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Ultrasonics Sonochemistry 15 (2008) 274–278

Short Communication

Effects of ultrasonic waves on the interfacial forces betweenoil and water

Tarek Hamida 1, Tayfun Babadagli *

University of Alberta, Department of Civil and Environmental Engineering, School of Mining and Petroleum, 3-112 Markin CNRL-NREF,

Edmonton, AB, Canada T6G 2W2

Received 20 November 2006; received in revised form 11 June 2007; accepted 23 September 2007Available online 1 October 2007

Abstract

The effect of ultrasound on flow through a capillary using the pendant drop method was investigated. Water was injected into a0.1 mm Hastelloy C-276 capillary tube submersed into several mineral oils with different viscosity, and kerosene. The average drop rateper minute was measured at several ultrasonic intensities. We observed that there exists a peak drop rate at a characteristic intensity,which strongly depends on oil viscosity and the interfacial tension between water and the oil. The semi-quantitative results reveal thatthe remarkable change in the interfacial forces between oil and water could be the explanation to the enhancement of oil recovery whenthe ultrasonic waves are applied.� 2007 Elsevier B.V. All rights reserved.

PACS: 43.35.Zc; 43.35.Ns; 43.35.Ei; 43.35.-c; 68.03.Cd; 68.03.Kn

Keywords: Interfacial tension; Ultrasonic waves; Oil–water interface; Capillary forces; Pendant drop; Oil recovery

1. Introduction

Field tests and laboratory investigations have demon-strated that high intensity acoustic stimulation mayenhance oil recovery in rocks [1–8]. Despite a vast bodyof empirical and theoretical support, this technology lackssufficient understanding to make meaningful, consistentengineering predictions. This is in part due to the complexnature of the physical processes involved, as well as theshortage of fundamental/experimental research.

Our recent research showed that the ultrasonic wavesinfluence capillary forces in porous media [9–11] causingremarkable changes in the shape of the interface betweentwo immiscible fluids [12,13]. More efforts are needed inclarifying the type and level of this influence on the interfa-cial properties such as interfacial tension and wettability.

1350-4177/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.ultsonch.2007.09.012

* Corresponding author. Tel.: +1 780 492 9626.E-mail address: tayfun@ualberta.ca (T. Babadagli).

1 Now with Alberta Research Council, Edmonton, AB, Canada.

Many researchers have investigated the rise of liquid levelwithin a capillary tube subjected to an ultrasonic field. Mostof the investigations attribute this rise to ultrasonic cavita-tion at the tip of the capillary. A collapsing vapor bubblecan exert thousands of pounds per inch of pressure locally,causing a temporary increase in phase pressure. When suchbubbles collapse rapidly, the cumulative pressure increaseresults in a rise in fluid level. It was also shown that the wet-tability of liquids on solid ultrasonically driven surfaces isenhanced [14]. Therefore, applying ultrasonic radiationonto a capillary may increase capillary forces, and alterthe wetting properties of a vibrating capillary.

The drop formation at a capillary tip has been studiedextensively. Two types of flow regimes have been identifiedto describe fluid ejection from a capillary. At high rates,drops exit the capillary in form of a jet (connected set ofdrops), while at low rates, distinct droplets form at thetip. An excellent experimental treatise on this topic maybe found in the thesis of Cramer [15]. Drop formation isusually divided into two stages: (a) early static growth at

Glass vesselFilled with water

Ultrasonichorn

Ultrasonicgenerator

Constant head pressure

camera

computercapillary

Glass beaker

Rubber stopper

Fig. 1. Experimental setup of the pendant drop experiments.

Fig. 2. Snap shots of pendant drops just before detachment. A wide rangeof viscosity and interfacial tension was investigated by using: (a) lightmineral oil, (b) heavy mineral oil, (c) kerosene, and (d) high viscosityprocessed mineral oil (N350). Numbers at the top of each image indicatesthe ultrasonic setting [0 = no ultrasound; 5 = high ultrasound]. Afterinspecting the shape of the drops visually, it can be readily concluded thatinterfacial tension does not noticeably change with ultrasonic intensity.

T. Hamida, T. Babadagli / Ultrasonics Sonochemistry 15 (2008) 274–278 275

the capillary, and (b) necking/detaching. These mecha-nisms are described in greater detail elsewhere [16–18].

Graham [19,20] and Graham and Higdon [21,22] stud-ied the motion of fluid droplets in harmonically forced cap-illary tubes. They observed remarkable enhancement inmobility for large droplets (droplet diameter exceedingthe diameter of the capillary) when oscillatory forcing isstrong, and the drop capillary number is low. They attrib-uted this enhancement to the increased droplet deforma-tion and observed that frequency, amplitude and type ofwaveform play a critical role on it as well.

Zaslavskii [23] performed experiments to study themotion of small droplets in a capillary. They applied a sta-tic pressure on the droplet to control the level of the fluidwithin the capillary, and altered the frequency of vibration.They noticed a hysteretic dependence of surface tensionforces on the velocity of the meniscus.

In a series of experiments, Tamura et al. [14] showedthat liquids may adhere to flat ultrasonically driven sur-faces. The shape of the droplet depends on the amplitudeof vibration. After investigating the effects of intrinsic sur-face tension and intensity, they developed a simple modelto predict drop shapes. The predicted drop dimensionsmatched closely with the experimental observations.

In this study, a series of pendant drop experiments wereperformed to explore the effect of ultrasound on drop ratewithin narrow capillaries. The drop rate is a strong func-tion of both viscosity and interfacial tension. The dropexperiments may explain various observations made duringthe imbibition experiments. In essence, the capillary repre-sents a pore throat. When applying ultrasound to porousmedia, a series of physical mechanisms, such as change inthe wettability and vibrations of ganglia at the pore throat,may induce a higher capillary snap-off threshold. Thismechanism may considerably improve the percolation ofoil ganglia into adjacent pores, and ultimately lead tohigher oil recovery.

2. Experimental setup

Equipment: A Hastelloy C-276 0.1 mm capillary wasused to inject water at a constant pressure into various oleicfluids. A tear drop shaped glass vessel was utilized to main-tain injection at a constant hydrostatic head pressure of10 cm H2O (0.1422 psi). A 3/4’’ ultrasonic horn wasemployed to deliver continuous ultrasound at a broadrange of intensities at an angle of approximately 45�. Theoleic phase was contained in a 2 mm thick glass cylinder(5 cm diameter) which was sealed with a rubber stopperat the bottom end. The glass container served to protectthe capillary from the intense cavitation zone generatedby the horn, and ensured that the observed drop rate wasentirely due to ultrasound alone. It also reduced the levelof emulsification which may alter the rheological propertiesof the oleic phase. One inch of the capillary was immersedinto the oleic phase. Images of the falling drops wererecorded using a Canon Xi video camera, and digitized

using Windows Moviemaker. The experimental setup ispresented in Fig. 1. Snap shots of the droplets just beforedetachment are shown in Fig. 2 for increasing ultrasonicintensities.

Fluid properties: Three types of refined mineral oils andkerosene were used in the experiments. The fluid propertiesare shown in Table 1. Fluids were chosen to cover a broadrange of interfacial tension and viscosity. N350 (CanonInstruments calibration oil) is a high viscosity, low IFT

Table 1Properties of the fluids used in pendant drop experiments

Fluid Densitya (g/cc) Viscosityb (cp) Surface tensionc (dynes/cm) Interfacial tensionc (dynes/cm)

Light mineral oil 0.8383 ± 0.0050 46.5 ± 0.5 9.9 ± 0.2 61.8 ± 1.2Heavy mineral oil 0.8508 ± 0.0050 167.0 ± 1.7 46.2 ± 0.9 51.0 ± 1.0Kerosene 0.768 ± 0.0050 2.9 ± 0.03 53.2 ± 1.1 40.7 ± 0.8N350 0.8885 ± 0.0014 1112.0 ± 1.8 8.6 ± 0.2 35.3 ± 0.7

a Weight–volume method using a 5 cc syringe.b Rotational viscometer (Fann 35 A) at 300 rpm; viscosity values for S60, S200, N350 and S600 were obtained from Canon Instrumentation Company

(A2LA Certificate#1262.01).c Du Nouy typetensiometer (Fisher Scientific Tensiometer Platinum–lridium ring).

276 T. Hamida, T. Babadagli / Ultrasonics Sonochemistry 15 (2008) 274–278

oil. Light mineral oil and heavy mineral oil are low viscos-ity, high IFT oils. Kerosene has very low viscosity, andintermediate IFT to water. De-aerated water was used asthe injected phase, primarily to prevent emulsification athigher intensities of ultrasound, as well as to increase thecavitation threshold due to the absence of dissolved air.

Procedure: After applying a constant fluid pressure of10 cm H2O onto the capillary, we allowed the flow ratethrough the capillary to stabilize to a constant rate. Ordi-narily, a constant flow rate was achieved after 2 min ofdripping. Once a constant flow rate was maintained, we ini-tiated ultrasound at gradually increasing power settingsand a frequency of 20 kHz. Experiments at a particular set-ting usually last more than 10 min to assure a consistenttime averaged drop rate to come to equilibrium, and pro-vide a large enough count to be statistically meaningful.In order to get an accurate drop count, we used a CanonXi camera to record the process. The analog film was thendigitized, and drops were counted during the period oftime. Drop rate per minute was estimated by dividing thenumber of drops over the time interval at which these

0

5

10

15

20

25

30

0 10 20Ultrasonic transd

Dro

p c

ou

nt

per

min

ute

Keroseneμo = 2.9 cpIFT = 40.7 dynes/cm

Fig. 3. Water drop count per minute versus ultrasonic t

drops were measured. For example, if we counted 43 dropsin 10 min and 32 s, the resulting drop rate is 4.23 drops/min. In order to avoid emulsification of the oil–water sys-tem, we carefully monitored the texture of the liquids anddid not exceed the emulsification intensity.

3. Results and discussion

Fig. 3 shows the results of all experiments. The dropcount increases up to a critical threshold after which it lin-early decreases with increasing intensity. It is apparent thatthe dripping rate is dependent on both viscosity and inter-facial tension between the oil and water. The peak droprate tends to shift to higher intensities as viscosity isincreased, and shifts to lower intensity settings withdecreasing interfacial tension as pointed by the arrows inFig. 3.

As the viscosity of the continuous phase increases, ahigher intensity is required to maximize the water expulsionrate through the capillary. Lower intensities are necessary

30 40 50 60ucer amplitude [μm]

Light mineral oilμo = 46.5 ± 0.5 cpIFT = 61.8 dynes/cm

Heavy mineral oilμo = 167.0 ± 1.7 cpIFT = 51.0 dynes/cm

N350 (processed mineral oil)μo = 1112.0 ± 1.8 cpIFT = 35.3 dynes/cm

Viscosity

IFT

ransducer excitation amplitude for various oil types.

T. Hamida, T. Babadagli / Ultrasonics Sonochemistry 15 (2008) 274–278 277

when interfacial tension is reduced. One of the reasonsbehind such observation is that in the presence of ultra-sound, the event of neck-breaking between the capillarytip and the droplet is enhanced. Since the rate of dropletformation is directly related to neck-breaking, which inturn is a function of the mechanical oscillations at the neck,it is expected that one observes a lower drop rate as inten-sity is increased. At low intensity, however, oscillations atthe neck are reduced, resulting in a more steady state drop-let formation. This causes droplets to form at a higher rate.It was also observed that the shift in peak drop rate pla-teaus at high viscosity, and therefore does not change con-siderably from 167 cp to 1112 cp.

It has also been shown that fluids tend to adhere toultrasonically driven surfaces [14]. Therefore, as oneincreases the intensity, vibration of the capillary due tothe ambient acoustic field causes an increase in wetting ofthe water onto the capillary. Increased water wettingresults in higher positive capillary pressure, thereby retard-ing the flow of water through the capillary. Apparently,this effect is magnified at high intensity. At low intensities,on the other hand, such process may be reversed. Waterwetting is reduced, hence reducing the capillary pressure.As a consequence, more water is expelled from the capil-lary. One key feature, namely the peak water rate at a char-acteristic intensity value, indicates a switch in the physics ofthe process.

A less likely explanation, which should still be consid-ered, is the formation of micro-bubbles within the capil-lary. Micro-bubbles may form during cavitation, andmay stick to the inner wall of the capillary. This effectresults in a lowered effective diameter of the capillary,resulting in a reduced flow rate of water. Nevertheless,

0

5

10

15

20

25

30

0.001 0.01 0.1

Modified Capillary Num

Ave

rag

e d

rop

co

un

t (d

rop

s/m

inu

te)

Keroseneμo = 2.9 cpσ = 40.7 dynes/cm

LMOμo = 46.5 ± 0.5 cpσ = 61.8 dynes/cm

Fig. 4. Average drop rate per minute versus mod

the de-aeration of the water considerably reduced the prob-ability of this to happen.

The hypothesis that interfacial tension between thewater and the oil was altered under ultrasound is question-able. Comparing the drop shape at different ultrasonicintensities (Fig. 2), it can be readily concluded that the dropshape did not noticeably vary with increasing intensity.Since the images taken are of limited resolution, it wasnot possible to estimate interfacial tension via the geomet-ric method, since the error due to digital processing wouldhave exceeded the variation.

To further quantify the change in capillarity as ultra-sound is introduced to the submersed capillary, we intro-duce a density corrected capillary number Ca, defined asfollows:

Ca ¼ lAfr� q0

qwð1Þ

where l is the oil viscosity, A is the vibration amplitude inmeters, f is the frequency in 1/s, and r is the interfacial ten-sion between the oil phase and water. Fig. 4 presents theresulting plot of average drop rate per minute versus mod-ified capillary number for all oil–water fluid pairs. The sys-tematic shift of peak drop rate with increasing viscosity aswell as the increasing trend with decreasing interfacial ten-sion is clearly noticeable. The similar trend over five ordersof magnitude of capillary number illustrates the consis-tency of the phenomenon. Even after density correction,it was not possible to collapse these curves into one univer-sal non-dimensionalized curve, implying that there is anadditional mechanism that was unaccounted for. Wettabil-ity may play an important role in the process.

1 10 100

ber (density corrected)

HMOμo = 167.0 ± 1.7 cpσ = 51.0 dynes/cm

N350μo = 1112 ± 1.8 cpσ = 35.3 dynes/cm

ified capillary number for all oil–water pairs.

278 T. Hamida, T. Babadagli / Ultrasonics Sonochemistry 15 (2008) 274–278

At low capillary numbers, the average drop rate remainsrelatively unchanged. At some critical capillary numberwhich depends on the properties of the oleic phase, thedrop rate accelerates. This may be caused by low amplitudevibrations at the tip, which improves the expulsion of thewater from the capillary. This process continues to highercapillary numbers until a maximum drop rate is reached,after which the drop rate dramatically falls off. The originof this fall-off is not certain, and may be caused by a super-position of effects such as improved fluid wettability on thevibrating capillary, reduced interfacial tension, and poreblocking due to cavitating bubbles.

4. Conclusions

1. Ultrasound affects the dripping rate of water through acapillary into various oleic phases.

2. The dripping rate reaches a maximum at a characteristicintensity, which depends on interfacial tension and oilviscosity (viscosity ratio). Increase in viscosity resultsin a shift of the peak to higher intensities. Reductionin interfacial tension causes a shift of the peak to lowerintensities.

3. We speculate that ultrasound affects the breaking ofdroplets from the capillary tip, causing mechanicalvibrations at the fluid which remains attached. Thesevibrations may slow down the development of consecu-tive drops. Another mechanism may be improved adher-ence of water films onto the inner wall of the capillary,thereby increasing the capillary pressure. Increase incapillary pressure reduces the ability of water to exitthe capillary. At low intensities, this effect does not seemto be considerable.

4. A density corrected modified capillary number is intro-duced, and applied to further illustrate the effect ofultrasonic excitation on capillary dripping dynamics.

5. The interference of trapped air bubbles within the capil-lary (generated during cavitation) is not believed to bethe source of our observations.

6. These promising results require further validation.

Acknowledgements

This work was partly funded by an NSERC Grant (No:G121210595). The funds for the equipment used in theexperiments were obtained from the Canadian Foundationfor Innovation (CFI) (Project # 7566) and the Universityof Alberta. We gratefully acknowledge these supports.We are also thankful to Dr. Peter R. Toma for his valuableadvice and fruitful discussion during this study. During thereview process of the manuscript, one of the -anonymous-reviewers suggested the use of dimensionless variables togeneralize the experimental results. This suggestion led usto propose a modified capillary number (Eq. 1). We thankhim for this insightful contribution.

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