Journal of Petroleum Science and Engineering 58 (2007) 201 – 206 www.elsevier.com/locate/petrol

Effect of advancing velocity and fluid viscosity on the dynamic contact angle of petroleum hydrocarbons A.A. Keller ⁎, V. Broje, K. Setty Bren School of Environmental Science and Management, University of California, Santa Barbara, USA Received 2 August 2006; received in revised form 5 December 2006; accepted 6 December 2006

Abstract The goal of this research was to study the effect of advancing velocity and liquid viscosity on the dynamic contact angle between a solid surface and various hydrocarbons. The Wilhelmy plate technique was used to measure the dynamic contact angle at advancing velocities between 20 and 264 μm/s. In addition to hydrocarbons, two silicon oils were also tested for comparison purposes. The results indicate that advancing velocity and oil viscosity have a significant effect on the dynamic contact angle for both hydrocarbon and silicon oils. For example, the advancing contact angle for viscous (1540 mPa s at 15 °C) oils was up to 2 times higher at 200 μm/s than at 20 μm/s. As a result, it is recommended that when values for dynamic contact angles are reported, the advancing velocity at which they were measured be indicated. This will ensure correct data interpretation, meaningful comparison between various studies, and better prediction of multiphase flow or adhesion processes that depend on the dynamic contact angle. © 2007 Elsevier B.V. All rights reserved. Keywords: Wilhelmy plate; Petroleum hydrocarbons; Silicon oil; Multiphase flow; Adhesion; Contact angle; Oil recovery; Advancing velocity

1. Introduction Contact angles of liquids on solid surfaces are widely used to predict wetting and adhesion properties of liquid/solid combinations. This method has been widely discussed in the literature (e.g. Good, 1977; Wake, 1982; Mittal, 2000). When a drop of liquid is deposited on the solid surface, an angle θ can be formed. The affinity of the solid for the liquid increases with decreasing θ. The size ⁎ Corresponding author. 3420 Bren Hall, Bren School, UCSB, Santa Barbara, CA 93106–5131, USA. Tel.: +1 805 453 1822; fax: +1 805 456 3807. E-mail addresses: [email protected] (A.A. Keller), [email protected] (V. Broje), [email protected] (K. Setty). 0920-4105/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.petrol.2006.12.002

of this angle is determined by the equilibrium of surface forces including liquid surface tension (γLV), solid surface energy (γSV) and surface tension at liquid–solid interface (γSL). The relationship between forces can be described by Young's equation (Young, 1805): gLV cos h ¼ gSV −gSL

ð1Þ

The theory of contact angle measurements is based on the equilibrium of an axisymmetric sessile drop on a flat, horizontal, smooth, homogeneous, isotropic, and rigid solid. In practice, a static contact angle does not give a correct representation of the wetting and spreading processes. Using the Wilhelmy plate technique and measuring a dynamic contact angle provides more accurate description of the behavior of a liquid at a

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liquid/solid interface. Static contact angle devices measure a contact angle at a single point of contact between liquid and solid. The dynamic contact angle (DCA) analyzer overcomes this limitation by measuring the contact angle over a larger surface area. This eliminates the effect of local compositional and textural imperfections on the measured angle. The DCA analyzer operates by holding a plate in a fixed vertical position attached to a microbalance and moving a test liquid contained in a beaker at constant rate up and down past the plate. Microbalance measures the force exerted by the moving contact line in the advancing and receding directions. The forces acting on the plate consist of the weight of the plate, the buoyancy of the submerged part of the plate, and the surface tension of liquid in contact with the plate. This can be expressed as (Della Volpe et al., 1998):
F ¼ qp lwt g−ðql hwt Þg ð2Þ þ 2ðw þ t Þgl cos h where F = force measured by DCA, ρp is density of plate, w = width of plate, l = length of plate, t = thickness of plate, g = acceleration due to gravity, ρl = density of liquid, h = length of submerged part, γl = surface tension of liquid, and θ = contact angle between liquid and plate. The first term in Eq. (2) is eliminated by zeroing the balance after the plate is attached to it. The second term can be neglected because the force exerted on the microbalance by moving line measured during advancing of the plate through the liquid, is extrapolated to find a force acting on the plate at the moment when only lower edge of solid is in a contact with the liquid. At this point, the plate is not affected by buoyancy. The remaining force acting on the plate is the surface tension of the meniscus. This is illustrated in Fig. 1. For this case, Eq. (2) can be simplified to: F ¼ 2ðw þ t Þgl cos h

ð3Þ

Knowing the force (F) exerted by a meniscus on the plate and the surface tension of the liquid, the contact angle can be found by the following relationship: cos h ¼ F=2ðw þ t Þgl

ð4Þ

Eq. (4) is the modified Young's equation or Wilhelmy equation (Kaya et al., 2000). The assumption behind this equation is that the solid surface is smooth and nonporous. This equation is used by the DCA to eval-

Fig. 1. Static and dynamic contact angles between liquid and solid surface.

uate the dynamic contact angle between a liquid and a solid. The dynamic contact angle is widely used to describe wetting, spreading, and adhesion processes between a liquid and a solid. A number of scientists have observed that a dynamic contact angle measured at high advancing/receding velocities and with viscous fluids differs significantly from the static value and is dependent on measurement parameters (Ranabothu et al., 2005; Tavana and Neumann, 2006). Fig. 1 illustrates the difference between the static and dynamic contact angle. Reorientation of molecules at the solid–liquid–air interface (Hansen and Miotto, 1957; Elliott and Riddiford, 1966; Cherry and Holmes, 1969), surface heterogeneity (Cherry and Holmes, 1969; Johnson et al., 1977), and drag (viscous and capillary) force (Strom et al., 1990; Morra et al., 1992; Giannotta et al., 1993; Sauer and Kampert, 1998) were among the factors affecting the magnitude of the dynamic contact angle measured at higher velocities. Although a large number of liquids and solids have been studied previously, to our knowledge, none of these previous studies examined the magnitude of the dynamic contact angle change for crude oils or hydrocarbon products. Adhesion and wetting properties of crude oils have been studied by a number of researchers interested in crude oil behavior in reservoir formations (Gloton et al., 1992; Akhlaq et al., 1996, 1997; Liu and Buckley, 1999; Drummond and Israelachvili, 2002, 2004; Freer et al., 2003). There have been several papers reporting dynamic contact angles between crude oils and various surfaces. Most of these experiments used a drop technique to measure the advancing and receding angles. The rate of advance of the interface was not specified in many studies (Mennella et al., 1995; Yang et al., 1999; Rao, 2001; Xie et al., 2002).

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We believe that a dynamic contact angle value is meaningful so long as the rate at which it was measured is reported as well. This will ensure correct data interpretation and allow meaningful comparison between various studies. This study helped to quantify the effect of oil viscosity and advancing rate on the magnitude of the measured dynamic contact angles for crude oil and hydrocarbon products. 2. Methods A DCA utilizing the Wilhelmy plate technique (Cahn Radian 315, Thermo Electron Corporation) was used for all measurements. This fully automated equipment is capable of measuring adhesion-related parameters such as dynamic contact angle, surface tension, surface free energy, surface polarity and the amount of adhered oil with high accuracy. The technical characteristics of this DCA are presented in Table 1. This DCA has been successfully used by other researchers to study wetting and adhesion properties of various surfaces (e.g. Lee et al., 1998; Della Volpe et al., 1998; Della Bona et al., 2004). The experiments were performed in a temperature-controlled room. The temperature was controlled with a precision of ± 1 °C. All hydrocarbons were tested at 15 °C and 25 °C. In addition, two silicon oils were tested at 25 °C for comparison purposes. 2.1. Materials Fisherbrand® Microscope Cover Glass slides (12– 541-B) with dimensions of 22 × 22 mm and thickness of 0.15 mm were used as a test surface. The following liquids were used in the experiment: • Pt. McIntyre crude oil — an Alaskan crude; • HydroCal 300 — a hydrotreated naphthenic medium grade lubricant oil; • IFO-120 — intermediate fuel oil; • Two silicon oils (poly(dimethyl siloxane)) with viscosities of 50 and 975 cP — Brookfield viscosity standard fluids. Properties of the tested liquids measured in the temperature-controlled room at 15 °C and 25 °C are listed in

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Table 2 Properties of tested liquids Test liquid

Viscosity (mPas)

Pt. McIntyre crude HydroCal 300 IFO-120 Silicon oil 50 Silicon oil 1000

Surface tension (mN/m)

15 °C

25 °C

15 °C

25 °C

24 342 1540 n/a n/a

14 162 487 50 975

28.3 32.5 32.4 n/a n/a

27.3 31.8 31.8 19.7 20.9

Table 2. The surface tension was measured using the DCA with the du Noüy ring technique. The viscosity was measured with a Digital Viscometer DV-II+ Pro (Brookfield). 2.2. Test procedure The contact angle measurement process can be summarized as follows: A pre-cleaned glass slide was measured using calipers for exact width and thickness with an accuracy of ± 0.01 mm. The glass slide was then briefly exposed to a propane-torch flame to remove any residues, cooled for a few seconds, and placed in the test chamber above the test liquid. The lower edge of the slide was positioned parallel to the liquid surface. Once the trial was initiated, the test surface was automatically submerged into the test liquid at a pre-selected rate (20, 80, or 264 μm/s) until 7 mm of the slide was submerged, and then it was withdrawn at the same velocity. The DCA measured the force on the test surface while it entered the liquid, was submerged, and then retracted. The DCA software calculated the corresponding advancing and receding angles. In order to ensure the accuracy of the data, an average of five measurements was taken for each liquid–solid combination. During the receding phase, a thin film of the residue remained on the glass surface adding some weight to the force measured by the DCA. The DCA software could not separate this additional weight from the force exerted by the meniscus leading to an error in the estimated value of the receding angle. Since a film of liquid was remaining on the surface after receding phase, representing

Table 1 Technical characteristics of Cahn Radian 315 dynamic contact angle analyzer Surface tension range

Contact angle range

Surface tension precision

Contact angle precision

Balance precision

Max sample weight

Max sample diameter

Min fiber diameter

1–1000 mN/m

0–180°

±0.001 mN/m

±0.01°

1 μg

100 g

75 mm

0.1 mm

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Table 3 Advancing contact angles between a glass slide and different hydrocarbon oils Temperature

15 °C

25 °C

Advancing velocity (μm/s)

HydroCal 300

Pt. McIntyre

Cos(θ) ±0.01

Angle (°)

Cos(θ) ±0.01

Angle (°)

Cos(θ) ±0.01

Angle (°)

20 80 175 264 20 80 175 264

0.8394 0.7506 0.6479 0.6206 0.829 0.8059 0.8055 0.8065

33 41 50 52 34 36 36 36

0.9884 0.9543 0.9288 0.9231 0.979 0.9557 0.9299 0.9276

9 17 22 23 11 17 22 22

0.8969 0.6183 0.4154 0.3896 0.916 0.7481 0.6141 0.5878

26 52 65 67 24 42 52 54

complete wetting, the receding angle was considered to be equal to 0°. 3. Results and discussion Results of tests with hydrocarbons at 15 °C and 25 °C are summarized in Table 3. The silicon oils were tested at 25 °C. The results for silicon oils are presented in Table 4. In both tables cos(θ) refers to the cosine of the advancing angle. The graphic representation of these data is shown in Fig. 2. Fig. 2 shows that both oil viscosity and advancing velocity significantly affect the magnitude of the advancing contact angle. At velocities below 150 μm/s, the slopes of the curves are proportional to oil viscosity. At velocities between 200 and 260 μm/s for viscous oils, and at about 170 μm/s for the light Alaskan oil, the contact angle curves reach a plateau, and the advancing contact angle does not change as the advancing velocity is increased. The dynamic behavior of silicon oils was similar to that reported in the literature (Strom et al., 1990). Results from Strom et al. (1990) are plotted along with our data in Fig. 3. Silicon oils appear to reach a plateau at velocities greater than those tested in this experiment. The behavior presented in Fig. 2 is similar to that reported in the literature for other solid/liquid combinations. For example, Elliott and Riddiford (1966), using optical means to measure the dynamic contact angles,

IFO-120

found a dependency between the contact angle and the interfacial velocity. They studied water interacting with siliconed glass plates, water with polyethylene plates, and a water-saturated hydrocarbon oil with siliconed glass plates at 22 °C. At very low rates (up to about 17 μm/s), the dynamic contact angle as essentially equal to the equilibrium contact angle. Above this critical velocity, the contact angle increased linearly with velocity. At higher rates (over 167 μm/s) the rate of change decreased and eventually reached a limiting value. HydroCal oil was the only oil that exhibited a pattern slightly different from the other tested liquids. At 25 °C it showed no change in the value of contact angle with increasing advancing velocity for velocities exceeding 80 μm/s. This could be explained by the fact that this oil was refined and artificially modified to enhance its lubrication properties. As a commercially available lubricant oil, it contains some additives affecting its

Table 4 Advancing contact angles between a glass slide and two silicon oils Temperature

25 °C

Advancing velocity (μm/s)

Silicon oil 50

Silicon oil 975

Cos(θ) ± 0.005

Angle (°)

Cos(θ) ± 0.005

Angle (°)

20 80 175 264

1.0000 1.0004 0.9812 0.9682

0 1 11 14

0.8983 0.7206 0.5368 0.4723

26 44 58 62

Fig. 2. Effect of advancing velocity on advancing contact angle.

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silicon oil. The data from Hoffman (1975) for silicon oils also follow the same pattern. Although from Fig. 2 the behavior of silicon and hydrocarbon oils appears similar, the analysis in Fig. 3 indicates that there are in fact two trends, due to differences in chemical composition of these liquids. This is particularly important at low capillary numbers. This suggests that silicon oils should not be used to model the wetting behavior of hydrocarbon oils without correcting for these differences. 4. Conclusions

Fig. 3. Relation between capillary number and advancing contact angle.

spreading properties. The exact chemical composition of this oil is not reported. At 15 °C, the behavior of HydroCal was comparable to other viscous oils. Pt. McIntyre and IFO-120 exhibited patterns of behavior similar to the silicon oil. These experiments clearly indicate the importance of specifying oil viscosity and advancing velocity when reporting advancing contact angle values. For light Pt. McIntyre oil, the measured advancing angle varied between 9–23° at 15 °C and 11–22° at 25 °C, depending on the advancing speed. For the more viscous IFO-120, the contact angle varied between 26–67° at 15 °C and 24–54° at 25 °C, depending on the advancing speed. A number of scientists (Hoffman, 1975; Strom et al., 1990; Sauer and Kampert, 1998; Petrov et al., 2003; Barraza et al., 2002; Mourik van et al., 2005; Carré and Woehl, 2006) used the dimensionless capillary number, Ca, to describe relative effect of surface tension, viscosity and advancing velocity on the dynamic contact angle. The capillary number is the ratio of viscous to capillary forces at the liquid/gas interface, and it is calculated by (Mourik van et al., 2005): Ca ¼ l⁎ V =r

ð5Þ

Where μ is the viscosity of the liquid, σ is the surface tension and V the advancing velocity. A low capillary number implies that viscous forces are relatively unimportant in comparison with capillary forces. Fig. 3 presents the relationship between log10Ca and advancing contact angle for the tested hydrocarbon and silicon oils, along with data from Strom et al. (1990) for

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