직류 전기장에 의한 오일-오일 에멀젼계에서의 점도 저하

하종욱, 양승만

한국과학기술원 화학공학과

대전광역시 유성구 구성동 373-1, 305-701

Viscosity Reduction of Oil-in-oil Emulsion Systems in a DC Electric Field

Jong-Wook Ha and Seung-Man Yang

Department of Chemical Engineering

Korea Advanced Institute of Science and Techonology, Taejon 305-701, Korea

INTRODUCTION

Recently, as a new kind of electrorheological (ER) fluids, the rheological responses of immiscible liquid blends or emulsion systems in an electric field have been reported [1-6]. When an electric field is applied to emulsion systems, the droplets of more conducting dispersed phase elongate along the field and form column-like morphology between the electrodes. According to Kimura et al. [3], two opposite effects are observed due to the connection of more conductive dispersed phase between electrodes under the electric field. The ER effect is positive when the viscosity of dispersed phase is larger than that of continuous phase. On the other hand, when the more conducting fluid formed a continuous phase, the ER effect did not appear.

However, all of aforementioned studies have considered mainly the responses of more conducting dispersed phase suspended in a less conducting medium. In general, when the continuous phase is more conducting than the drop phase, the equilibrium shape of the drop is oblate and otherwise prolate in an electric field [7]. Therefore, it is natural to conceive the idea that the rheological responses may be different from either case depending on the deformation type of dispersed phase.

EXPERIMENTAL

Two oily fluids, castor oil (Aldrich chemical Co.) and silicone oil (KF-96, Shinetsu silicone Korea, Inc.) were used to prepare oil-in-oil emulsions. The viscosity and dielectric constant of castor oil was 1.55Pa.s and 3.8, respectively. Because silicone oils have nearly constant electrical properties with various viscosity grades, we can observe the effect of viscosity ratio between dispersed and continuous phase on the ER effect independently. Although Kimura et al. [3] studied the effect of viscosity ratio on the ER effect by changing temperature, it may give some ambiguous or even incorrect conclusion because the temperature dependence of the conductivity is not negligible. In general, the electrical conductivity varies 3 to 5%/C whereas dielectric constant varies about 0.1%/C for poorly conducting dielectric fluids [8].

Six kinds of silicone oils with a wide range viscosity from 0.13 to 3.50Pa.s, respectively, were used in order to form model emulsions as listed in Table 1. These are prepared by mixing commercially available lower viscosity oil with higher one at various fraction. The conductivity of castor oil was higher than that of silicone oil about ten times. To examine the effect of oil conductivity more thoroughly, we used modified castor oil by adding small amounts of organic material such as ethyl alcohol. The volume fraction of the dispersed phase was fixed at about 20% for all emulsions.

The rheological properties of emulsions with or without electric field were measured by ARES fluid rheometer (Rheometric Scientific Inc.). The parallel plate fixture of 5cm in diameter, which was also worked as electrode, was used. The gap between two plates was 1mm. High voltage power supply (Glassman High Voltage, Inc., series EH) was connected to these plates. The ER response of the emulsions was measured at room temperature (15±1℃) with dc applied electric fields E0 = 0∼2kV/mm. The electric current was also measured by sensitive ammeter after the field had been applied. The current was divided by the area of the electrode to give the current density. The conductivity (or resistivity) was then obtained by dividing the current density by the electric field strength.

RESULTS

A. Steady shear rate sweep

In Fig.1, the rheological behaviors of EV1, which has lowest viscosity ratio in our experimental runs, are represented up to the electric field strength of 2kV/mm. Here, viscosities are normalized by the viscosity of emulsion in the absence of electric field. When the shear flow is weak, the positive ER effect is observed. The magnitude of the viscosity enhancement is increased as the electric field strength is increased. However, the positive ER effect is vanished instantaneously as increasing shear rate and followed by the very small (about 5%) negative ER effect. Finally, when the shear flow becomes more stronger, the contribution from electric field to the viscosity of emulsion is not considerable. The rheological responses of emulsions possessing higher viscosity ratio is not different from those of EV1 in qualitative point of view, but more drastic viscosity reduction (about 55%) in the intermediate shear rate region is observed as shown in Fig. 2. Furthermore, although positive ER effect is observed in the low shear rate region, there is an electric field strength above which the magnitude of positive ER effect is decreased. In the case of EV5, the electrorheological responses of emulsion are also diminished with increasing shear rate.

As shown in Figs. 1 and 2, it is obvious that there are three regions exhibiting distinctive rheological responses in the presence of electric field. At least conceptually, these can be classified by considering relative magnitude between shear flow and applied electric field. In conventional, in the studies of ER fluids, the dimensionless Mason number has been used to represent the relative strength of shear flow and electric polarization force. When the Mason number is smaller than unity, i.e., the electric polarization force is the dominant effect, the rheological behaviors of emulsions are determined by microstructures, if present, induced by electric field. On the other hand, the Mason number is much larger than unity, the microstructures governed by shear flow and thus the effect of electric field becomes trivial. In the intermediate value of the Mason number, two competing mechanisms are both responsible for the apparent viscosities of emulsion systems.

B. Step application of electric field

Now, let us consider the transient response of emulsion systems to a step electric field. In Figs. 3(a) and (b), the rheological responses of the emulsions EV4 and RV3 at a fixed shear rate of 0.2s-1 to a successive turn on and off of electric field strength of 1.2kV/mm is shown. As mentioned earlier, the viscosity ratio of RV3 is slightly larger than that of EV4 and the resistivity ratio of RV3 is much higher than that of EV4. Under these circumstances, both the emulsions exhibit positive ER effect.

For the emulsion EV4, the measured shear viscosity rises rapidly to a much larger value within a short period time on the order of few seconds and a slower transient relaxation process follows subsequently. The viscosity decreases with time and finally approaches a steady value which is still substantially larger than the corresponding value in the absence of the electric field. After turning off the electric field, the rheological response of the emulsion is reduced the original state with dynamic relaxation process. Among a sequence of tests on a sample under the same electric field, the first application of the electric field usually leads to the strongest response as observed by Pan and McKinley [2] for the chlorinated paraffin oil/silicone oil emulsions. In this case, the dispersed phase was more conducting than the continuous phase. They also found that the longer the zero-field time interval, the closer the following response strength to the first one. Although not great, viscosity undershoot is observed when the electric field is turn on. On the other hand, as can be seen from Fig. 3(b) in the transient responses of RV3, the viscosity overshoot or undershoot is not observed. Instead, after turning off the electric field, more longer relaxation time is required to return to the original state.

CONCLUSIONS

In this study, we considered the rheological behaviors of emulsion systems consisted of a less conducting dispersed phase and a more conducting continuous phase in a dc electric field. Depending on the relative strength of electric field and shear flow, three distinctively different behaviors are observed. When the shear flow is weak, the positive ER effect attributed to the formation of unstable electric-field-induced microstructures. When both the electric field and shear flow play an important role, the apparant negative ER effect or viscosity reduction is produced. The magnitude of viscosity reduction depends on the viscosity and resistivity ratios between the dispersed and continuous phases. As increasing viscosity ratio of emulsion systems or decreasing conductivity of continuous phase, the viscosity reduction becomes outstanding. When the shear flow is dominant effect on the rheological responses of emulsions, the zero-field-viscosity is recovered.

REFERENCES

[1] Tajiri, K., K. Ohta, T. Nagaya, H. Orihara, Y. Ishibashi, M. Doi and A. Inoue, J. Rheol. 41, 335 (1997)

[2] Pan, X.-D. and G. H. McKinley, J. Colloid Interface Sci. 195, 101 (1997)

[3] Kimura, H., K. Aikawa, Y. Masubuchi, J. Takimoto, K. Koyama and T. Uemura, J. Non-Newtonian Fluid Mech. 76, 199 (1998)

[4] Tajiri, K., H. Orihara, Y. Ishibashi, M. Doi and A. Inoue, J. Rheol. 42, 1175 (1998)

[5] Orihara, H., Y. Hosoi, K. Tajiri, Y. Ishibashi, M. Doi and A. Inoue, J. Rheol. 43, 125 (1999)

[6] Tajiri, K., H. Orihara, Y. Ishibashi, M. Doi and A. Inoue, J. Rheol. 43, 137 (1999)

[7] Taylor, G. I., Proc. R. Soc. Lond. A 291, 159 (1966)

[8]Turnbull, R. J., Phys. Fluids, 11, 2588 (1968)

	Viscosity ratio	Resistivity ratio
EV1	0.08	∼5
EV2	0.26	∼5
EV3	0.70	∼5
EV4	0.83	∼5
EV5	2.30	∼5
RV1	0.64	∼50
RV2	0.62	∼100
RV2	1.04	∼500

Table 1. Properties of emulsion systems used in experimental runs.

Fig. 1 Reduced viscosity of EV1          Fig. 2 Reduced viscosity of EV4

Fig. 3 Resposes of EV4 to step                Fig. 4 Resposes of RV3 to step

applications of electric field                    applications of electric field

(E0 = 1.2kV/mm) at 0.2s-1.                     (E0 = 1.2kV/mm) at 0.2s-1.