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Subdrop ejection from double emulsion drops in shear flow
Journal of MembraneScience, Elsevier
Science
Publishers
26 (1986) B.V.,
231
231-236
Amsterdam
-
Printed
in The Netherlands
Short Communication SUBDROP FLOW
EJECTION
FROM DOUBLE
MADAPUSI
PALAVEDU
Department
of Chemical Engineering,
(Received
July 7, 1985;
SRINIVASAN
accepted
EMULSION
and PIETER
DROPS IN SHEAR
STROEVE
University of California, Davis, CA 95616
in revised
form
October
(U.S.A.)
25, 1985)
Introduction Double emulsions have been used as separation media to extract valuable metals or to remove toxins from wastewater. Because of the high surface area to volume ratio, short diffusion distances, and the high permeabilities of liquid systems, double emulsions have been studied in a wide variety of separation processes [1,2]. The double emulsions are usually of the water/ oil/water type (w/o/w). The suspended drops are made of an oil phase (membrane phase) and contain a large number of small subdrops of an aqueous reagent phase which serves to sequester chemical species. A solute to be extracted from the continuous phase diffuses through the oil phase of the double emulsion drops and then reacts with the reagent in the subdrops. The transformed solute remains encapsulated in the subdrops if its permeability properties are poor so that it does not diffuse out of the drops. Double emulsion separation systems are also known as unsupported liquid surfactant membranes. When using double emulsions as separation systems in industrial processes, it is desirable to understand the phenomena of drop stability [1,3]. In mixing equipment, hydrodynamic forces act on the drops which may cause the breaking of drops. When not properly formulated, double emulsions show instability which results in loss of internal emulsion phase under agitation conditions during the extraction operation. Consequently, the systematic study of drop breakup is an important scientific endeavor. Hochhauser and Cussler [4] observed the breakage of double emulsion drops by measuring the release of the reagent chemical from the subdrops into the continuous phase as a function of the surfactant concentration, oil viscosity and the volume fraction of the subdrops inside the double emulsion drops. Martin and Davies [5 1 reported similar studies on the stability of drops under different stirring conditions. These studies did indicate significant breakup of drops but did not offer details on the mechanism of drop breakup nor the phenomenon of subdrop or reagent release. Experimental studies by Stroeve and Varanasi [6] showed that double
0376-7388/86/$03.50
@ 1986
Elsevier
Science
Publishers
XV
232
emulsion drop breakup in uniform shear flow followed the breakup classification of Rumscheidt and Mason [7]. In order to systematically study the mechanisms of drop breakup, the use of a well-defined flow field is common practice [8]. Stroeve and Varanasi [6] have given predictive correlations (in graphical form) for emulsion drop breakup, but the mechanism of reagent phase or subdrop release was not investigated. The studies were carried out at dilute double emulsion drop concentrations (GO.5 vol.%). Double emulsion extractions are often carried out at moderate drop concentrations, about 1 to 5 vol.%. The purpose of this communication is to report a new type of drop breakup at moderate concentrations and a mechanism of subdrop release to the continuous phase. Experimental Materials Double emulsions were prepared by dispersing a water-in-oil (w/o) emulsion into a continuous aqueous phase. The oil phase for the w/o emulsion consisted of 36 vol.% kerosene, 54 vol.% Indopol H-25, a polybutene polymer (Amoco), and 10 vol.% Span 80, the surfactant sorbitan monooleate (ICI Americas). The aqueous phase of the w/o emulsion was 50 vol.% glycerin and 50 vol.% water. The presence of glycerin in the aqueous phase was found to be advantageous to get a reproducible apparent viscosity for the w/o emulsion. The viscosities and normal stress differences of the oil phase and the w/o emulsion were measured as a function of the shear rate with the Weissenberg rheogoniometer (Model R16). The interfacial tension between the oil and continuous phase was measured with the sessile drop technique [9]. Interfacial tensions were measured over time and back extrapolated to zero time to obtain the dynamic interfacial tension. The w/o emulsion was made by adding the aqueous phase slowly to the well-stirred oil phase. The aqueous phase vol.% in the w/o emulsion was 62. The w/o/w double emulsion was made by dispersing a small amount of the w/o emulsion into a continuous and immiscible aqueous phase. With a disposable pipet, a sufficient amount of w/o emulsion was added to corn syrup (Karo) to make a 5 vol.% dispersion. The 62 vol.% emulsion drops were fully packed with the aqueous subdrops. This volume content of subdrops in the oil drops was the highest value attainable in the system used here. Apparatus The cone-and-plate viscometer used in this study has been described by Fisher et al. [lo] and Stroeve and Varanasi 161. The viscometer was mounted on an inverted microscope. The transparent plate and transparent cone have a diameter of 3.0 cm and a gap angle of 2 degrees. A mechanical drive caused the cone and plate to rotate at equal but opposite speeds. Because of the small gap angle, a w/o/w sample placed in the gap was subjected to a uniform shear rate throughout the gap. In these studies, the initial
233
double emulsion drops were approximately 20-100 pm in size. During shear, there was a conical surface in the midposition of the gap where drops were stationary with respect to the observer. In order to record the drops during shear, the inverted microscope was coupled to a 16-mm movie camera (Photosonics, Inc.). Technical Pan film 2415 (Kodak) was used and the film was exposed at 100 frames per second. The magnification was 300X. All experiments were carried out at 23 + 1°C. Results and discussion The corn syrup used as the continuous phase for the double emulsion was found to be Newtonian in its rheological behavior with a viscosity of 9.74 N-sec/m2. The oil phase and the glycerol/water solution were also Newtonian with a viscosity of 0.089 N-see/m2 and 0.0055 N-sec/m2, respectively. The w/o emulsion was found to be shear thinning and to generate a first normal stress difference as shown in Fig. 1. The dynamic interfacial tension between the oil phase and the corn syrup was found to be 4.5 mN/m, and was within experimental error (* 0.2 mN/m) from the equilibrium interfacial tension. The viscosity and interfacial tension are important physical parameters to characterize the double emulsion system in breakup. In a separate study on double emulsion drop breakup, Srinivasan [ll] found the double emulsion system used here to behave identically to the breakup behavior of the w/o/w emulsion employed by Stroeve and Varanasi [6]. The correlation of the breakup parameter versus the viscosity ratio, previously found by Stroeve and Varanasi [6], is applicable to this double emulsion system [ 111. The breakup parameter is the dimensionless ratio of the applied viscous shearing force to the surface tension force and the viscosity ratio is the apparent viscosity of the w/o emulsion at the imposed shear divided by the continuous phase viscosity. It should be noted that the breakup studies were conducted at a double emulsion drop concentration of 0.5 vol.% which is considerably diluter than the concentration employed for the cinemicrophotography.
01
s-1
Y Fig. 1. Viscosity at 23°C.
100
10
1 and first
normal
stress
difference
versus
shear rate for the w/o emulsion
234
Double emulsion drop breakup, as observed from the tine films, occurred by dumbbell formation and by slender body formation. In dumbbell breakup, the drop first elongates and then becomes unstable when a surface deformation (Rayleigh wave) causes a local thinning of the drop. The nodal point will continue to become thinner until the drop fractures into two double emulsion drops and several much smaller satellite drops. The satellite drops are generated when the nodal point becomes very thin and extended like a liquid thread, which itself becomes unstable when a second wave with a much shorter wave length forms on its surface. Breakup due to slender body formation occurs when the double emulsion drops are pulled to long cylindrical shapes and then become unstable when a wave forms along the drop’s surface creating many nodal points. Again, the nodal points become thin and the drop fractures into main drop fragments and much smaller satellite drops. This drop breakup behavior was also observed by Stroeve and Varanasi [6] and is consistent with the breakup behavior for homogeneous Newtonian drops [ 71. For the moderate double emulsion drop concentrations employed here, a third type of drop breakup was observed. A sequence of frames obtained from cinemicrophotography is shown as an example in Fig. 2. The imposed shear rate of the sample in this sequence was 40 set-’ and the time difference between each succeeding frame was 0.03 sec. The double emulsion drop in the lower center broke up due to particles streaming from the tip. Although the phenomenon appears at first to be similar to tip streaming described by Rumscheidt and Mason [7], there is an important difference. In tip streaming, the stream of small drops breaks away from the pointed end of the elongated tip. The stream of particles in Fig. 2 was observed to be subdrops escaping from the center portion of the drop and which issued out from the pointed end of the tip. Like bullets leaving a machine gun, the subdrops shot out of the double emulsion drop in a queue of particles. Since in Fig. 2 the subdrops are still observable in the continuous phase, a thin film of oil phase must cover them; otherwise they would have mixed with the continuous phase immediately upon release. Before the shearing experiments, the double emulsion drops were fully packed with subdrops. After double emulsion drop breakup at a given shear rate had occurred, the drops did not break up any further since they were smaller than the critical size 161. The remaining double emulsion drops were observed to be no longer fully packed. If the shear rate was increased again, additional subdrop ejection would occur and correspondingly smaller double emulsion drops with fewer subdrops were observed. At the highest shear rates employed in this study (100 see-‘), small oil drops were observed with only a few subdrops inside. Although difficult to observe from Fig. 2, in the tine films the aggregate of subdrops were seen to move in a circulatory flow inside the double emulsion drop. Because of the moderate concentration of double emulsion drops (5 vol.%), the shear field around the double emulsion drops was probably
235
not uniform. Other drops passing by in the neighborhood of a drop would disturb the local shear field. As shown in Fig. 2, several drops were in close proximity to the double emulsion drop which was exhausting its subdrops.
4
5
6
8
9
Fig. 2. A typical sequence of frames obtained from cinemicrophotography showing the breakup of a double emulsion drop. The time difference between each frame is 0.03 sec. The length of the bar represents 100 pm. The shear rate is 40 set-’
236
Therefore, this type of breakup was probably associated with unsteady shear. The flow within the double emulsion drops must be complex for the subdrops to leave the interior of the double emulsion drop sequentially. The loss of subdrops from double emulsion drops may have important consequences in separation processes where drop breakup occurs. The subdrops ejected into the continuous phase were presumably unstable since only a thin oil film covered them. Small subdrops were not found in the continuous phase after shearing was terminated. The loss of encapsulated chemicals to the continuous phase due to subdrop instability would lead to a reduced extraction efficiency. Acknowledgement This study Grant.
was supported
in part by a University
of California
Research
References
9 10
11
P. Stroeve and P.P. Varanasi, Transport processes in liquid membranes: Double emulsion separation systems, Sep. Purif. Methods, 11 (1982) 29. R. Marr and A. Kopp, Liquid membrane technology - A survey of phenomena, mechanisms and models, Int. Chem. Eng., 22 (1982) 44. T.H. Maugh, Liquid membranes: New techniques for separation, purification, Science, 193 (1976) 134. A.M. Hochhauser and E.L. Cussler, Concentrating chromium with liquid surfactant membranes, AIChE Symp. Ser., 71 (1975) 136. T.P. Martin and G.A. Davies, The extraction of copper from dilute aqueous solutions using a liquid membrane process, Hydrometallurgy, 2 (1976/77) 315. P. Stroeve and P.P. Varanasi, An experimental study on double emulsion drop breakup in uniform shear flow, J. Colloid Interface Sci., 99 (1984) 360. F.D. Rumscheidt and S.G. Mason, Particle motions in sheared suspensions, J. Colloid Sci., 16 (1961) 238. H.P. Grace, Dispersion phenomena in high viscosity immiscible fluid systems and applications of static mixers as dispersion devices in such systems, Chem. Eng. Commun., 14 (1982) 225. J.F. Padday,, Surface tension. Part II. The measurement of surface tension, Surf. Colloid Sci., l(l969) 101. T.M. Fisher, M. Stohr and H. Schmidt-Schonbein, Red blood cell (RBC) microrheology: Comparison of the behavior of single RBC and liquid droplets in shear flow, AIChE Symp. Ser., 182 (1978) 38. M.P. Srinivasan, Double emulsion drop breakup in uniform shear flow, M.S. Thesis, University of California, Davis, 1985.
Science
Publishers
26 (1986) B.V.,
231
231-236
Amsterdam
-
Printed
in The Netherlands
Short Communication SUBDROP FLOW
EJECTION
FROM DOUBLE
MADAPUSI
PALAVEDU
Department
of Chemical Engineering,
(Received
July 7, 1985;
SRINIVASAN
accepted
EMULSION
and PIETER
DROPS IN SHEAR
STROEVE
University of California, Davis, CA 95616
in revised
form
October
(U.S.A.)
25, 1985)
Introduction Double emulsions have been used as separation media to extract valuable metals or to remove toxins from wastewater. Because of the high surface area to volume ratio, short diffusion distances, and the high permeabilities of liquid systems, double emulsions have been studied in a wide variety of separation processes [1,2]. The double emulsions are usually of the water/ oil/water type (w/o/w). The suspended drops are made of an oil phase (membrane phase) and contain a large number of small subdrops of an aqueous reagent phase which serves to sequester chemical species. A solute to be extracted from the continuous phase diffuses through the oil phase of the double emulsion drops and then reacts with the reagent in the subdrops. The transformed solute remains encapsulated in the subdrops if its permeability properties are poor so that it does not diffuse out of the drops. Double emulsion separation systems are also known as unsupported liquid surfactant membranes. When using double emulsions as separation systems in industrial processes, it is desirable to understand the phenomena of drop stability [1,3]. In mixing equipment, hydrodynamic forces act on the drops which may cause the breaking of drops. When not properly formulated, double emulsions show instability which results in loss of internal emulsion phase under agitation conditions during the extraction operation. Consequently, the systematic study of drop breakup is an important scientific endeavor. Hochhauser and Cussler [4] observed the breakage of double emulsion drops by measuring the release of the reagent chemical from the subdrops into the continuous phase as a function of the surfactant concentration, oil viscosity and the volume fraction of the subdrops inside the double emulsion drops. Martin and Davies [5 1 reported similar studies on the stability of drops under different stirring conditions. These studies did indicate significant breakup of drops but did not offer details on the mechanism of drop breakup nor the phenomenon of subdrop or reagent release. Experimental studies by Stroeve and Varanasi [6] showed that double
0376-7388/86/$03.50
@ 1986
Elsevier
Science
Publishers
XV
232
emulsion drop breakup in uniform shear flow followed the breakup classification of Rumscheidt and Mason [7]. In order to systematically study the mechanisms of drop breakup, the use of a well-defined flow field is common practice [8]. Stroeve and Varanasi [6] have given predictive correlations (in graphical form) for emulsion drop breakup, but the mechanism of reagent phase or subdrop release was not investigated. The studies were carried out at dilute double emulsion drop concentrations (GO.5 vol.%). Double emulsion extractions are often carried out at moderate drop concentrations, about 1 to 5 vol.%. The purpose of this communication is to report a new type of drop breakup at moderate concentrations and a mechanism of subdrop release to the continuous phase. Experimental Materials Double emulsions were prepared by dispersing a water-in-oil (w/o) emulsion into a continuous aqueous phase. The oil phase for the w/o emulsion consisted of 36 vol.% kerosene, 54 vol.% Indopol H-25, a polybutene polymer (Amoco), and 10 vol.% Span 80, the surfactant sorbitan monooleate (ICI Americas). The aqueous phase of the w/o emulsion was 50 vol.% glycerin and 50 vol.% water. The presence of glycerin in the aqueous phase was found to be advantageous to get a reproducible apparent viscosity for the w/o emulsion. The viscosities and normal stress differences of the oil phase and the w/o emulsion were measured as a function of the shear rate with the Weissenberg rheogoniometer (Model R16). The interfacial tension between the oil and continuous phase was measured with the sessile drop technique [9]. Interfacial tensions were measured over time and back extrapolated to zero time to obtain the dynamic interfacial tension. The w/o emulsion was made by adding the aqueous phase slowly to the well-stirred oil phase. The aqueous phase vol.% in the w/o emulsion was 62. The w/o/w double emulsion was made by dispersing a small amount of the w/o emulsion into a continuous and immiscible aqueous phase. With a disposable pipet, a sufficient amount of w/o emulsion was added to corn syrup (Karo) to make a 5 vol.% dispersion. The 62 vol.% emulsion drops were fully packed with the aqueous subdrops. This volume content of subdrops in the oil drops was the highest value attainable in the system used here. Apparatus The cone-and-plate viscometer used in this study has been described by Fisher et al. [lo] and Stroeve and Varanasi 161. The viscometer was mounted on an inverted microscope. The transparent plate and transparent cone have a diameter of 3.0 cm and a gap angle of 2 degrees. A mechanical drive caused the cone and plate to rotate at equal but opposite speeds. Because of the small gap angle, a w/o/w sample placed in the gap was subjected to a uniform shear rate throughout the gap. In these studies, the initial
233
double emulsion drops were approximately 20-100 pm in size. During shear, there was a conical surface in the midposition of the gap where drops were stationary with respect to the observer. In order to record the drops during shear, the inverted microscope was coupled to a 16-mm movie camera (Photosonics, Inc.). Technical Pan film 2415 (Kodak) was used and the film was exposed at 100 frames per second. The magnification was 300X. All experiments were carried out at 23 + 1°C. Results and discussion The corn syrup used as the continuous phase for the double emulsion was found to be Newtonian in its rheological behavior with a viscosity of 9.74 N-sec/m2. The oil phase and the glycerol/water solution were also Newtonian with a viscosity of 0.089 N-see/m2 and 0.0055 N-sec/m2, respectively. The w/o emulsion was found to be shear thinning and to generate a first normal stress difference as shown in Fig. 1. The dynamic interfacial tension between the oil phase and the corn syrup was found to be 4.5 mN/m, and was within experimental error (* 0.2 mN/m) from the equilibrium interfacial tension. The viscosity and interfacial tension are important physical parameters to characterize the double emulsion system in breakup. In a separate study on double emulsion drop breakup, Srinivasan [ll] found the double emulsion system used here to behave identically to the breakup behavior of the w/o/w emulsion employed by Stroeve and Varanasi [6]. The correlation of the breakup parameter versus the viscosity ratio, previously found by Stroeve and Varanasi [6], is applicable to this double emulsion system [ 111. The breakup parameter is the dimensionless ratio of the applied viscous shearing force to the surface tension force and the viscosity ratio is the apparent viscosity of the w/o emulsion at the imposed shear divided by the continuous phase viscosity. It should be noted that the breakup studies were conducted at a double emulsion drop concentration of 0.5 vol.% which is considerably diluter than the concentration employed for the cinemicrophotography.
01
s-1
Y Fig. 1. Viscosity at 23°C.
100
10
1 and first
normal
stress
difference
versus
shear rate for the w/o emulsion
234
Double emulsion drop breakup, as observed from the tine films, occurred by dumbbell formation and by slender body formation. In dumbbell breakup, the drop first elongates and then becomes unstable when a surface deformation (Rayleigh wave) causes a local thinning of the drop. The nodal point will continue to become thinner until the drop fractures into two double emulsion drops and several much smaller satellite drops. The satellite drops are generated when the nodal point becomes very thin and extended like a liquid thread, which itself becomes unstable when a second wave with a much shorter wave length forms on its surface. Breakup due to slender body formation occurs when the double emulsion drops are pulled to long cylindrical shapes and then become unstable when a wave forms along the drop’s surface creating many nodal points. Again, the nodal points become thin and the drop fractures into main drop fragments and much smaller satellite drops. This drop breakup behavior was also observed by Stroeve and Varanasi [6] and is consistent with the breakup behavior for homogeneous Newtonian drops [ 71. For the moderate double emulsion drop concentrations employed here, a third type of drop breakup was observed. A sequence of frames obtained from cinemicrophotography is shown as an example in Fig. 2. The imposed shear rate of the sample in this sequence was 40 set-’ and the time difference between each succeeding frame was 0.03 sec. The double emulsion drop in the lower center broke up due to particles streaming from the tip. Although the phenomenon appears at first to be similar to tip streaming described by Rumscheidt and Mason [7], there is an important difference. In tip streaming, the stream of small drops breaks away from the pointed end of the elongated tip. The stream of particles in Fig. 2 was observed to be subdrops escaping from the center portion of the drop and which issued out from the pointed end of the tip. Like bullets leaving a machine gun, the subdrops shot out of the double emulsion drop in a queue of particles. Since in Fig. 2 the subdrops are still observable in the continuous phase, a thin film of oil phase must cover them; otherwise they would have mixed with the continuous phase immediately upon release. Before the shearing experiments, the double emulsion drops were fully packed with subdrops. After double emulsion drop breakup at a given shear rate had occurred, the drops did not break up any further since they were smaller than the critical size 161. The remaining double emulsion drops were observed to be no longer fully packed. If the shear rate was increased again, additional subdrop ejection would occur and correspondingly smaller double emulsion drops with fewer subdrops were observed. At the highest shear rates employed in this study (100 see-‘), small oil drops were observed with only a few subdrops inside. Although difficult to observe from Fig. 2, in the tine films the aggregate of subdrops were seen to move in a circulatory flow inside the double emulsion drop. Because of the moderate concentration of double emulsion drops (5 vol.%), the shear field around the double emulsion drops was probably
235
not uniform. Other drops passing by in the neighborhood of a drop would disturb the local shear field. As shown in Fig. 2, several drops were in close proximity to the double emulsion drop which was exhausting its subdrops.
4
5
6
8
9
Fig. 2. A typical sequence of frames obtained from cinemicrophotography showing the breakup of a double emulsion drop. The time difference between each frame is 0.03 sec. The length of the bar represents 100 pm. The shear rate is 40 set-’
236
Therefore, this type of breakup was probably associated with unsteady shear. The flow within the double emulsion drops must be complex for the subdrops to leave the interior of the double emulsion drop sequentially. The loss of subdrops from double emulsion drops may have important consequences in separation processes where drop breakup occurs. The subdrops ejected into the continuous phase were presumably unstable since only a thin oil film covered them. Small subdrops were not found in the continuous phase after shearing was terminated. The loss of encapsulated chemicals to the continuous phase due to subdrop instability would lead to a reduced extraction efficiency. Acknowledgement This study Grant.
was supported
in part by a University
of California
Research
References
9 10
11
P. Stroeve and P.P. Varanasi, Transport processes in liquid membranes: Double emulsion separation systems, Sep. Purif. Methods, 11 (1982) 29. R. Marr and A. Kopp, Liquid membrane technology - A survey of phenomena, mechanisms and models, Int. Chem. Eng., 22 (1982) 44. T.H. Maugh, Liquid membranes: New techniques for separation, purification, Science, 193 (1976) 134. A.M. Hochhauser and E.L. Cussler, Concentrating chromium with liquid surfactant membranes, AIChE Symp. Ser., 71 (1975) 136. T.P. Martin and G.A. Davies, The extraction of copper from dilute aqueous solutions using a liquid membrane process, Hydrometallurgy, 2 (1976/77) 315. P. Stroeve and P.P. Varanasi, An experimental study on double emulsion drop breakup in uniform shear flow, J. Colloid Interface Sci., 99 (1984) 360. F.D. Rumscheidt and S.G. Mason, Particle motions in sheared suspensions, J. Colloid Sci., 16 (1961) 238. H.P. Grace, Dispersion phenomena in high viscosity immiscible fluid systems and applications of static mixers as dispersion devices in such systems, Chem. Eng. Commun., 14 (1982) 225. J.F. Padday,, Surface tension. Part II. The measurement of surface tension, Surf. Colloid Sci., l(l969) 101. T.M. Fisher, M. Stohr and H. Schmidt-Schonbein, Red blood cell (RBC) microrheology: Comparison of the behavior of single RBC and liquid droplets in shear flow, AIChE Symp. Ser., 182 (1978) 38. M.P. Srinivasan, Double emulsion drop breakup in uniform shear flow, M.S. Thesis, University of California, Davis, 1985.