Effect of Interfacial Dilational Rheology on the Breakage of Dispersed Droplets in a Dilute Oil Water Emulsion 2014 Colloids and Surfaces a Physicochemical and Engineering Aspects

7/27/2019 Effect of Interfacial Dilational Rheology on the Breakage of Dispersed Droplets in a Dilute Oil Water Emulsion 2014

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Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 4350

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Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l homepage: www.elsevier .com/ locate /colsur fa

Effect ofinterfacial dilational rheology on the breakage ofdispersed

droplets in a dilute oilwater emulsion

Wei Wang, Kai Li, Pengyu Wang, Shuai Hao,Jing Gong

Beijing Key Laboratory of Urban Oil& GasDistribution Technology, Department ofMechanical andTransportation Engineering, China University of

Petroleum, 18 no. Fuxue Road, ChangpingDistrict,102249 Beijing, China

h i g h l i g h t s

Interfacial dilational properties are

introduced to explain for drop break-

age in stirred vessels. Chord length distributions measured

with a FBRM instrument are com-

pared. Strong relationship between interfa-

cial dilational elasticity and dispers-

ing modality is demonstrated.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:

Received 25 August 2013Accepted 29 August 2013

Available online xxx

Keywords:

Interfacial dilational modulus

Breakage

Chord length

Square weighted mean diameter

a b s t r a c t

Surfactants reduce the interfacial tension ofoilwater and hence favor emulsification; however, attention

has rarely been focused on the effect ofinterfacial dilational rheology on drop breakage. The aim ofthiswork is to rheologically characterize the interfacial properties of oilwater droplets and the effect on

drop breakage during emulsification. The dispersed holdups are fixed in dilute dispersion where breakage

dominates during mixing. The total number ofdroplets, the square weighted (sqr-wt) mean diameter and

the chord length distribution (CLD) are measured online using the focus beam reflectancemethod (FBRM).

Variations in the dispersed droplets are further analyzed with the interfacial dilational modulus. For

emulsions with surfactant concentrations below the critical micelle concentration (CMC), the interfacial

tension decreases as the surfactant concentration increases, whereas the mean drop diameter initially

shows an increasing trend and then decreases. However, for concentrations above the CMC, the interfacial

tension rarely decreases, whereas the mean diameter ofthe dispersed phase continues to decrease, and

thetotal number ofdroplets increases asthe concentration ofSpan 80 continues to increase. This indicates

that the interfacial dilational elasticity is a key factor that influences the pure breakage emulsification

process and the related dispersed modality.

2013 Elsevier B.V. All rights reserved.

1. Introduction

Oilwater dispersions play a significant role in petroleum pro-

duction processes wheretwo immiscible liquids are mixed, and one

Abbreviations: Sqr-wt, square weighted; CLD, chord length distribution; FBRM,

focus beam reflectance method; CMC, critical micelle concentration; RPM, revolu-

tions perminute; mM, mmol/L. Corresponding author. Tel.: +86 10 89733804.

E-mail addresses:[email protected] (W. Wang), [email protected] (J. Gong).

phase is dispersed into the other phase. In mature oilfields, oil is

often obtained mixed with water that wasinjected into the well to

improve oil recovery. Oil produced from reservoirs always contains

certain quantities of asphaltenes, resins and polymers, which are

acknowledged as surface active components, or surfactants. When

sheared in a turbulent multiphase flow, a stable oilwater emul-

sion is formed due to the major contribution of such surfactants.

The emulsification process of oilwater dispersions has been the

subjectof many studies, due to its importance to several industries

including petroleum, cosmetics, pharmacy, and food production.

Much attentionhas been paid to thephenomenology andmodeling

0927-7757/$ see front matter 2013 Elsevier B.V. All rights reserved.

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44 W.Wang et al./ Colloids andSurfaces A: Physicochem. Eng. Aspects 441 (2014) 4350

of droplet breakage and coalescence processes [14]. Al-Wahaibi

[5] studied the CLD of horizontal oilwater flows and developed

correlations for drop breakage in a turbulent flow field. Birouks

work [6] investigatedthe effects of liquid viscosityand crossairflow

velocities on the primary break-up mechanisms of a viscous liquid

jet. In regard to the breakage of droplets in emulsions, however,

the influence of surfactants adsorbed onto the oilwater interface

must be considered.

Surfactants reduce the interfacial tension and facilitate emul-

sification of such dispersions [79]. This can be explained by the

reduced Gibbs freeenergy dueto the surfactantmolecules adsorbed

onto the oilwater interfaces. Tcholakova et al. [10] studied the

effects of oil viscosity, oil volume fraction, and interfacial tension

on the dispersing modality of high volume fraction O/W emul-

sions. The reduced interfacial tension is not the only explanation

for the stability of an emulsion. Emulsions of certain lower paraf-

fin hydrocarbons do not remain stable even when the interfacial

tension is extremely low. Additionally, emulsions stabilized by

large molecule surfactants, such as asphaltenes and resins, can

remain stable for years and have interfacial tensions as high as

3040 mN/m [11].

Recently, the importance of interfacial dilational rheology on

emulsion stability has been widely recognized [1215]. Work by

many researchers [1619] demonstrates that the surface rheolo-

gical properties of the adsorption layers are the most important

factor for determining the stability of emulsions. The equilibriums

and dynamic properties of surfactants at fluid interfaces, which

modify the dilational viscoelasticity and hence the stability of

emulsions, have been explored [2023]. However, work has rarely

been focused on the effect of interfacial dilational rheology dur-

ing emulsion formation, which is where drop breakage mainly

occurs. Work by Bak et al. [24] has investigated the role of the non-

ionic surfactants Tween 20 and Tween 80 on drop breakage in an

oilwater dispersion. The influence of surfactants on drop break-

age is taken into consideration with the idea of reduced interfacial

tension. However, this is insufficient to give a proper explanation

for the case of certain stablyformed emulsions with relatively high

interfacial tensions. Hence, we argue that the factors that affectemulsification include the interfacial dilational properties, espe-

cially the dilational elasticity. If this holds, then a correlation could

be demonstrated between the interfacial dilational properties and

the dispersing modality of the emulsion.

To prove the above hypothesis, a set of experiments on interfa-

cial properties and oilwater mixing processes were conducted to

find an explicit relationship between the interfacial dilationalprop-

erties and the dropsize distribution of the dispersion. The dispersed

hold-ups were restricted to less than 10% in dilute dispersions to

eliminate the influence of drop coalescence, so that the influences

of interfacial properties would be mainly on drop breakage.

2. Theoretical background

2.1. Droplet breakage mechanisms

The breakage of droplets experiencing turbulently shearing flow

is discussed extensively in the literature, and several mechanisms

have been proposed. Liao [3] reviewed most of the mechanisms

of fluid particle breakup and classified them into four categories

including:

(1) Breakup due to turbulent fluctuation and collision: Drop

shape is balanced between the turbulent pressure of the surround-

ing fluid and interfacial interactions. It is characterized by Weber

numberWe, whichis theratio ofdynamicpressureto surface stress;

(2) Breakup due to the viscous shear forces: It is the viscous shear

forces that cause velocity gradient near the surface and deform the

droplet. The capillary number Ca is proposed as a judgment. (3)

Breakup due to the shearing-off process: The shearing-off process is

a resultof thevelocitydifference acrossthe interface, which is com-

mon for foams with bubbles dispersed in bulk liquid. (4) Breakup

due to interfacial instability.

Despite the fact that the mechanisms mentioned above vary

from each other and the conditions may differ, they are strongly

related to the ability of an interface to resist deformation. Thus,

drop breakage is essentially demanded to be studied from the per-

spective of interfacial dilational rheology.

2.2. Interfacial dilational rheology

Interfacial dilational rheology manifests the ability of an inter-

face to resist changes in the interface area and is described by

the interfacial dilational modulus () [25]. The interfacial dilationalmodulus is defined as the surface tension increase after a small

increase in area of a surface element [12] and is expressed by

=d

d ln A= ||exp

i

(1)

whereis the interfacial tension,A is theinterface area, andis the

phase angle, which indicates a hysteresis of interface deformationresponse to the change in interfacial stress.

Once surfactant molecules are introduced into the dispersion,

they will transfer from the bulk onto the interface to maximally

reduce the Gibbs free energy of the system. When adsorption

equilibrium is reached, the interface shows some content of vis-

coelasticity. This property means that when the interfacial area

undergoes a periodic change, relaxation processes, including dif-

fusion exchange between the surface layers, will occur, and the

interfacial dilational modulus can be divided into two parts, with

the real part representing the elastic/storage modulus and the

imaginary part representing the viscous/loss modulus [26,27],

= + i = d + id (2)

where is thefrequency of the oscillations, andd is the interfacialdilational viscosity.

The phase angle can be obtained by

tan=dd

(3)

The elasticity or rigidity of the interface filmleads to high emul-

sion stability[28] becauseit dampens breakage and the coalescence

of the dispersed droplets. This observation can be explained by the

Marangoni effect, which is induced by the interfacial tension gra-

dient generated in the process of mass transfer or heat exchange

[29]. Surfactant molecules contribute in two aspects: First, when

equilibrated at low bulk surfactant concentration, molecules tend

to adsorb homogeneously onto the surface, as is shown in the

left picture ofFig. 1. When the surface is elongated, a concentra-tion gradient occurs, and molecules rearrange due to the resulting

interfacial tension gradient; thus, the surface restores its deforma-

tion. Second, when saturated adsorption is reached, the diffusion

of excess molecules in the bulk to the depleted region (see the

right picture of Fig. 1) will take charge of the process, result-

ing in a weakened Marangoni effect. The above mechanism was

further confirmed by Wang [30], who explored the interfacial dila-

tional properties of surface-active fractions at oilwater interfaces.

A highly elastic interface resists deformation and produces a larger

driving force forsurfactantsto diffuse tothe depleted region, which

decreases the film compressibility and increases the resistance to

the change in surface area that occurs in drop breakage or coales-

cence [31]. This may further explain the emulsification process and

the related drop size distribution in a turbulently stirred vessel.


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W. Wang et al. / Colloids andSurfaces A: Physicochem. Eng. Aspects 441 (2014) 4350 45

Fig. 1. Schematic diagram of surfactant molecule absorption and rearrange-

ment/diffusion during drop deformation.

Table 1

Properties of test fluids (at20 C) fraction.

Exxsol D80 oil Deionized water

Density (kg/m3) 792.6 992.8

Viscosity (mPa s) 1.78 0.93

Interfacial tension (mN/m)a 42.87

a

Measured interfacial tension between Exxsol D80 oil and deionized water byspinning drop method.

3. Experimental facility

3.1. Fluid properties

The experiment is conducted in a 400 mL beaker

(width = 80mm, height= 110mm). A four pitched blade impeller

is used (diameter D=50mm, width 7.5 mm, height 5.0mm, incli-

nation angle 45). The height of the impeller is fixed at 8 mm

from the bottom of the beaker. The temperature is maintained at

200.5 C. Deionized water and Exxsol D80 oil are used as the test

fluids; their properties are shown in Table 1.

Deionized water is used as the second phase of the dispersion,and its surface tension is checked before the experiment to assure

a high purity. The surfactant used is sorbitan monooleate (Span

80), which is oil soluble. The molecular formula is C24H44O6, and

the molecular weight is 428.6g/mol. The molecule structure of

the surfactant is shown in Fig. 2. The density used for Span 80 is

1.068 g/cm3. The surfactant is dissolved in D80 oil before being

mixed together with water for seven different concentrations.

McClements [32] proposed an equation concerning the minimum

amount requiredto prepare a stableemulsion,however,it is unnec-

essaryto decidewhether this minimum value is reached ornot here

since the aim is to discuss the correlation between drop break-

age andinterfacial properties. Hence a wide range of concentration

from 0.00037 to 0.556mM (mmol/L) is elaborately selected.

Note that the current work particularly focuses on the dropbreakage process during mixing instead of the stability of emul-

sions, Span 80 is introduced as the surface active component

Fig. 2. Moleculestructure of Span 80.

between oiland waterinterface andno moreemulsifier or stabilizer

is used.

3.2. Interfacial properties

The investigation of interfacial tension and interfacial dilational

rheological properties is performed with spinning drop tensiome-

ter (Data physics SVT20N). The spinning drop method determines

the interfacial properties by analyzing the size and profile of thedispersed drop being elongated by a high-speed rotating motor.

A camera was used to obtain the curvature radii of the elon-

gated drop and interfacial tension could be calculated according

to YoungLaplace equation [33]:

1

R1+

1

R2

=

2

R0+22 (4)

where R0 is the sphere radius and R1, R2 represent the two ortho-

gonal maximum curvature radii of the elongated drop, is theangular velocity of the motor and is the potential distance.

Elastic/viscous modulus is determined by applying a sinusoidal

speed oscillation to the motor, with the base speed of the motor

being 10,000 rpm and the amplitude being 3000 rpm. The frequen-

cies of the oscillations are 0.05, 0.1, or 0.2 Hz according to themeasurement conditions of the tensiometer.

3.3. Chord length measurement

The chord length distribution is measured in-situ with the

focus beam reflectance method (FBRM D600L, Mettler Toledo).

The probe is placed 30mm under the liquid surface and 20mm

away from the vertical middle line of the vessel. The probe

can detect a minimum drop size of approximately 397.5nm and

measures the time the beam is reflected and determines the

chord length by the product of the time and the laser scan

speed.

The numberof chord lengths obtained is subjectedto a moving-

average filter, which averages a specified number ofJmax pastmeasurements by giving equal weight to each measurement. The

moving-average filter can be expressed as

Yt=1

J

t=tJ+1

XJ (5)

whereJis thenumber of past data pointsavailable. At thebeginning

of the measurement or when restarting the averaging,Jis set to 1

because there is only one past measurement available. With each

new measurement,J increases until it reaches Jmax. In the present

research,Jmax is set to 10.

D80 oil, water and surfactant are prepared at the experimen-

tal temperature in a beaker. The total volume of the dispersion is

300mL, with the dispersed oil phase volume fraction making upapproximately 0.510% of the dispersion. Emulsification is initi-

ated with a stirring rate of 500 rpm for all the experiments, which

generates a fully developed turbulent flow (Re= 20,833). The mix-

ing timeis fixedat 30min to allow a nearly stabilized chord length

distribution.

To make sure that the dispersions are O/W, conductivity is

measured. As the water phase is deionized, the measured con-

ductivity of the dispersion is limited, with a value of about

0.9s/cm, indicating that with oil holdup low enough (no more

than 10%), O/W dispersion is obtained. This is also validated

by the fact that as the volume fraction of oil phase exceeds

70%, the conductivity of the system becomes zero and stays

unchangedas theoil holdupcontinues to increase,indicating aW/O

dispersion.


4/8


Fig. 3. O/Wequilibrium interfacial tension versus Span 80 concentration.

4. Results and discussion

4.1. Interfacial dilational properties

To investigate the influence of surfactants on the emulsification

process of dilute D80 oil/water dispersions, the interfacial prop-

erties including interfacial tension and dilational modulus were

determined.

Fig. 3 shows the equilibrium interfacial tension between D80

oil and water phases at different Span 80 concentrations. Without

surfactant the interfacial tension measured between oil and water

phases is 42.87 mN/m at 200.5 C. With a surfactant concentra-

tion below the CMC, the interfacial tension decreases significantly

as the Span 80 concentration increases. When the Span 80 concen-

tration exceeds the CMC, the interfacial tension remains stable at

a value of 5.67mN/m. It can be concluded from the figure that the

CMC for the studied system is about 0.4 mM.

Figs. 46 show the interfacial dilational total modulus, the

elastic modulus and the phase angle as a function of surfac-

tantconcentration at different oscillation frequencies, respectively.

The total modulus and the elastic modulus first increase to a

peak and then decrease as the surfactant concentration increases

for all three oscillation frequencies. The maximum is obtained

with a concentration of approximately 103102 mM. This can

be explained by two exclusive relaxation processes: molecular

rearrangements within the surface layer or diffusion exchange

Fig. 4. Interfacial dilatational modulus Eas a function of surfactant concentration

at different frequencies.

Fig. 5. Interfacial dilatational elastic modulus E as a function of surfactant concen-

tration at different frequencies.

of molecules between the surface layer and the bulk solution.At low surfactant concentrations, the unsaturated adsorption of

molecules on the interface increases, and molecular rearrange-

ments dominate as the concentration increases, which reflects an

increased Gibbs elasticity of the interface due to the strength-

ened Marangoni effect. Simultaneously, the diffusion exchange

of molecules between the surface layer and the bulk solution is

strengthened as the concentration continuesto increase, whichbal-

ances theinterfacialtension gradient andweakens theabilityof the

interface to resist deformation. Thus, the total and elastic modulus

initially increases and then decreases with continuously increasing

surfactant concentration. For surfactant concentrations above the

CMC, saturatedadsorption on theinterface is reached, andmicelles

exist in the bulk oil phase. Further increasing the bulk concentra-

tion only influences the diffusion exchange and results in a limited

decrease ofthe elastic modulus.The figuresalsoshowthatthe mod-

ulus increases as the oscillation frequencies increases. Phase angle

for each concentration shown in Fig. 6 is a relative ratio of vis-

cous modulus to elastic modulus. The data shows a non-monotone

increasing trend while the sudden decrement for 0.05Hz may be

attributed to the formation of micelle as surfactant concentration

reaches CMC.

Fig. 6. Interfacial dilatational phase angle as a function of surfactant concentration

at different frequencies.


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Fig.7. Square weightedmean diameterversussurfactantconcentration at different

oil volume fractions.

4.2. Influence on the dispersed modality

Square-weighted mean diameter, Sauter mean diameter,

counted chord length and total counts are parameters chosen to

characterize the dispersed modality of oilwater mixing system.

Detailinformationaboutthe above four parameters could be found

from Wang et al. [34].

The measured sqr-wt mean diameters (chord length) at differ-

ent surfactant concentrations are shown in Fig. 7, with oil phase

volume fractions of 0.5%, 1%, 2.5%, 5% and 10%, respectively. At

such low dispersed hold-ups, drop breakage dominates during the

stirring process, and the influence of coalescence can be ignored,

which provides a clearer perspective fordiscussingthe role ofinter-

facial dilational elasticity on pure breakage. The measured mean

diameter is square weighted according to the following equation:

yi = wi ni (6)

whereyi is the weighted channels, ni is the corresponding counts,

andwi is the weights obtained from the channel midpointsMi via

wi =M2

iNj=1

M2j

N (7)

The sqr-wt mean diameter, a surface average property mainly

for adsorption experiments, is used because interfacial dilation

properties are planar, and they affect the emulsification process

through changes in the surface area. The mean diameters are cal-

culated and plotted in Fig. 7 based on an average value of 40 points

gathered at 30min after the end of stirring. The sqr-wt diameters

experience a first increase then decrease trend with the increment

of surfactant concentration in Fig. 7, with a maximum reachedbetween 103 and 102 mM. From pure system to surfactant con-

centrations of 0.00037 and 0.0037mM, as shown in Fig. 7, the

increase trend of sqr-wt diameter is not in accordance with the

result of IFT in Fig. 4 which suffers a mono decreasing trend, where

the elongation and breakage of dispersed drop becomes enhanced

at low IFT. Instead, the increase of dilational elastic modulus in the

commented concentrations in Fig. 5 is correlated with the sqr-wt

diameters change in Fig. 7.

The trend of molecular rearrangement dominates at surfac-

tant concentrations below the peak point of 3.71103 mM. The

increased dilational elasticity reflects a strengthened resistance to

drop deformation and a counteractive effect for droplet breakage.

Thus, the mean diameter first increases as the surfactant con-

centration increases, despite the fact that the interfacial tension

Fig.8. Sautermean diameterversus surfactantconcentrationat differentoil volume

fractions.

decreases. When the concentration of the surfactant exceeds the

peak value in Fig. 4, the resistance to deformation is reduced dueto the gradually dominated molecular diffusion effect from the

bulk phase, which in turn results in a decreasing mean diameter.

When the concentration increases over the CMC, both the inter-

facial tension and the elastic modulus of the interface are lower,

which favors deformation, and droplets are further broken to even

smaller droplets. In Fig. 7, the mean diameters show a rising trend

as the dispersed hold-ups increases.

The Sauter mean diameter (chord length), d32, sampled after

30min of mixing is compared in Fig. 8, where the diame-

ter is found to be nearly stable. Sauter mean diameter, d32 =ki=1

nid3i/k

i=1nid

2i

, which links the area of dispersed phase to

its volume, is widely used in the characterization of liquidliquid

dispersions. According to Leng [35] and Pacek [36], the equilibrium

size of the largest stable droplet, dmax, is proportional to the Sautermean diameter, which gives dmaxd32. A maximum is observed

at the 3.71103 mM surfactant concentration, which manifests a

strong Marangoni effect and stabilization characteristics. A similar

trend ofd32 is observed in the measured sqr-wt mean diameter,

which further manifests the influence of interfacial elasticity on

emulsification.

Comparing the variations of sqr-wt mean diameter in Fig. 7 and

the d32 in Fig. 8 with the interfacial tension change in Fig. 3 and

the elastic modulus variation in Fig. 5, it can be seen that although

theinterfacialtension remains constant above the CMC, thediame-

ters decrease as the surfactantconcentration increases. The slightly

decreased elasticity in Fig.5 provides an insight into thedecreasing

diameters.

The measured number density of the chord length furtherdemonstrates the above finding. Data from the 0.5%, 1%, 2.5%, 5%,

and 10% oil holdups are obtained and compared; only that of the

0.5% is chosen fordetaileddemonstration,and a setof comparisons

of droplet numbercounts persecond at various chord lengthscopes

are shown in Fig. 9(ae). The number counts of the small chord

length scopes (i.e.,


6/8


Fig. 9. Comparison of droplet counts per second at various chord length scopes for different surfactant concentrations, 0.5% oil volume fraction.

which corresponds to the maximum elastic modulus shown in

Fig. 5.

As the surfactantconcentrationcontinues to increase, the counts

ofsmallchordlengthbeginto increase andthe countsof large chord

length decrease. Withsurfactant concentrationsabove the CMC,the

counts of small drops continue to increase, and the number counts

forlarge drops are nearly zero, which indicates that the interface of

large drops is not stable when undergoing an external deformation

effect, breaking the drops into smaller ones. This agrees with the

evolution of the interfacial dilationalmodulus, especiallythe elastic

modulus.

In Figs. 10 and 11, the total counts per second of all seven

concentrations are compared, with oil holdups of 2.5% and 5%,

respectively. The numerical counts at each time point vary, but

general trends can be observed. The two figures show that the

dispersions with a 3.71103 mM surfactant concentration have

the least total counts per second and that the 5.56mM surfactant

dispersions have the most total counts per second. This suggests


7/8


Fig.10. Comparison of total droplet counts per second for differentsurfactant con-

centrations, 2.5% oil volume fraction.

Fig.11. Comparison of total droplet counts per second for differentsurfactant con-

centrations, 5% oil volume fraction.

that the droplets of dispersions with smaller elastic modulus could

break easily, which results in the system having the most droplet

counts.

The dispersing modality of emulsions with surfactant concen-

trations above the CMC is to be discussed further based on the

experimental data shown in Figs. 711 because it differs from that

of concentrations below the CMC as a result of micelle formation.

Research on emulsion stabilitypresented stabilized emulsionswith

surfactant concentrations far above the CMC. This is explained by

the effect of the adsorbed surfactant, which hinders droplet coales-

cence. The interfacial dilational elastic modulus is greatly reducedat these higher concentrations; large droplets could still be broken

into smaller ones, but the steric repulsion force will strongly resist

the Laplace pressure due to drop deformation and prevent the film

drainage process and coalescence of smaller one. The more uni-

form and stabilized drop size distributions at these concentrations

further demonstrates the effect of the interfacial dilational elastic

modulus on droplet breakage in emulsions.

5. Conclusions

Surfactant added into oilwater dispersions is believed to pro-

mote the emulsification process, and the influence of surfactants

on drop breakage is caused mainly by the reduced interfacial ten-

sion. However, the interfacial dilational properties, especially the

dilational elastic modulus, are responsible for the droplet breakage

process because they are important parameters for characterizing

the stability of emulsions. A set of experiments are conducted to

find an explicit relationship between the interfacial dilational prop-

ertiesand thedrop size distributionof thedispersions, with a broad

range of surfactant concentrations.

The experimental results for the interfacial dilational rheo-

logical properties indicate that as the surfactant concentration

increases, the elasticityof the oilwater interface initially increases

due to the dominant rearrangement relaxation of the surfac-

tant molecules adsorbed onto the interface; thus, the ability of

the droplet to resist deformation strengthens. The diffusion of

molecules between the bulk and the interface then takes charge,

and the dilational modulus decreases as the concentration contin-

ues to increase, which results in a weakened droplet stability.

The sqr-wt mean diameter, Sauter mean diameter, counted

chord length, andtotalcounts aredetermined to examine theeffect

of the interfacial dilational modulus on drop breakage during the

mixing process. The dispersing modality are in consistent with the

trends observed in the interfacial dilational modulus, which sug-

gests that as the elasticity of the interface increases, the ability

of the dispersed droplets to resist deformation is strengthened,

whereas interfaces with low dilational elasticity are broken rela-

tively easily. These results verifythe argument thatthe mainfactors

effecting emulsification include the interfacial dilational proper-

ties, especially dilational elasticity.

Acknowledgments

The authors wish to thank the National Natural Science Founda-

tion of China (51104167), the Program for New Century Excellent

Talents in University (NCET), the Foundation for the Author of

National Excellent Doctoral Dissertation of PR China(FANEDD),and

the Science Foundation of China University of PetroleumBeijing

(BJ-2011-02) for providing support for this work.

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