field and Rayleigh limit of charge). 42 In the present case, since the breakup occurs in the presence of electric field, modes 1 and 3 are expected to be at play. When a neutral drop is placed in a dielectric fluid in a uniform electric field, it undergoes breakup above a critical Ca. When a very high electric field was applied the formed lobes break by the charged lobe disintegration mode 38 leading to emulsification (figure 6(a), t = 44.8 ms). Figure 6(b) shows that a drop near an electrode (negative in this case, t = 378 ms) translates towards that electrode and acquires a charge (negative in this case). The drop then migrates towards the opposite electrode. The critical field required for the breakup of 14 ACS Paragon Plus Environment

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t (ms)

0

43.4

44.8

(a) 378.0

389.3

475.3

-

+ (b)

t ( s) 3.38

3.46

3.50

3.83

3.96

4.38

5.54

5.58

6.08

6.13

6.25

9.21

0.13

0.29

2.92

3.21

(c) 0.38

(d) 3.25

2

3

0.50

1

2

3.29 1

2

1

2

1

4

3.42

After contact

3.50 1

2

3

3.54

1,3

1

4

3.63 1,3,4

4 1,3 coalescence

1,3,4 coalescence

(e)

Figure 6: Drop breakup mechanisms. (a) Neutral drop breakup by charged lobe disintegration mode (E0 = kV). (b) Charged drop breakup (Rayleigh breakup) (E0 , t) = (17.5 kV/cm, 4 s). (c) Chain formation (E0 , t) = (7.5 kV/cm, 10 s). (d) Electrospray (E0 , t) = (17.5 15 kV/cm, 4 s). The black shadingACS at the edges of the images shows the electrodes. (e) Charge Paragon Plus Environment transfer and coalescence of drops (E0 , t) = (7.5 kV/cm, 10 s). (Scale bar = 200 µm).

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a charged drop is reduced by an extent proportional to the net charge on the drop. 42 The charged drop (figure 6(b), t = 389.3 ms and 475.3 ms) then undergoes asymmetric breakup (tear shaped drop) that manifests in an electrospray as shown in figure 6(b) (t = 475.3 ms) leading to emulsification. Another interesting kind of breakup is caused by the cooperative action of many drops (figure 6(c)). Conductor drops align in the direction of applied field due to free charge based dipolar interaction between the drops. Few such drops are seen in figure 6(c), t = 3.38 s. The proximity of these drops can lead to formation of thin liquid bridges between them, leading to a beaded, interconnected structure as shown in figure 6(c), t = 3.50 s and 3.83 s. The drops seem to move towards the electrodes depending upon their polarity (figure 6(c), t = 3.83 s is contact, and t = 3.96 s and 4.38 s, movement towards left). This process leads to thinning of the interconnecting bridge, which could break (figure 6(c), t = 5.54 s) and reform (figure 6(c), t = 5.58 s), admitting more drops, such as in figure 6(c) t = 6.08 s. These drops can continue to exhibit to and fro motion along the reformed liquid bridge (figure 6(c), t sequence from 6.08 s to 6.25 s). The movement of the drop is shown with a white arrow. The shedding of drops (figure 6(c), t = 4.38 s) and breakup of liquid bridge, leads to emulsification. Old position New position

E0 t A

1

2

3

t1 > t B

1

1'

2

2'

3

3'

3''

3'

t 2 > t1 > t C

1''

1'

2''

2'

t3 > t 2 > t 1 > t D

1'''

1''

2'''

2''

3'''

3''

Figure 7: Schematic showing sequence of bead-like chain of water droplet formation and its movement along the liquid bridge. 16 ACS Paragon Plus Environment

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This whole sequence of bead-like chain of water droplet formation and its movement along the liquid bridge is shown schematically in figure 7. Let us consider the initial position of the drops in the liquid bridge as shown in figure 7 (A) at time t. At time t1 (> t), the drops move towards the negative electrode without disturbing the liquid bridge (figure 7 (B)). Note that in figure 7, the drops shown with the dashed lines are the new positions corresponding to their previous positions, shown as drops with continuous lines. Once the drop (drop 30 in figure 7(B)) touches the electrode it acquires charge and transfers it to the other drops (drop 10 and 20 ) in the liquid bridge. After acquiring charges the droplets are repelled and travel towards the opposite electrode until they touch the other electrode (figure 7 (C) and (D) for t2 (> t1 > t) and t3 (> t2 > t1 > t) respectively). At high electric fields one end of the formed chain was observed to be connected to the electrode (negative electrode, (figure 6(d), t = 0.13 s) while the other end was found to emanate tiny droplets on to the electrode (towards positive electrode, figure 6(d), t = 0.29 s) leading to emulsification by a mechanism similar to that of DC electrospraying from a pendant drop attached to a needle electrode. 31,43 The reaction force of the emanated tiny droplets push the mother droplet away from the positive electrode (figure 6(d), t = 0.38 s and t = 0.50 s). The electrospray is shown with a white arrow (figure 6(d), t = 0.38 s). Another mechanism observed during the breakup of the droplets was charge transfer and coalescence of the droplets. This mechanism with a sequence of snapshots of droplets labeled 1, 2, 3 and 4 is shown in figure 6(e). We conjecture that droplet 2 is negatively charged while droplet 1 is neutral in figure 6(e) t = 2.92 s. Droplet 1 then approaches droplet 2 by induced charge and dipolar interaction and on contact undergo charge transfer and partial coalescence 44 such that droplet 1 is now negatively charged. It moves towards the positive electrode and encounters the drop pair 3,4 with which it coalesces in two steps (figure 6(e) at t = 3.54 s and t = 3.63 s respectively). The final droplet (figure 6(e), t = 3.63 s), because of its increased size after coalescence, deforms further and translates towards the positive electrode. After contacting the positive electrode it acquires charge and charged


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drop breakup is observed leading to emulsification.

Other observations During the drop breakup in a coarse emulsion apart from the above discussed breakup mechanisms several other observations were observed (figure 8). Although the applied field is uniform DC, droplet shape oscillations were observed as shown in figure 8(a). The drops indicated as 1 and 2 exhibited oscillatory prolate drop deformations similar to that observed in an alternating electric field of low frequency. The drops kept on oscillating until they reached the electrode where they acquired charge from the electrode and charged drop breakup was observed as discussed in the previous section (figure 6(b)). The oscillations in the drop deformation are plotted in the figure 8(b). When the droplets in a coarse emulsion were emulsified, several tiny droplets were formed and it was found that most of them resided for longer time at the negative electrode (figure 8(c), t = 3.178 s). After residing at the negative electrode, a plume of droplets was then repelled towards positive electrode (figure 8(c), t = 3.708 s) where they acquired positive charge and almost instantaneously returned back to the negative electrode again (figure 8(c), t = 4.721 s). It suggest that there might be difference in the charging mechanisms and corresponding charging time of the droplets at the positive and the negative electrodes (figure 8(c)).

Conclusions The average diameter of the emulsion is found to decrease with the applied electric field as well as the duration of application of electric field. It is found that the process of emulsification is almost complete by t = 4 s. Different breakup mechanisms responsible for the formation of fine emulsions are identified that include charged lobe disintegration, charged drop breakup, formation and breakup of bead-like structure of water droplets interconnected by


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0.5 Drop 1 Drop 2

0.4

t (ms)

0.3

319.0

325.3

333.0

339.7

347.0

353.0

1

D 0.2

f = (2

= (2

)/ t

)/ (14 x 10

-3 )

0.1 f = 448 Hz

14 ms

0.0 310

2

320

330

340

350

360

t , (ms)

(a)

(b) Eo

t ( s) 2.092

+

3.708

3.178

4.061

4.721

(c)

Figure 8: Other observations. (a) Drop shape oscillations (E0 , t) = (17.5 kV/cm, 4 s). (b) Degree of deformation of drop 1 and drop 2 (shown in figure 8(a)) with respect to time. (c) Droplets residing for more time at the negative electrode as compared to the positive electrode (E0 , t) = (12.5 kV/cm, 4 s). The white arrows show direction of movement of droplets. The black shading at the edges of the images shows the electrodes (Scale bar = 200 µm).


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thin water bridges, DC electrospraying and coalescence of water droplets after charge transfer. This indicates the significance of charged drop disintegration in an otherwise neutral, initial coarse emulsion. The most significant finding of the present work is that single drop emulsification experiments, although give a very good idea about the basic mechanism, could be very limited in scope in understanding the emulsification of coarse emulsions. This clearly underlines the importance of both many body interactions and charged drop breakup in an initial neutral emulsion. Several such mechanisms have been observed and reported in this work. The prediction of temporal as well as the electric field dependence of drop size distribution is therefore going to be a challenge and the behavior and design is therefore expected to be highly system dependent.

Acknowledgement The authors would like to thank Department of Science and Technology, New Delhi, India for funding the work. The authors also would like to thank Dr. Manu Vashishtha for Spinning drop video tensiometer experiments.

Supporting Information Available This material is available free of charge via the Internet at http://pubs.acs.org/.

References (1) Sjöblom, J., Ed. Encyclopedic handbook of emulsion technology; Marcel Dekker: New York, 2001. (2) Banker, G. S., Siepmann, J., Rhodes, C., Eds. Modern Pharmaceutics, 4th ed.; Taylor & Francis, 2002.


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Graphical TOC Entry Emulsification of Perfectly Conducting Drops E0

Coarse Emulsion Before application of E0

3 kV/0.4 cm

Effect of E0, Time duration of application of E0

Fine Emulsion After application of E0

7 kV/0.4 cm

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Breakup Mechanism


Electroemulsification in a Uniform Electric Field - ACS Publications

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