전기장하에서의 다중에멀젼 방울의 변형과 파괴

하종욱, 양승만

한국과학기술원 화학공학과

Deformation and Breakup of Multiple Emulsion drops

in an Electric Field

Jong-Wook Ha, Seung-Man Yang

Department of Chemical Engineering,

Korea Advanced Institute of Science and Technology

INTRODUCTION

Recently, our research group has investigated about the breakup modes of drops in an electric field and found that there are two principal mechanisms of drop breakup: namely, pinch-off and tip-streaming. Generally speaking, the pinch-off (burst of drop with formation of bulbous ends and a waist) is a prevalent breakup mode of Newtonian drops immersed in an insulating Newtonian fluid. On the other hand, when the drop interface is contaminated by surface-active agents [1], or when at least one of two phases is non-Newtonian fluid [2], tip-streaming is observed. Although details of the feature of tip-streaming are slightly different from each other in the aforementioned two cases, small sub-drops or thin liquid threads are ejected from the pointed ends of the drop. In this case, the drop is not highly elongated but remains ellipsoidal shape. In this study, we have taken into account the electric-field-induced breakup of multiple emulsion drops. By doing this, we are able to examine the effect of nonuniformity of the drop phase on the drop deformation and breakup modes experimentally.

EXPERIMENTAL

We have prepared o/w and w/o emulsion drops in order to examine their deformation and breakup in an electric field. The results are compared with those of homogeneous Newtonian drops possessing similar electric properties. Since the volume fraction of the inner phase is relatively low, emulsions used in the experiments have shown Newtonian behaviors in the rheological tests. Accordingly, differences observed experimentally between the homogeneous Newtonian drops and inhomogeneous emulsion drops may be caused by the effect of nonuniformity of the drop phase.

The o/w emulsion was made by adding an oil phase slowly to an aqueous phase with rigorous stirring. The aqueous phase was a mixture of water and ethyl alcohol dissolving 1wt% polyvinylpyrrolidone. The oil phase consisted of the mixture of hexane, chlorobenzene and 5wt% of the oil-soluble nonionic surfactant, Span85. After adding dispersed phase to continuous phase, the emulsion system was continuously agitated by mechanical homogenizer more than 24 hours. In the case of w/o emulsion, the aqueous phase was a mixture of water and ethyl alcohol and the membrane oil phase was castor oil. 5wt% of water-soluble nonionic surfactant, Brij58, was added to the aqueous phase. The resulting o/w emulsions did not show phase separation at least within 2 days and the w/o emulsion was more stable.

The experimental apparatus and procedure have been also reported in detail in literature [1,2]. In order to examine the electric-field-induced deformation and breakup of emulsion drops, a single isolated drop of sample emulsion was placed in the middle of two copper electrodes. The drop volume was controlled such that the undeformed radius was 1.3mm. Then, the uniform dc electric field was applied. The deformation of emulsion drops was observed through CCD camera equipped with microscopic lenses and continuously recorded by video cassette recorder. For each emulsion drop, the critical value of electric field strength above which the steady-state drop shape ceased to exist were determined at first, and then the breakup modes of the emulsion drop were studied at higher electric field strengths.

RESULTS AND DISCUSSION

In Fig. 1, the steady deformation of a homogeneous single phase Newtonian drop is shown as a function of the electrical Weber number. In the plot, the degree of deformation D is defined as D = (L-B)/(L+B), in which L is a half-length of drop axis parallel to the electric field and B is one perpendicular to the electric field. The solid line stands for the numerically obtained deformation curve for the perfectly conducting drop [3]. In addition, both the numerical solution and the experimental data contained in Fig. 1 were obtained until the steady-state drop shape did not exist. It can be seen that the drop phase can be treated as a conducting drop and the critical Weber number which is a limiting point of the drop stability is well agreed with the theoretical value.

In order to show the effect of viscosity ratio on the deformation and the critical Weber number of emulsion drop, silicone oils of various grades of viscosity are used as a continuous phase. In Figs. 2 and 3, the degree of deformation is plotted as a function of the Weber number for 10vol% and 30vol% o/w emulsions, respectively. Obviously, the viscosity ratio is not an important factor. This result is also consistent with that of Newtonian conducting drops.

Next, we consider the effect of nonuniformity of the drop phase by varying the volume fraction of the inner drop phase. The results are plotted in Figs. 4 and 5. When the volume fraction of oil phase is relatively low, the degree of deformation of the emulsion drops is slightly deviated from that of homogeneous Newtonian drops. This clearly indicates that the emulsion drops are broken up at lower Weber number than the homogeneous drops. Obviously, as the amounts of the inner drops increases, the drop becomes more deformable.

The behaviors of w/o emulsions in an electric field have been also investigated. Since the continuous phase used is a viscous oil, the resulting system is a w/o/o emulsion. Although this configuration of the emulsions may not be found frequently in industrial operations, we can deduce some information about the deformation or breakup of inhomogeneous leaky dielectric drops from it. The results about the steady deformation of leaky dielectric emulsions are shown in Fig. 6. The presence of the dispersed drops does not give a great influence. This is due to the fact that the nonuniformity of the emulsion is not considerable to produce any appreciable effect. However, similar to the case of o/w emulsions, slight destabilization of the emulsion drop is observed. That is, the critical Weber number is decreased by the nonuniformity.

In the present study, the breakup mode of o/w emulsion drops is dependent upon the fraction of the inner drop phase. The pure membrane phase drop is broken up via pinch-off, which is typical of Newtonian drops. The breakup mode of very dilute emulsions is identical to that of homogeneous Newtonian drops. In the case, nonuniformity of the drop phase is not enough to change the breakup mode. Further increase in the volume fraction of the inner drops phase leads to the tip-streaming. The drop sustains ellipsoidal shape. From the pointed ends, small emulsion drops are ejected continuously. After some amount of the emulsion drops is removed, tip-streaming is ceased due to the decreased electrical Weber number by tip-streaming. At this point, the drop still has pointed ends. Of course, further increasing of electric field strength restarts tip-streaming process.

CONCLUSION

In this study, we have studied the deformation and breakup of emulsion drops in a uniform electric field. The emulsions prepared in this study exhibited Newtonian behaviors in the rheological responses. Since the conductivities of the assumed o/w emulsions are much higher than that of continuous phase, the drop can be supposed as a conducting drop effectively. Thus, the problem is closely related to the deformation and breakup of conducting drops with nonuniformity. When the emulsion is very dilute, the deformation and breakup mode are nearly identical to those of conducting Newtonian drops. As nonuniformity increases, deviations from the behaviors of homogeneous Newtonian drops are pronounced and breakup mode is also changed. In conclusion, the nonuniformity of the drop phase renders drop more unstable and leads to tip-streaming. For the w/o emulsion drops, slight destabilization due to the inhomogeneity of the drop is also observed. In this case, however, the breakup mode is identical to that of homogeneous Newtonian drops. This is because the spatial inhomogeneity is not sufficient to change breakup modes.

REFERENCES

[1] Ha, J.-W. and Yang, S.-M., J. Colloid Interface Sci., in press (1998)

[2] Ha, J.-W. and Yang, S.-M., J. Fluid Mech. submitted (1998)

[3] Feng, J. Q. and Scott, T. C. J. Fluid Mech. 311, 289 (1996)

Fig.  Deformation of a homogeneous Newtonian single drop

Fig . Deformation of 10% o/w emulsion drops in a various viscosity grades silicone oil

Fig.  Deformation of 30% o/w emulsion drop in a various viscosity grade silicone oil

Fig.  Effect of volume fraction of emulsion drops in silicone oil

Fig. 6 Deformation of w/o emulsion drop in a various viscosity grades silicone oil

Fig. 5 Effect of volume fraction of emulsion drops in silicone oil