Effect of Dispersion Stability on Electrorheology of Water-Based ZnO

Jun 29, 2016 - The stabilized nanofluids were then subjected for measuring of electrorheological behavior using a rotating viscometer attached to a cu...
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Effect of Dispersion Stability on Electrorheology of Water-based ZnO Nanofluids Muhammad Adil, Hasnah Mohd Zaid, Lee Kean Chuan, and Noor Rasyada Ahmad Latiff Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01116 • Publication Date (Web): 29 Jun 2016 Downloaded from http://pubs.acs.org on July 5, 2016

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Effect of Dispersion Stability on Electrorheology of Water-based ZnO Nanofluids. Muhammad Adil,*,† Hasnah Mohd Zaid,† Lee Kean Chuan,† and Noor Rasyada Ahmad Latiff† †

Fundamental and Applied Sciences Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 32160 Tronoh, Perak, Malaysia ABSTRACT: Untreated nanoparticles possess huge surface areas compared to their mass, resulting in strong inter-particle interactions in saline water. This induces a strong tendency of particles’ agglomeration, rapid sedimentation and consequently reduced the mobility of nanoparticles in the aquatic environment, which ultimately lowering the effective viscosity of the nano system. This study aimed to investigate the effect of stabilizers on the stability of dielectric nanofluid, to provide a better electrorheological characteristic for Nano-EOR purposes. In this research, zinc oxide (ZnO) was employed as dielectric nanoparticles under various nanoparticles concentration (0.1, 0.05, 0.01 wt. %). Anionic surfactants (SDS, SDBS, and Oleic acid) were compared in an attempt to prepare the homogeneous dispersions with long-term stability at high temperature (~ 95°C). The laboratory experiments were designed to evaluate the sedimentation behavior of nanoparticles using visualization method; whereas UV–vis spectrophotometry was employed to quantitatively characterize the stability of the nanoparticle dispersions. Further, dynamic light scattering (DLS) were also used to determine the size distribution of dispersed nanoparticles. The stabilized nanofluids were then subjected for measuring of electrorheological behavior using a rotating viscometer attached to a custom-built solenoid coil. From the experimental results, it is concluded that the most stable aqueous dispersion of ZnO nanoparticles is obtained at 0.1 wt. % with the aid of 0.025 wt. % SDBS under the conditions of 60 min of ultrasonication, adjusted at the pH value of 2. The ZnO/SDBS dispersion having a hydrodynamic size of 240.9 nm exhibits extreme stability at high temperature of 95°C, with the supernatant ZnO concentration decreasing only 19% compared with a decrease of 100% for the bare ZnO/Brine system. The rheological measurements indicated that all the nanofluids exhibit pseudoplastic (shear thinning) behavior. While the 0.1 wt. % ZnO/SDBS dispersion provide an enhancement in the relative viscosity of nanofluid up to 11% compared to brine as a basefluid, indicating the role of stability to achieve an electrorheological effect by activating dielectric ZnO nanoparticles. Additionally, the viscosity ZnO nanofluid increased with the increase of particle concentration under an applied field, which shows the strong dependence of viscosity on particle loading. The combined treatment with the surfactant, pH and ultrasonication is recommended to enhance the electrorheological characteristics of ZnO nanofluid. Hence, the mobility of a stabilized nanofluid can be efficiently controlled by regulating the applied field for EOR purposes.

INTRODUCTION Ultrafine and nanosized single-metal oxide powders have gained a lot of interests as the front runners in the technologically important material. For practical applications, suspension of nanodimensional particles in liquid (termed as nanofluid) can be of huge advantages due to; (i) enhanced sedimentation stability due to the counterbalance of force of gravity by the surface forces; (ii) thermal, optical, rheological, electrical and/or magnetic properties that strongly influenced the morphology of as-synthesized nanoparticles [1, 2]. Rheology is a key topic of interest among the above-mentioned advantages, which investigates the flow and deformation of materials under external forces [3, 4]. The rheological properties of nanofluids (NFs) evaluate the quality of those fluids; hence their influence for further

application. Many researchers have investigated the rheological properties of metal oxide NFs using rotating rheometers [5-8]. Among the several types of metal oxides, development of dielectric materials for novel application in the field of enhanced oil recovery such as viscosification, interfacial tension reduction, emulsion and foam stabilization, etc. has been recently proposed [9-11]. Dielectric materials have been broadly classified into insulators and semiconductors based on dielectric constant [12]. A dielectric material possesses electric charge carriers of typical arrangement that can be displaced by an electric field. In order to compensate for the electric field, the charges become polarized such that the positive and negative charges travel in opposite directions. A low dielectric constant is often desirable for several applications. However, for enhanced oil recov-

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ery application, high loss dielectrics will require that rendering the particles as the surface-active agent. The surface active agent suspended in a liquid undergo polarization and re-orientation when an external field is applied, providing the fluid rheology alteration called electrorheological effect [13 - 15]. This effect causes the viscosity of the suspension to increase due to the formation of fibrillated network when polarized molecules are reoriented along the direction of the applied electric field. In enhanced oil recovery, the mobility of oil to the production well will increase, when the viscosity ratio between the displacing fluid and oil to be recovered were reduced. However, despite their intriguing performance under extreme reservoir conditions such as high temperature and/or high salinity, the difficulty of transportation and high retention of nanoparticles due to high surface energy and strong inter-particle interactions are still debatable without a suitable surface coating. Derjaguin, Verway, Landau, and Overbeek (DVLO) developed a theory [16, 17] which involved the inter-particle interaction among the nanoparticles due to their vast surface area. DLVO theory implies that the stability of a particle in suspension is evaluated by the sum of van der Waals attractive [18] and electrical double layer repulsive forces [18] which exist between particles as they move towards each other due to the Brownian motion [20] they have underwent. The attractive forces induce the strong tendency of agglomeration, rapid sedimentation and subsequently restricted the flow of nanoparticles in the porous medium. This is because of Brownian motion that causes solid bridging between particles in close contact at room temperature, due to van der Waals attraction forces [21]. However, the particles aggregate depends on the zeta-potential, i.e. attraction and repulsion between nanoparticles. Therefore, if the particles have sufficient repulsion force between the electric double layers to offsets the attraction between particles, the suspensions will exist in a stable state [19]. With the increase in temperature, the Brownian motion (kT) of nanoparticles turn stronger, and the collision frequency of nanoparticles will increase. Hence, the stronger collision leads to the dissociation of the aggregated clusters; and consequently a stable dispersion. The aggregates change not only the hydrodynamic size but also the morphology, which ultimately lowering the effective viscosity of the colloidal system [22]. Previous observations indicated that the properties of rocks including porosity and permeability can also be altered due to NPs aggregation, which leads to pore-throat blocking and increment in pressure drop [23]. This observation is similar to that described by Ogolo et al. [24] and Ehtesabi et al. [25]. Therefore, one of the major challenges in the application of nanoparticles is the dispersion of the nanoparticles into liquids, because uniformly dispersed nanoparticles will improve not only the properties of nanofluids but also the overall injection process for EOR [26, 27].

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Additionally, there is a lack of investigation of the performance of nanofluids at high temperature and high salinity, which can broaden the potential application areas of nanofluids. Hence, the successful preparation of stable nanofluids becomes the fundamental step towards the application of nanoparticles for the enhancement of oil recovery at drastic reservoir conditions. There are three techniques for the stabilization of dispersion of nanoparticles in liquid including, (i) surface modification of particle by adding surfactants [28], (ii) pH alteration of the basefluid [29] and (ii) ultrasonic vibration [30]. The modification of the particle surface involves adsorbing organic groups on nanoparticles by employing surfactants [31], or the in-situ modification of nanoparticle surface where the organic groups are introduced during the synthesis of nanoparticles [32]. The second technique is to change the pH values of the basefluid of the nanofluids. The pH value of basefluid is related to zeta potential which directly influences the stability of nanofluids [30]. Thus altering the pH value can affect the dispersion stability of nanofluids. Finally, the dispersion can also be stabilized by the aid of supersonic waves, which generates cavitation oscillations and consequently affects the dispersion [33, 34]. On the other hand, the increase in salinity may also affect the stability of nanofluid in two ways; aggregating the particles by decreasing the electrostatic double layer forces, or by reducing the surfactant solubility in brine [35]. It has been established based on the studies to date that the amount and the charge of nanoparticles in the nanofluid, and their interaction with the dispersant directly affect the stability of the suspension. Therefore in this study, an attempt to prepare aqueous dispersions of ZnO NPs in the presence of anionic surfactants (SDS, SDBS, Oleic acid) is made, which resulted in the generation of stable homogeneous dispersions. The laboratory experiments were designed to evaluate the sedimentation behavior of ZnO nanoparticles using visualization method along with UV–vis spectrophotometry. The average particle size of the nanofluids was also measured using dynamic light scattering technique. The spectrophotometric measurements were employed to characterize quantitatively the stability of the nanoparticle dispersions which systematically studied the effects of the aqueous suspension pH, dispersant concentration, period of ultrasonication and the weight fraction of the dispersed nanoparticles on the enhanced stability ratio at a relatively high temperature of 95°C. An optimal pH value and optimal dispersant concentration can result in the highest stability of the nanofluids. Hence, expected to provide guidance to design nanofluids with excellent performance to exploit the enhanced dielectric behavior of ZnO NPs to achieve an electrorheological effect for EOR purpose.

EXPERIMENTALS Materials. ZnO nanoparticles were synthesized using solgel auto-combustion method [36], and were used after calcining at 500°C having an average particle size of 55 nm.

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Sodium dodecyl sulfate (SDS), sodium dodecylbenzenesulfonate (SDBS) and oleic acid (OA) from Sigma Aldrich were used as stabilizers. These anionic surfactants were of analytic grade and used without further purification. Deionized water (with σ = 18 MΩ) was used as a solvent. NaCl obtained from Fisher Scientific, was employed as salt to prepare brine of a concentration of 30000 ppm (equivalent to sea water concentration). The pH value of system was adjusted with HCl and NaOH solution by precise pH meter (Mettler Toledo, FE20-Basic).

Nanofluid Preparation. The nanofluids (NF) were prepared using two steps method: (1) to add the as-synthesized nanoparticles in the basefluid (brine) and (2) to prepare the dispersion of nanoparticles in the basefluid by adding the surfactant, ultrasonic vibration, and altering the pH value. Nanoparticles with weight concentration of 0.1 wt. % were dispersed in brine as the basefluid and magnetically stirred for 1 hour to produce nanoparticles suspension. In this step, the pH was adjusted below PZC to support the adsorption of anionic surfactant by providing the high zeta potential of nanoparticles that provide a high electrostatic repulsion, leading to a more stable and uniform dispersion. The role of PZC can be explained by the help of a model as shown in Fig.1. In the second step, the stabilizing agent was added to the suspension which enables long-term stability in spite of high nanoparticle concentration. The anionic surfactant used as the stabilizer in this study were SDS, SDBS, and oleic acid; mixed with different concentrations in the nano dispersion. The surfactant concentrations were selected using CMC determination methods at the experimental condition of 95°C (tabulated in Table 1). Then the suspension was agitated in an ultrasonic bath at ambient temperature for an optimum time to get a homogenous suspension in the basefluid. Stability Evaluation Methods. Sedimentation Technique. The traditional batch sedimentation approach and photographic method were used

Figure 1. The point of zero charge model.

Table 1. Concentrations of anionic surfactants adopted in this work. Conc. #1

Conc. #2

Conc. #3

(= CMC)

(> CMC)

(>> CMC)

mM

wt. %

mM

wt. %

mM

wt. %

SDS

4

0.115

4.9

0.14

5.72

0.165

SDBS

0.5

0.017

0.7

0.025

1.75

0.050

Oleic Acid

0.02

0.0006

0.035

0.001

0.05

0.0015

Surfactant

to investigate the settling characteristics of ZnO suspensions by recording the sediment volume against the sediment time. Li et al. [37] used the same method to study the stability of Cu-H2O nanofluids with and without the addition of a dispersant. In the current study, the investigation was conducted at a fixed concentration of 0.1 wt. % of ZnO nanoparticles in the brine-surfactant mixture. The pH of the dispersion was adjusted in the range of 6-1 based on autotitration (Fig. 2), which was below the PZC value of ZnO suspension. The autotitration shows the influence of pH on the zeta potential, along with a measure of the average size of particle size distribution. The pH significantly altered the average size of pure ZnO NPs; with the minimum size of 427.9 and 502.3 nm measured at the pH value of 2 and 12 respectively. These dispersions were allowed to settle under gravity in the oven maintained at 95°C and observed the settling behavior. For autotitration, the ZnO sample was dispersed in deionized water due to the limitation of Zetasizer at a high salt concentration of 3 wt. % NaCl. Therefore, a preliminary sedimentation test was conducted throughout the pH range in the presence of 3 wt. % NaCl, which revealed a complete sedimentation for all values of pH due to charge screening. On the other hand, the surface of reservoir rocks such as sandstone (consists mainly of quartz, feldspar, and clays) is negatively charged [38]. This suggests that the nanoparticles must have negative or neutral surface charge to

Figure 2. Autotitration of ZnO nanoparticles suspended in deionized water with acid and base over the pH spectrum of pH 2 to pH 12; where the blue line shows the isoelectric point.

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flow through the porous medium without retention. Hence, the pH range of 6-1 were chosen in order to manipulate the surface charge of nanoparticles by the adsorption of anionic surfactant. The change in surface charge was performed not only to al-low the particles to flow through without retention, but also to achieve stable dispersion.

Spectral Absorbency Analysis. Spectral absorbency analysis is another efficient method to study the stability of nanofluids. In this method, the stability of 0.1 wt. % dispersion was evaluated by measuring the sediment concentration of 50-ml dispersion versus the sediment time. The samples were left under ambient temperature and also under uniform heating at 95°C for 20 days. In aqueous solution, the absorption of ZnO-NPs appeared at 240.9 nm. In general, the intensity of absorbance follows a linear relationship with the concentration of nanoparticles in fluid. This relationship was used as a basis for the determination of ZnO-NPs concentration in the dispersion. ZnO dispersions were prepared without any additives in the range of 0.1– 0.005 wt. % and then examined using a UV-Vis scanning spectrophotometer (Biochrom Libra S60) at a wavelength of 240.9 nm. Based on these results, a linear calibration curve between ZnO-NPs concentration and absorbance was plotted as depicted in Fig. 3. This similar standard curve is then used to perform spectral analysis of ZnO-surfactant dispersions; where the same concentration of aqueous surfactant solution was employed as the reference to eliminate the absorbance of surfactant in the suspensions. Thus, only the absorbance of ZnO-NPs was obtained, which used to determine the concentration of ZnONPs in the sediment by fitting the absorbance to the calibration curve (Fig. 3). Dynamic Light Scattering Measurement. The average hydrodynamic particle size of the nanofluids with and without surfactants were measured by a dynamic light scattering instrument (Zetasizer Nano-ZS, Malvern Instruments Inc., UK). The nanofluids were aged in the oven

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maintained at 95°C, for the equal period as in other stability evaluation techniques. Each measurement was repeated at least three times to calculate the average value of the experimental data. The particle size measured via a DLS system is the diameter of the ideal sphere which diffuses at the same rate of the particle being measured. The DLS instrument uses a correlation to determine the size distribution based on different populations existing in the sample, representing a separate peak for each population. Whereas, if only one peak is found, then it shows that a large majority of the particles have a common average diameter.

RESULTS AND DISCUSSION Characterization of ZnO Nanoparticles. The untreated ZnO NPs are practically insoluble in water, as shown by its low solubility of 57, 22 and 10 mg/L for ZnO particles with average size of 4, 15 and 241 nm respectively, at pH 7.5 [39]. In spite of that, the surface atoms of the precipitates tend to coordinate with ions due to the presence of unsaturated bonds to form Zn–OH groups in water. These Zn–OH groups become either positively (Zn–OH2+) or negatively (Zn–O-) charged by joining with or liberating a proton [40], depending on pH. For the ZnO NPs used here, an isoelectric point of pH = 7.37 was obtained as in Figure 3, and the pH of the NPs dispersion in brine (pH = 6.76) was measured to be 6.98. When dispersed in aqueous solution of SDS, SDBS and Oleic acid, an initial pH of NFs is tabulated in Table 2. Effect of pH. The pH of nanofluid is an important factor affecting the dispersion stability. The dispersion stability of ZnO in the brine-surfactant mixture, with different pH values, was determined by visual inspection (Fig. 4). All the nanofluids were found to have deposited at the bottom of tubes except the samples at the pH value of 2 and 1. The reason might be the isoelectric point of nano-dispersion that proved to have a critical effect on the dispersion stability of nanofluids. Near the isoelectric point, the particle Table 2. Concentrations of anionic surfactants adopted in this work. Concentration Surfactant

SDS

SDBS

Oleic Acid Figure 3. The calibration curve of ZnO nanoparticles in aqueous solution at λ = 240.9 nm.

Initial pH

mM

wt. %

Brine + Surfactant

Brine + Surfactant + NPs

4

0.115

6.47

7.18

4.9

0.14

6.49

7.36

5.72

0.165

6.47

7.41

0.5

0.017

5.25

6.19

0.7

0.025

5.16

6.22

1.75

0.050

5.77

7.23

0.02

0.0006

6.13

6.14

0.035

0.001

6.44

7.00

0.05

0.0015

6.21

7.06

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Figure 4. The contrast photo of sedimentation technique.

surface is electrically neutral that lead to a weak electrostatic stabilization and hence a poor stability of nano-dispersion. The dispersion stability of ZnO nanofluid was lower when the pH value was 6 due to the isoelectric point of ZnO nanoparticles at the pH value of 7. However when the pH value was far from the isoelectric point, i.e. 2, the positive zeta potentials on the surface of nanoparticles were larger, which in turn increased the adsorption of anionic surfactant providing a stronger electrostatic repulsion between the nanoparticles. Hence, with lower pH value in the basefluid, more amount of charge exist on the surface of nanoparticle which suppresses cluster formation thus enhanced stability. Since the stability of the nanofluid depends on the pH of the basefluid, hence the trends are similar for all the surfactant concentrations.

Effect of Surfactants. The effect of anionic surfactants and their concentrations on dispersion stability were further studied at the optimized pH value of 2, using spectral absorbency analysis. Here, the concentration of ZnO-NPs in the sediment was determined by fitting the absorbance from the spectral analysis to the calibration curve (Fig. 3). At high temperature. Fig. 5 illustrates the ZnO concentration in the sediment suspension versus the sediment time under the condition of 95°C, with and without anionic surfactants. The inherent mechanism in the process of nanofluids preparation by the addition of surfactant can be described as follows: (i) reduced surface tension of nanoparticles (ii) enhanced affinity with water, (iii) improved wettability of the particle surface, (iv) reduced attraction between the particles, (v) effective steric hindrance between nanoparticles, and (vi) improved repulsive energy between the nanoparticles; thus preventing the agglomeration of nanoparticles. The results showed that for the nanoparticles with the different properties, there were big differences of the dispersion stability under the action of different concentrations of anionic surfactants. When ZnO nanofluid was added with the surfactant concentration of 0.14 wt. % SDS, 0.025 wt. % SDBS and 0.001 wt. % oleic acid, the total sedimentation was the lowest and the dispersion stability was the best. The main reason for these different dispersion effects

Figure 5. ZnO concentration vs. sediment time for 0.1 wt. % ZnO suspensions at pH 2 under the condition of 95°C.

is the adsorption of free-surfactant on the dispersed nanoparticles, which might have different acting mechanisms. The negative ions dissociated from anionic surfactant in water were adsorbed by ZnO nanoparticles having the positive charge at pH 2 and neutralizes the charge of ZnO particles. As a result, Van Der Waals forces between nanoparticles were overcome by the action of electrostatic stabilization. The surfactants’ concentrations above CMC at 95°C, shows a decrement in dispersions stability as the additional free-surfactant dissociates at high temperature decreases the stability of nanoparticles. The adsorption mechanism of SDBS on ZnO particles is illustrated in Fig. 6.

At ambient temperature. Since the CMC values used in this experiment were determined at an elevated temperature of 95°C. Therefore, as shown in Fig. 7, nanofluid showed a poor dispersibility at the ambient conditions in the presence of anionic surfactants. Influence of Ultrasonication. Based on Fig. 5, the surfactant concentration of 0.025 wt. % SDBS and 0.001 wt. % oleic acid were selected to study the effect of the period of ultrasonication on dispersion stability. While SDS was neglected, due to the lowest stability among the others; and more importantly the lack of economic feasibility. In this experiment, the impact force generated by the cavitation of the ultrasonic were the key acting force to distribute and disperse nanoparticles. Therefore, ultrasonic conditions would essentially have an effect on the dispersion stability of nanofluids. The effects of different ultrasonication time on dispersion stability of ZnO nanoparticles in the brinesurfactant mixture at pH 2 are shown in Fig.8. The figure shows that the total concentration of ZnO nanoparticles in the sediment first increased then decreased,

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Figure 6. The adsorption of SDBS on ZnO nanoparticles.

Figure 7. ZnO concentration vs. time for 0.1 wt. % ZnO suspensions at pH 2 under the ambient condition.

and later increased again as the ultrasonication time increased. When the ultrasonication time was 60 min, the sedimentation was at the smallest and the stability was the strongest. The total sedimentation of 0.019 wt. % of ZnO nanoparticles was observed out of 0.1 wt. % in case of SDBS. While 0.021 wt. % sediment was quantified for oleic acid. This indicated that the ultrasonication is more beneficial to the dispersion stability within a specified time period. Hydrodynamic particle size. The average particle size of ZnO–brine nanofluids with and without surfactants are presented in Table 2. According to Stokes–Einstein equation, the average particle size of the nanoparticles in the nanofluid is greater than the actual size [41]. However, from the table, it is observed that the average particle sizes

Figure 8. Effect of the ultrasonication period on 0.1 wt. % ZnO suspensions at pH 2 under 95°C.

of the nanoparticles in the nanofluids with the surfactant are smaller than that without surfactant, due to the obvious agglomeration of the nanoparticles. The average particle size of the nanofluid with 0.025 wt. % SDBS is observed to be lowest among all the surfactant added, which is also corroborated from absorbency test. Effect of Concentration. Fig. 9 shows the percentage stability of ZnO nanofluids with different concentration of weight fraction of nanoparticles; where the other parameters used (pH, surfactant dosage, ultrasonication) were similar as of 0.1 wt. % ZnO. The figure portrays that the sample with 0.05 and 0.01 wt. % particles suffer the most agglomeration, while the sample with 0.1 wt. % nanoparticles reached better suspension. These results are in

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Table 3. Average particle size of ZnO nanofluids in brine (3 wt. % NaCl) as well as brine–surfactant mixture at a fixed concentration of 0.1 wt. % ZnO. Surfactant added

Concentration (wt. %)

Average particle size (nm)

-

-

955.4

0.115 SDS

SDBS

Oleic Acid

the Table 4. Comparison of average particles size of ZnO nanofluids at varying concentration in the presence of brine and 0.025 wt. % SDBS. Average particle size (nm)

Concentration of ZnO NPs (wt. %)

Brine (3 wt. % NaCl)

0.025 wt. % SDBS

310.4

0.1

955.4

240.9

0.14

261.7

0.05

824.9

208.4

0.165

268.1

0.01

615.1

195.7

0.017

293.5

0.025

240.9

0.050

350.2

0.0006

380.9

0.001

265.8

0.0015

402.2

agreement with the hydrodynamic particle size of nanofluids proportional to the different weight fraction of ZnO nanoparticles, as presented in Table 2. The maximum settling rate was obtained for the nanofluid with 0.01 wt. % particles in the presence of 0.025 wt. % SDBS. This was due to the over loading of dispersant which agglomerates the particles. In order to make the nanoparticles disperse steadily in aqueous solution, the added surfactant’s dosage should fulfill the condition of forming the double layer saturation adsorption on the nanoparticle’s surface. The adsorption quantity of surfactant on the particle surface is increased by the increment of surfactant’s dosage, before the dosage being saturated. Thus, the electrostatic repulsion between nanoparticles increased and the dispersion stability was enhanced. However, once the surfactant’s adsorption quantity reached saturation, excessive surfactant introduced into the liquid phase; while ions from the electrolyte solution increased the ionic strength of the system and changed the stability of nanofluids. Meanwhile, the bridging of nanoparticles’ coupling effect was enhanced by the free liquid electrolyte concentration, which consequently make the dispersion stability get worse. Therefore 0.1 wt. % ZnO is considered as the suitable concentration of nanoparticles for a stable dispersion, owing to the agglomeration and precipitation of ZnO nanofluid with 0.05 and 0.01 wt. % at 0.025 wt. % SDBS. Viscosity under EM waves. Viscosity is one of the major factors which could manipulate to alter the mobility of reservoir fluid. Currently, the quantitative information of viscosity of nanofluids is vital for industrial applications, especially in heavy oil recovery. Many factors such as particle concentration, size distribution, basefluid, pH value, temperature, and surfactants influence the viscosity. These factors strongly affect the morphology of suspended nanoparticles/nanofluids [42, 43] by altering the structure of electrical double layer around particles/aggregates [44] and

Figure 9. Effect of different concentrations of ZnO nanoparticles on percentage stability of nanofluids.

interaction between particles/aggregates due to attractive van der Waals force and repulsive electrostatic force (DLVO forces) [45]. A rotating viscometer (Brookfield DV-I+) attached to a custom-built solenoid coil was used to measure the viscosity of nanofluid. The schematic diagram of measuring setup is depicted in Fig. 10. The electric field perpendicular to the stainless steel (SS) sample chamber was generated by the solenoid surrounding the spindle. The UL adapter spindle was chosen to be used with the viscometer, which required 16 mL of nanofluid for every measurement. Nanofluid was sheared from 10 s-1 to 122.4 s-1 considering the instrument restrictions. The torque during the experimental measurement was kept within 10-90 % of the maximum torque. The suggested torque during the experimental measurement was maintained within 10–90 % of the maximum torque. However, the low-viscous fluids cannot be measured in this range, especially at low shear rate. Therefore, the viscosity measurements at low shear rates were conducted in less than 10 % torque with more measurement points and average technique to have better accuracy, as recommended by Behi and Mirmohammadi [46].

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(a)

Figure 10. Schematic diagram of experimental setup for measuring nanofluid viscosity under applied electromagnetic field.

Effect of Stability on Viscosity. In this section, two types of ZnO NFs with varying solid contents (0.1, 0.05, and 0.01 wt. %) were prepared. One type of NFs with optimal stability parameters; while the other dispersions were prepared in brine only under optimum pH of 2. Fig. 11 shows that the both types of NFs exhibit the non-Newtonian (shear- thinning) behavior. In the colloidal suspension study, the reason that the Newtonian viscosity of a basefluid is mostly modified to be non-Newtonian by the addition of nanoparticles, which comes from the complex interactions between the fluid and particles and between particles themselves. It can be seen that the shear rate depends non-linearly on the viscosity (especially from 0 to 25 s-1), indicating the non-Newtonian behavior of ZnO NFs under the condition of this work. With an increase in shear rate, the interaction between particles become weaker and are broken down; thus, nanofluids show Newtonian behavior. As indicated by Fig. 11a, ZnO–SDBS mixture demonstrated an increase in relative viscosity under EM waves, similar to that of brine alone (Fig. 11b) due to electrorheological effect. It is noted that the applied electric field had a greater effect on the relative viscosity of ZnO–Brine NFs than ZnO–SDBS. However, the apparent viscosity of ZnO–SDBS (0.1 wt. %) under EM wave is recorded to be 56 cP at 0.36 s-1, as compared to 40 cP for ZnO–Brine which shows the increment of viscosity up to 28.5 %. The electrorheological effect on SDBS-modified ZnO NFs is depicted in Fig. 12,

Figure 12. The polarization of (a) nanoparticle and (b) anionic surfactant under applied electric field.

(b)

Figure 11. The relative viscosity of ZnO nanofluids under EM waves; where the ZnO particles are dispersed in (a) SDBS and (b) Brine at different particle concentrations.

where both the nanoparticle and the anionic surfactant are polarized. As shown in Fig. 12a, the positive and negative surface polarization gathers at both sides of the nanoparticle and the surfactant. The surfactant polarization causes the electrically opposite charge to accumulate at the inner side of the surfactant which partially neutralizes the surface polarization charge of ZnO crystal; while an equal amount of charge distributes at the outer side of the surfactant (Fig. 12b). Hence, weakens the electrorheological effect which leads to a small change in relative viscosity of ZnO–SDBS mixture. Since the viscosity of basefluid was not affected by an applied electric field, the viscosity of NFs is determined by the properties of dielectric nanoparticles. Under an electric field, dielectric nanoparticles are polarized and the dipole

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moments rotate, leading to the occurring of rotational/orientational polarization at the interfaces of nanoparticles. This consequently creates the most stable chain structure of NFs by aligning the dipole moment parallel to the electric field. Then, a share rate is applied along with an external electric field to NFs. At this point, the relative viscosity at 0.36 s-1 for ZnO NPs (0.1 wt. %) dispersed in SDBS and brine is recorded to be 3.11 and 6.67 under EM waves, as compared to 1.33 and 2.33 respectively. With the increase in share rate, the direction of the chains changes in the direction of electric dipole moment, which makes the chains weaker. However, the oriented microstructure of ZnO/SDBS NFs provides a higher resistance to the increasing shear rate, due to the particle-particle interaction of stable NFs. Thus, ZnO/SDBS NFs under electric field presents a greater relative viscosity of 1.26, 1.22, and 1.1 at the share rate of 61.2, 73.44, and 122.4 s-1 respectively; which is significantly higher as compared to 1.12, 1.10, and 1.04 for ZnO/Brine system. Fig. 11 also shows the relative viscosity as the function of shear rate for different concentration of ZnO nanoparticles (0.1 – 0.01 wt. %). It was observed that the nanofluids with higher concentration (0.1 wt. %) of nanoparticles exhibit higher viscosity than of lower concentrations (0.05 and 0.01 wt. %). It seems that the possible existence of particle aggregations at the lower concentration of nanoparticles (as depicted in Fig. 9) can be responsible for the present results. According to [47, 48], the viscosity of nanofluids has been affected by the aggregations where the sizes of aggregates are between 3 and 4 times the diameter of nanoparticles. These particle aggregations can explain the nonNewtonian viscosity of nanofluids, i.e. the presence of a Newtonian plateau is followed by a shear-thinning behavior at the high shear rate where the aggregate is possibly destroyed under shear. Therefore in such cases, the low shear rate viscosity (LSV) is always different in comparison to high shear rate viscosity (HSV). In an oil reservoir, the fluid flows at a high flux during the waterflooding, generates high shear rate in consequence. Thus, the high shear rate viscosity (HSV) is more practical and vital from the engineering perspective.

dispersion reach closely to the optimized value, the particle surface charge rises because of constant attacks to the surface hydroxyl groups and phenyl sulfonic group by potential-determining ions (H+, OH- and phenyl sulfonic group). Therefore, the combined treatment with both the pH and chemical dispersant is suggested to improve the dispersion stability which subsequently enhances the high shear rate viscosity (HSV) of nanofluids. Results have shown that the viscosity of ZnO NFs displayed the strong dependence on particle loading, as well as the stability of the dispersion. The viscosity of NFs increases with the increase of particle concentration under EM waves. While the ZnO/SDBS dispersion provide an enhancement in the relative viscosity of NFs between 5 to 11 % at the relatively high shear rate of 61.2, 73.44, and 122.4 s-1, compared to brine as a basefluid. This indicates the role of stability to achieve an electrorheological effect by activating dielectric ZnO nanoparticles.

AUTHOR INFORMATION Corresponding Author *Tel.: (+60) 19-524 5700. Email: [email protected]

Author Contributions The manuscript was written through contributions of all authors.

ACKNOWLEDGMENT The authors would like to acknowledge Ministry of Higher Education Malaysia (FRGS Grant No: 0153AB–I47) and Department of Fundamental and Applied Sciences, Universiti Teknologi PETRONAS for funding the research.

ABBREVIATIONS CMC, critical micelle concentration; DLS, dynamic light scattering; NPs, nanoparticles; NFs, nanofluids; SDS, sodium dodecyl sulfate; SDBS, ), sodium dodecylbenzene-sulfonate; OA, oleic acid; PZC, point of zero charge; SS, stainless steel; LSV, low share rate viscosity; HSV, high shear rate viscosity.

REFERENCES

CONCLUSIONS This study concerned with the stability of ZnO dispersion in the brine-surfactant mixture under different pH values, surfactant concentrations and ultrasonication periods. The results of the experiments concluded that the most stable aqueous dispersion of ZnO nanoparticles is obtained at 0.1 wt. % with the aid of 0.025 wt. % SDBS under the conditions of 60 min of ultrasonication, adjusted at the pH value of 2. The ZnO/SDBS dispersion exhibits extreme stability at high temperature of 95°C, with the supernatant ZnO concentration decreasing only 19% compared with a decline of 100% for the bare ZnO/Brine system. This suggests that the nanofluids can be tailored by adjusting the concentration of surfactant based on CMC value at any given temperature. As the pH and SDBS concentration of nano-

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