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Dispersion stability and surface tension of SDS-Stabilized saline nanofluids with graphene nanoplatelets
Colloids and Surfaces A 592 (2020) 124584
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Dispersion stability and surface tension of SDS-Stabilized saline nanofluids with graphene nanoplatelets
T
Suhaib Umer Ilyasa,*, Syahrir Ridhaa,b, Firas Ayad Abdul Kareemc a
Institute of Hydrocarbon Recovery, Universiti Teknologi PETRONAS, 32610 Seri Iskandar, Perak Darul Ridzuan, Malaysia Petroleum Engineering Department, Universiti Teknologi PETRONAS, 32610 Seri Iskandar, Perak Darul Ridzuan, Malaysia c Shale Gas Research Group (SGRG), Institute of Hydrocarbon Recovery, Universiti Teknologi PETRONAS, 32610 Seri Iskandar, Perak Darul Ridzuan, Malaysia b
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Dispersion stability Graphene nanoplatelets Nanofluid Saline fluid SDS Surface tension
The dispersions of engineered nanomaterials in fluids have envisioned numerous industrial applications. Despite less experimental data available on the surface tension of nanofluids, it is one of the critical parameters to define thermal configurations and two-phase transport properties. This study focuses on the benchmark experimental investigation of the surface tension behavior in graphene nanoplatelets-based saline nanofluids in ambient air using the pendant drop method. Graphene nanoplatelets are dispersed in saline fluid (30PPT) using optimum anionic SDS stabilizer and ultrasonication, exhibiting excellent stability for minimum 72 h. Different characterizations are performed for nanoplatelets and nanofluid stability such as electron microscopy, FTIR, XRD, DLS and sedimentation analysis. Surface tension measurements are taken at varying concentrations of graphene nanoplatelets range of 0-0.25 wt% and temperature range of 25−65 °C. The obtained results from this research exhibit that the addition of nanoplatelets drops the surface tension of saline fluid by 21 %. However, the increase in concentration from 0.05 wt% to 0.25 wt% does not have a considerable implication on the overall surface tension behavior. It is observed that the surface tension of the saline fluid and the nanofluid decrease with the elevation in temperature.
1. Introduction Nanofluids are composed of conventional liquids containing smart
⁎
nanomaterials having at least one demission less than 100 nm. Since the pioneering study by Choi [1] in the 90 s, nanofluids have been subjected to numerous investigations and implemented in many
Corresponding author. E-mail address: [email protected] (S.U. Ilyas).
https://doi.org/10.1016/j.colsurfa.2020.124584 Received 12 November 2019; Received in revised form 10 February 2020; Accepted 11 February 2020 Available online 12 February 2020 0927-7757/ © 2020 Elsevier B.V. All rights reserved.
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showed a slight decrease up to 2 wt% loading and then increased. Similarly, Vafai et al. [27] found ambiguous results for surface tension at different nanoparticle loadings. Few studies [28–31] used surfactants, such as SDBS and SDS, to stabilize nanoparticles in the fluid before experimental measurement of nanofluid surface tension. In general, the surfactant concentration is kept low to avoid foaming and disintegration at high temperature operation. However, few studies [32,33] revealed that the surface tension could be lowered by introducing surfactants or stabilizing agents. Cabaleiro et al. [34] experimentally investigated surface tension of graphene oxide and reduced graphene oxide aqueous nanofluids. The impact of graphene functionalization and nanomaterial concentration (0.0005-0.1 vol%) was studied on the surface tension of aqueous-based nanofluids. It was found that the surface tension decreased by increasing graphene loading, with maximum diminutions of 3% for 0.1 vol% nanofluid. In another study [35], surface tension of phase change material nano–emulsions (paraffin-inwater) were experimentally determined in a concentration range of 2−10 wt.%. It was reported that the surface tension reduces up to 50 % with the presence of paraffin droplets. Saline solutions (also known as salt water) have significance in many applications under medical, electrolysis, microbiology, food, agriculture, etc., sectors. However, the intention of this research is focused on petroleum recovery applications, where a heavy amount of saline solutions is utilized in gas wells during hydraulic stimulation or fracking operation. In a study by Chen and Carter [36], it was concluded that only 14 states of the USA consume 9.30 Mm3 of water (in 2008–2014) for hydraulic fractured wells. One of the alternatives to reduce the consumption of fresh water is utilizing Enhanced Oil Recovery (EOR) and Improved Oil Recovery (IOR) methods, where brine or saline solutions with different compositions are profoundly injected as drilling fluid to alter the wettability behavior of reservoirs [37–40]. The surface tension between the injected fluid and the trapped gas plays a significant role in petroleum recovery applications. The aim of the present research is to introduce graphene nanoplatelets in saline media since graphene has proven to be a significant EOR agent. To the author’s knowledge, this combination of nanofluid has not been subjected to surface tension investigation, hence establishing a benchmark study for saline nanofluids. The literature review reveals highly conflicting results for surface tension and identifying many unknown phenomena associated with nanofluid-air surface tension behavior. Different characterisations and analysis are performed for graphene nanoplatelets and the stability of different concentrations of nanofluids. Obtaining stable nanofluids is one of the most crucial challenges [41–44] for actual applications, where most of the studies are performed in aqueous nanofluids. An optimum concentration of SDS stabilizer is introduced in graphene nanoplatelets-based nanofluids and the stability is evaluated for 72 h after a series of mechanical mixing. Pendant drop method is utilized to measure surface tension of base saline fluid and nanofluids. In this research, the primary function of SDS inclusion in graphene nanoplatelets-based nanofluids is to stabilize nanomaterials only. Therefore, the surface tension of pure saline water and SDS-stabilized nanofluids are compared and presented with the temperature variance. The literature [32,33,35] reveals that the addition of surfactant in water drastically reduces interfacial tension of the fluid. Therefore, the present study aims to compare the combined impact of SDS and graphene nanoplatelets on the surface tension of saline water. The obtained results of the surface tension of SDS-stabilized nanofluids with different graphene nanoplatelets loadings are compared with the pure saline water (30PPT).
biomedical, petrochemical, energy conservation, oil recovery, manufacturing, lubrication, cooling, electronics and food processing applications due to a dramatic improvement in thermophysical properties [2–8]. Several studies have been reported on the effective thermophysical properties of nanofluids, especially density, viscosity, thermal conductivity and heat capacity. However, the studies on the surface tension and the contact angle are rather limited. Most of the reported studies on surface tension in literature correspond to liquids [9], demonstrating a wide research gap on the surface characteristics at the gas-nanofluid interface. The introduction of nanomaterials in liquids significantly impact the surface/interfacial tension, wettability characteristics and contact angle. A recent study by Hernaiz et al. [10] reveals the consensus on dealing the contact angle of nanosuspensions as a thermophysical property, where the droplet volume and temperature play a crucial role in heat transfer applications. The surface tension between liquid and gas have a critical significance to boiling and condensation heat transfer applications, where the bubble formation generates an interface between two mediums and highly depends on the density of two fluids. Surface tension is defined as the molecular force at the interface of two immiscible mediums. The intermolecular attractive forces between the molecules at the interface (also known as surface energy) causes a tension between two fluids [11,12]. A recent study by Estelle et al. [13] proves the importance of surface tension and wettability characteristics of nanofluids in heat transfer and energy conversion operations, especially boiling and condensation processes. The Bond number defines the dominance of surface tension forces and the buoyancy forces, which is an essential parameter in boiling heat transfer flows [14]. A lower value of surface tension is considered preferable for efficient boiling heat transfer. Estelle et al. [13] described the importance of surface tension in boiling heat transfer, flow boiling in microchannels, critical heat flux, external flow convection, thermosiphons and heat pipes. Heat transfer characteristics of these applications, especially boiling heat transfer coefficient and critical heat flux [15,16], are highly dependent on the dimensionless numbers involving surface tension of the fluid. These dimensionless paraments involve Bond number [17], Weber number [18], Capillary number [19] and Kutateladze number [20]. Surface tension of fluids can be measured in different types of tensiometers, such as Wilhelmy plate, maximum bubble pressure, spinning drop, Du Nouy ring, Capillary rise and Pendant drop [13]. In a recent study by Wanic et al. [21], the surface tension of ethylene glycol-based nanofluids was investigated using Du Nouy ring and pendant drop methods. It was reported that both methods did not influence the surface tension measurements nanofluids containing different types of nitrides. Few recent experimental studies have been conducted on the surface tension of nanofluids with air. Berrada et al. [15] stated that the surface tension is a function of particle loading, nature, shape, size, base-fluid type and the surfactant characteristics. They investigated surface tension of MWCNT dispersions in water- and water-PG (60:40)based nanofluids using the pendant drop method at different temperatures and nanotubes loadings. It was found that the surface tension decreased with temperature. It was also found that the surface tension decreased in water-based nanofluids. However, the opposite trend was observed in water-PG based nanofluids. Chinnim et al. [22] introduced Al2O3, ZnO, TiO2 and SiO2 nanoparticles in PG/Water (60:40) nanofluids and developed a correlation for the surface tension of nanofluids based on nanoparticle concentration, temperature and nanoparticle size. It was concluded that the reduction of surface tension primarily depends on temperature compared to other two parameters. Bhuiyan et al. [23] found an enhancement in surface tension for different waterbased nanofluids. Radiom et al. [24] reported a decreasing trend in surface tension at higher nanoparticle loadings. However, Moosavi et al. [25] concluded the opposite trend. Tanvir and Qiao [26] studied surface tension at ambient temperature for a wide range of MWCNT loadings. It was concluded that water-based nanofluids showed an increasing trend of surface tension. However, ethanol-based nanofluid
2. Experimental procedure 2.1. Materials Graphene nanoplatelets of surface area 750 m2/g, an average thickness of 10 nm and a molecular weight of 12.01 is acquired from 2
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Fig. 1. Transmission electron microscopy analysis of graphene nanoplatelets.
Sigma-Aldrich Malaysia. The average size of the nanoplatelet is < 2 μm and the relative gravity is 2–2.25 g/cm3. Sodium chloride NaCl (99.5 %) powder and sodium dodecyl sulfate SDS (≥ 99 %) are also acquired from Sigma-Aldrich Malaysia. De-ionized water is used in preparing saline media, which is taken from the laboratory. The transmission electron microscopy (TEM) analysis is performed using Zeiss Libra 200FE to study nanomaterial morphology using high-resolution imaging, presented in Fig. 1. Graphene nanoplatelets with minute concentration is dispersed in de-ionized water using probe-type ultrasonication for one hour to obtain homogenous dispersion. Then, the undispersed big agglomerates are allowed to settle down and the sample drop is taken from the non-aggregated zone. The sample is dropped on the copper grid and dried prior to TEM analysis.
2.3. Surface tension measurement The surface tension of nanofluids is experimentally measured by the tensiometer IFT 700 by Vinci Technologies, France. The equipment is capable of measuring surface tension, interfacial tension and contact angle at a different range of temperatures and pressures. However, the present research focuses on the surface tension of nanofluids with air medium at ambient pressure. The nanofluid droplet at different temperatures (25−65 °C) is exposed to air using the pendant drop method and the surface tension is measured. The equipment can measure a wide range (0.1–72 mN/m) of surface tension measurement at an operating temperature range of ambient-180 °C. The temperature control has an accuracy of ± 0.1 °C. In the pendant drop method, the threshold scroll enables the selection of the drop shape detection according to the surrounding light and the threshold value is used to determine drop border line and compute different parameters such as diameter, volume, Bond Number and surface tension. The horizontal limit (red line) is set at the end of the droplet needle and is kept similar for all measurements. A stainless-steel capillary needle with external diameter 4.2 mm and 0.4 mm material thickness is used for all samples. The drop analysis system software (provided by the manufacturer) allows fast computation of surface. A schematic model of IFT 700 is shown in Fig. 2(a). The equipment consists of a viewing cell with electrical heaters. Two manual pumps (equipped with temperature sensors) are used bulk fluid and drop fluid). A calibrated capillary needle is used to create a drop of nanofluid into bulk fluid. A similar needle is used for all measurements to avoid any ambiguity in results. In this research, the bulk fluid is referred to air at ambient conditions and the drop fluid is referred to the base fluid or nanofluids. The validation of the presented research is performed by measuring surface tension of de-ionized water at varying temperatures and comparison of results with the literature [45] are presented in Fig. 2(b). All measurements for water, saline fluid and nanofluids at varying temperatures are repeated ten times and an average value is presented with maximum deviations at each point (y-error bars). The maximum uncertainty for surface tension of water is found to be ± 0.6 mN/m. Similarly, a maximum error (based on standard deviation) of ± 0.71 mN/ m is observed for the nanofluid samples. The validation results from Fig. 2(b) exhibits that the measured surface tension of water is in agreement with the international tables given by Vargaftik et al. [45].
2.2. Characterizations and sample preparations The nanomaterials are subjected to different characterizations. Xray diffraction (XRD) of graphene nanoplatelets is performed in Bruker, D8 Advance to identify the crystalline phase. The XRD spectrum is measured at Kα radiation λ of 1.542 Å at 45 kV and 40 mA. The data is recorded in 2θ scan range of 15°-70° and a step size of 0.02. Infrared (IR) spectroscopy is performed using Perkin Elmer - Spectrum one Fourier transform infrared (FTIR) analysis in the range of 400-4000 cm−1. A saline media typically contains 30–50 PPT of NaCl salt. Therefore, the saline solution is prepared at 30 PPT by dissolving NaCl in de-ionized water using a magnetic stirrer for 1 h. Nanofluids are prepared using the two-step method by dispersing graphene nanoplatelets in saline media. A total of six mass concentrations are prepared; 0, 0.05 wt %, 0.1 wt%, 0.15 wt%, 0.2 wt% and 0.25 wt%. Four sets of samples are prepared to study the effect of surfactant (SDS) addition on the stability of nanofluids. Surfactant concentration is an important parameter to inspect and stabilize nanofluids [44]. SDS concentration in nanofluids are varied in different ratios to the graphene nanoparticle concentration i.e., 0:1 (no surfactant), 1:1, 1.5:1 and 2:1. The nanofluid components are mixed in the order of saline media-SDS surfactant-graphene nanoplatelets. A magnetic stirrer is used for 30 min at 500 rpm to break bigger agglomerates into smaller agglomerates. Then, the nanosuspension is subjected to ultrasonic homogenization (Biologics Inc. 150 V/T, 20 kHz) for 30 min. A pule of 70 % and a power of 80 % is applied for optimum operation of ultrasonication for all samples. The temperature of the ultrasonic homogenizer is controlled by a water circulation bath (Hahnshin, HS 3005 N). Visual sedimentation method is used to evaluate the stability of nanofluids with different concentrations. Dynamic light scattering (DLS) technique is utilized to quantify the particle size distribution in the stabilized sample using Malvern Zetasizer Nano s90.
3. Results and discussion 3.1. Nanoparticle characterizations Graphene nanoplatelets are subjected to different surface characterizations. This type of nanomaterial has naturally occurring carboxylic or hydroxylic functional group at the edges of platelets and may 3
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Fig. 2. (a) Experimental setup for pendant drop IFT700; (b) Experimental comparison of the present study with Vargaftik et al. [45] for the surface tension of water.
peak at 3420 cm−1 attributes to OeH stretching vibration [47]. The stretching vibration at peak 1580 cm−1 corresponds to C]C functional group, hence confirming the presence of graphite nanomaterial. Few additional stretching vibrations are also observed at 1722 cm−1 and 1080 cm−1 peaks, which are attributed to the C]O functional group. This group may be present due to the exposure of nanomaterial in the atmosphere, since the analytical equipment operates in open-air. Similar vibrations were also reported in the literature [48] for different graphite-based nanomaterials 3.2. Dispersion characteristics The dispersion characteristics or the stability of nanofluids have significance towards the actual application of nanofluids. The stability of nanofluids does not only depend on the nanomaterial and base fluid properties, but the temperature, preparation method, the nature of mechanical mixing and chemical techniques may have an important role. In the past decade, not much attention is given to the stability of nanosuspensions. However, it is a crucial factor and must be given primary importance prior to properties measurements. A combination of mechanical mixing and chemical technique is applied in this research to stabilize graphene nanoplatelets in saline media. Four sets of nanofluids are prepared with varying SDS loadings. The aim of this study is to utilize a minimum amount of surfactant in nanofluids because of few critical factors involved with surfactants such as surfactant disintegration at high temperature operations, foaming during turbulent flows, and the economic aspects. Sedimentation analysis is performed for all concentrations of graphene nanoplatelets-based saline nanofluids, shown in Fig. 5. Most of the studies in literature have studied dispersibility of different nanofluids for several hours to a few days [49–50], and some studies specified on 72 h [51–53]. Therefore, the dispersion behaviour of nanofluids are investigated for 72 h in this research at room temperature by following the methodology in literature [51–53]. It is a notable fact that the stability of nanofluids is degraded with the passage of time and the elevation in temperature. The well-dispersed nanomaterials in the freshly prepared nanofluids may re-agglomerate with the passage of time and the stability tends to diminution [54]. Four sets of nanofluids correspond to varying surfactant and nanomaterial (GNP) ratio. Set A with no surfactant completely loses stability within a few hours. However, Set B, C and D with 1:1, 1:1.5 and 1:2 (nanomaterial: surfactant) are exhibiting better stability than Set A. It attributes that the anionic surfactant is suitable to stabilize pristine graphene nanoplatelets in saline media. A surfactant ratio of 1.5 (to GNP) is chosen to achieve optimum stability and further investigate the effective surface tension. Low stability of Set B is visible after observation of 72 h. The maximum concentration (0.25 wt%) is found to be stable than all other concentrations. Similarly, some of the samples are showing bubble/
Fig. 3. XRD spectrum of graphene nanoplatelets.
react with the humidity in the atmosphere during exposure. Therefore, a minimum exposure of the nanomaterial in the environment has endeavoured. Fig. 3 describes XRD spectra of graphene nanoplatelets. A graphite characteristic sharp peak with high intensity is observed at 2θ = 26.4°. This peak is describing the layer-by-layer structure of graphene nanoplatelets. Similar trends for pristine graphene nanoplatelets were also reported in the literature [46]. IR Spectroscopy is performed using FTIR, shown in Fig. 4. The broad
Fig. 4. IR Spectroscopy of graphene nanoplatelets. 4
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Fig. 5. Dispersion analysis of different sets of graphene nanoplatelets-based saline nanofluid samples with varying SDS concentration (wt/wt).
measurement of base-fluid is rather lower than all nanofluid samples. It is also found that the surface tension decreases linearly with the rise in temperature ranging from 25 to 65 °C for all samples. Maximum surface tension of 40.5 mN/m is observed for the base fluid at 25 °C. However, a drastically decrease of 14 % is found in surface tension values from 25 to 65 °C. This decrement is observed to be lower in graphene nanoplatelets-based saline nanofluids. An average decrease of 2–2.7 % in surface tension values with temperature is recorded for fluids containing graphene nanoplatelets. This decrement in surface tension values with the rise in temperature is attributed to the weakening of interstratify forces between the molecules. Similar decreasing trends of surface tension with temperature were found in the literature in the case of Al2O3-based nanofluids [9,23,55] and TiO2-based nanofluids [56,57]. The addition of graphene nanoplatelets decreases surface tension of saline fluid. The impact of graphene nanoplatelets concentration on the surface tension characteristics of saline fluids is highlighted in Fig. 8. A percentage decrement in the surface tension of nanofluids is calculated using Eq. (1). Surface tension of saline nanofluids and saline base fluid are represented by σnf and σbf, respectively. The outcomes of this research clearly suggest that graphene nanoplatelets have a strong impact on the surface tension of saline media. A maximum decrease of 21.5 % is observed for the highest concentration of nanofluid at 25 °C. Ahammed et al. [28] exquisitely explained the facts for the changes in surface tension of fluids with the addition of nanomaterials. The wettability characteristics of nanomaterials are the most essential parameter in surface tension of nanofluids. The repulsion forces between the hydrophobic nanomaterial and the bulk saline fluid increases and nanomaterials lean towards the free surface zone. This phenomenon intensifies the intermolecular spacing at the interface zone and reduces surface tension. The addition of hydrophilic nanomaterials in water or
foam formation at the top of the container in Set D, whereas, Set C shows less foaming formation with similar dispersion characteristics. Therefore, Set C is found to be the optimum surfactant ratio and is proceeded with further investigation. The particle size distribution for the lowest concentration of saline-nanofluids from the chosen Set C is quantified using DLS technique after 72 h of visual observation. It is found that the average particle size does not reach up to the micron level and the nanofluid holds the stability. The average size of approximately 400 nm is found for the nanosuspension after 72 h displaying a better dispersion. The visual sedimentation study for nanofluids with 1:1.5 (nanomaterial: surfactant) surfactant concentration is extended for one week of preparation and it is found that the graphene nanoplatelets have settled down in saline media at the bottom of the container. Therefore, it is concluded that graphene nanoplatelets in saline media with SDS have achieved an ideal stability for 72 h. It is also evident from Fig. 5 (DLS) that the nanomaterial agglomerate is close to micron level and with the passage of time, it may cause sediment after one week of preparation. 3.3. Surface tension of saline nanofluids The surface tension of nanofluids is experimentally measured at varying temperatures. A sample visual reading of each concentration at 25 °C is shown in Fig. 6. It can be noticed that the nanofluid concentrations from 0.1 wt% to 0.25 wt% are showing completely black due to high concentration of graphene nanoplatelets. The experimental data for different concentrations of graphene-nanoplatelet-based saline nanofluids is plotted in Fig. 7. Each measurement is repeated ten times and the average values are reported. The uncertainty in the measurements based on the standard deviation is shown as y-error bars. It is observed from Fig. 6 that the average uncertainty in the repetitive 5
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Fig. 6. Visual comparison of sample reading for different concentrations of nanofluids at 25 °C using pendant drop method.
particle concentrations.
saline media tends towards the lingering of a majority of the nanomaterials in the bulk liquid. Thus, very few nanoparticles stay at the interfacial layer zone due to strong nanomaterial-fluid interaction. Hence, increasing the surface tension of the nanofluid. An interesting surface tension behavior is observed for nanofluids when graphene nanoplatelets concentration is increased from the lowest 0.05 wt% to the maximum 0.25 wt%. An insignificant increase from 21.25 % (0.05 wt%) to 21.5 % (0.25 wt%) is observed at 25 °C. All investigated saline nanofluid samples with varying graphene nanoplatelets loading at different temperatures exhibit similar phenomena. It is evident from Fig. 8 (a–e) that a nominal increase is found when the concentration of nanoplatelets are increased. This shows that the concertation of graphene nanoplatelets higher than 0.05 wt% does not correspond to a significant change in surface tension behavior. Therefore, surface tension of saline nanofluids in the studied range is primarily dependent on temperature only and least dependent on increasing nanoparticle concentrations. A recent study by Wanic et al. [21] showed a similar trend for ethylene glycol-based nanofluids containing nitride nanomaterials. A slight change in the surface tension behavior was observed within the uncertainty range for 1−5 wt%
σnf ⎞ σ Decrement (%) = ⎜⎛1 − ⎟ × 100 σbf ⎠ ⎝
(1)
The reported studies in literature [32,33] suggests that the addition of surfactant in the base fluid significantly decrease the interfacial tension of the fluid. For instance, Cabaleiro et al. [35] reported considerable diminution in interfacial tension between paraffin and water with the addition of SDS surfactant to the water. An addition of 0.25 wt % SDS in paraffin-water emulsions corresponded to a decrease in interfacial tension from 42 mN/m to 8.3 mN/m. The present research aims to investigate the combined effect of SDS and graphene nanoplatelets on the surface tension of saline water. The scope of the presented research is limited to the investigation of surface tension of nanofluidair system. However, the change in surface tension behaviour of the base-fluid by the addition of surfactants is of highly significance, and future studies should be performed with and without surfactants for better understanding.
Fig. 7. Surface tension of saline nanofluids containing graphene nanoplatelets at varying temperatures. 6
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Fig. 8. The decrement in surface tension of saline fluid and nanofluids at varying graphene nanoplatelets concentrations.
4. Conclusions
Declaration of Competing Interests
This research emphasizes the surface tension behavior of graphene nanoplatelets in saline medium. Pendant drop method is used, and the validation of the equipment is presented by testing surface tension of pure water, exhibiting a good agreement with the literature. Few surface characterizations are performed for the nanomaterial, showing the morphology, structure and highest purity. Graphene nanoplatelets are dispersed in 30 PPT saline media using SDS as a stabilizer agent and the ultrasonic homogenization. An optimum surfactant concentration is chosen using the visual stability examination. DLS study confirms that the agglomerate size of graphene nanoplatelets does not reach micron level. Surface tension of five different concentrations of graphene nanoplatelets dispersion in saline media is experimentally investigated for the first time in this study. The results show that surface tension of the saline fluid drastically decreases up to 14 % with temperature. Whereas, the nanofluids exhibit approximately only 2%. A maximum of 21.5 % decrement is observed in surface tension of saline fluid is perceived by the addition of graphene nanoplatelets at 25 °C. An intriguing outcome is observed when the increase in the concentration of nanomaterial from 0.05 wt% to 0.25 wt% in saline media shows an insignificant change in surface tension characteristics of saline fluid. It summarizes that surface tension is highly dependent on temperature compared to nanoparticle concentration. However, there remains an ambiguity in the available literature.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This research is supported by the Institute of hydrocarbon recovery at Universiti Teknologi PETRONAS. The financial assistance is provided by the Grant YUTP015LC0-101. References [1] S.U.S. Choi, Enhancing thermal conductivity of fluids with nanoparticles, Am. Soc. Mech. Eng. Fluids Eng. Div. FED (1995) 231. [2] B.A. Suleimanov, F.S. Ismailov, E.F. Veliyev, Nanofluid for enhanced oil recovery, J. Pet. Sci. Eng. 78 (2011) 431–437, https://doi.org/10.1016/j.petrol.2011.06.014. [3] O. Mahian, L. Kolsi, M. Amani, P. Estellé, G. Ahmadi, C. Kleinstreuer, J.S. Marshall, M. Siavashi, R.A. Taylor, H. Niazmand, S. Wongwises, T. Hayat, A. Kolanjiyil, A. Kasaeian, I. Pop, Recent advances in modeling and simulation of nanofluid flowsPart I: fundamentals and theory, Phys. Rep. 790 (2019) 1–48, https://doi.org/10. 1016/J.PHYSREP.2018.11.004. [4] M. Alhuyi Nazari, R. Ghasempour, M.H. Ahmadi, A review on using nanofluids in heat pipes, J. Therm. Anal. Calorim. 137 (2019) 1847–1855, https://doi.org/10. 1007/s10973-019-08094-y. [5] R. Taylor, S. Coulombe, T. Otanicar, P. Phelan, A. Gunawan, W. Lv, G. Rosengarten, R. Prasher, H. Tyagi, Small particles, big impacts: a review of the diverse applications of nanofluids, J. Appl. Phys. 113 (2013) 011301, , https://doi.org/10.1063/1. 4754271. [6] F. Mashali, E.M. Languri, J. Davidson, D. Kerns, W. Johnson, K. Nawaz, G. Cunningham, Thermo-physical properties of diamond nanofluids: a review, Int. J. Heat Mass Transf. 129 (2019) 1123–1135, https://doi.org/10.1016/J. IJHEATMASSTRANSFER.2018.10.033. [7] H.J. Lee, Design and development of anti-icing textile surfaces, J. Mater. Sci. 47 (2012) 5114–5120, https://doi.org/10.1007/s10853-012-6386-2. [8] M. Agista, K. Guo, Z. Yu, A state-of-the-Art review of nanoparticles application in petroleum with a focus on enhanced oil recovery, Appl. Sci. Basel (Basel) 8 (2018) 871, https://doi.org/10.3390/app8060871. [9] J. Chinnam, D.K. Das, R.S. Vajjha, J.R. Satti, Measurements of the surface tension of nanofluids and development of a new correlation, Int. J. Therm. Sci. 98 (2015) 68–80, https://doi.org/10.1016/J.IJTHERMALSCI.2015.07.008.
CRediT authorship contribution statement Suhaib Umer Ilyas: Conceptualization, Methodology, Writing original draft, Investigation. Syahrir Ridha: Project administration, Funding acquisition, Supervision, Resources, Writing - review & editing, Investigation. Firas Ayad Abdul Kareem: Visualization, Software, Investigation, Formal analysis, Validation, Investigation.
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Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Dispersion stability and surface tension of SDS-Stabilized saline nanofluids with graphene nanoplatelets
T
Suhaib Umer Ilyasa,*, Syahrir Ridhaa,b, Firas Ayad Abdul Kareemc a
Institute of Hydrocarbon Recovery, Universiti Teknologi PETRONAS, 32610 Seri Iskandar, Perak Darul Ridzuan, Malaysia Petroleum Engineering Department, Universiti Teknologi PETRONAS, 32610 Seri Iskandar, Perak Darul Ridzuan, Malaysia c Shale Gas Research Group (SGRG), Institute of Hydrocarbon Recovery, Universiti Teknologi PETRONAS, 32610 Seri Iskandar, Perak Darul Ridzuan, Malaysia b
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Dispersion stability Graphene nanoplatelets Nanofluid Saline fluid SDS Surface tension
The dispersions of engineered nanomaterials in fluids have envisioned numerous industrial applications. Despite less experimental data available on the surface tension of nanofluids, it is one of the critical parameters to define thermal configurations and two-phase transport properties. This study focuses on the benchmark experimental investigation of the surface tension behavior in graphene nanoplatelets-based saline nanofluids in ambient air using the pendant drop method. Graphene nanoplatelets are dispersed in saline fluid (30PPT) using optimum anionic SDS stabilizer and ultrasonication, exhibiting excellent stability for minimum 72 h. Different characterizations are performed for nanoplatelets and nanofluid stability such as electron microscopy, FTIR, XRD, DLS and sedimentation analysis. Surface tension measurements are taken at varying concentrations of graphene nanoplatelets range of 0-0.25 wt% and temperature range of 25−65 °C. The obtained results from this research exhibit that the addition of nanoplatelets drops the surface tension of saline fluid by 21 %. However, the increase in concentration from 0.05 wt% to 0.25 wt% does not have a considerable implication on the overall surface tension behavior. It is observed that the surface tension of the saline fluid and the nanofluid decrease with the elevation in temperature.
1. Introduction Nanofluids are composed of conventional liquids containing smart
⁎
nanomaterials having at least one demission less than 100 nm. Since the pioneering study by Choi [1] in the 90 s, nanofluids have been subjected to numerous investigations and implemented in many
Corresponding author. E-mail address: [email protected] (S.U. Ilyas).
https://doi.org/10.1016/j.colsurfa.2020.124584 Received 12 November 2019; Received in revised form 10 February 2020; Accepted 11 February 2020 Available online 12 February 2020 0927-7757/ © 2020 Elsevier B.V. All rights reserved.
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showed a slight decrease up to 2 wt% loading and then increased. Similarly, Vafai et al. [27] found ambiguous results for surface tension at different nanoparticle loadings. Few studies [28–31] used surfactants, such as SDBS and SDS, to stabilize nanoparticles in the fluid before experimental measurement of nanofluid surface tension. In general, the surfactant concentration is kept low to avoid foaming and disintegration at high temperature operation. However, few studies [32,33] revealed that the surface tension could be lowered by introducing surfactants or stabilizing agents. Cabaleiro et al. [34] experimentally investigated surface tension of graphene oxide and reduced graphene oxide aqueous nanofluids. The impact of graphene functionalization and nanomaterial concentration (0.0005-0.1 vol%) was studied on the surface tension of aqueous-based nanofluids. It was found that the surface tension decreased by increasing graphene loading, with maximum diminutions of 3% for 0.1 vol% nanofluid. In another study [35], surface tension of phase change material nano–emulsions (paraffin-inwater) were experimentally determined in a concentration range of 2−10 wt.%. It was reported that the surface tension reduces up to 50 % with the presence of paraffin droplets. Saline solutions (also known as salt water) have significance in many applications under medical, electrolysis, microbiology, food, agriculture, etc., sectors. However, the intention of this research is focused on petroleum recovery applications, where a heavy amount of saline solutions is utilized in gas wells during hydraulic stimulation or fracking operation. In a study by Chen and Carter [36], it was concluded that only 14 states of the USA consume 9.30 Mm3 of water (in 2008–2014) for hydraulic fractured wells. One of the alternatives to reduce the consumption of fresh water is utilizing Enhanced Oil Recovery (EOR) and Improved Oil Recovery (IOR) methods, where brine or saline solutions with different compositions are profoundly injected as drilling fluid to alter the wettability behavior of reservoirs [37–40]. The surface tension between the injected fluid and the trapped gas plays a significant role in petroleum recovery applications. The aim of the present research is to introduce graphene nanoplatelets in saline media since graphene has proven to be a significant EOR agent. To the author’s knowledge, this combination of nanofluid has not been subjected to surface tension investigation, hence establishing a benchmark study for saline nanofluids. The literature review reveals highly conflicting results for surface tension and identifying many unknown phenomena associated with nanofluid-air surface tension behavior. Different characterisations and analysis are performed for graphene nanoplatelets and the stability of different concentrations of nanofluids. Obtaining stable nanofluids is one of the most crucial challenges [41–44] for actual applications, where most of the studies are performed in aqueous nanofluids. An optimum concentration of SDS stabilizer is introduced in graphene nanoplatelets-based nanofluids and the stability is evaluated for 72 h after a series of mechanical mixing. Pendant drop method is utilized to measure surface tension of base saline fluid and nanofluids. In this research, the primary function of SDS inclusion in graphene nanoplatelets-based nanofluids is to stabilize nanomaterials only. Therefore, the surface tension of pure saline water and SDS-stabilized nanofluids are compared and presented with the temperature variance. The literature [32,33,35] reveals that the addition of surfactant in water drastically reduces interfacial tension of the fluid. Therefore, the present study aims to compare the combined impact of SDS and graphene nanoplatelets on the surface tension of saline water. The obtained results of the surface tension of SDS-stabilized nanofluids with different graphene nanoplatelets loadings are compared with the pure saline water (30PPT).
biomedical, petrochemical, energy conservation, oil recovery, manufacturing, lubrication, cooling, electronics and food processing applications due to a dramatic improvement in thermophysical properties [2–8]. Several studies have been reported on the effective thermophysical properties of nanofluids, especially density, viscosity, thermal conductivity and heat capacity. However, the studies on the surface tension and the contact angle are rather limited. Most of the reported studies on surface tension in literature correspond to liquids [9], demonstrating a wide research gap on the surface characteristics at the gas-nanofluid interface. The introduction of nanomaterials in liquids significantly impact the surface/interfacial tension, wettability characteristics and contact angle. A recent study by Hernaiz et al. [10] reveals the consensus on dealing the contact angle of nanosuspensions as a thermophysical property, where the droplet volume and temperature play a crucial role in heat transfer applications. The surface tension between liquid and gas have a critical significance to boiling and condensation heat transfer applications, where the bubble formation generates an interface between two mediums and highly depends on the density of two fluids. Surface tension is defined as the molecular force at the interface of two immiscible mediums. The intermolecular attractive forces between the molecules at the interface (also known as surface energy) causes a tension between two fluids [11,12]. A recent study by Estelle et al. [13] proves the importance of surface tension and wettability characteristics of nanofluids in heat transfer and energy conversion operations, especially boiling and condensation processes. The Bond number defines the dominance of surface tension forces and the buoyancy forces, which is an essential parameter in boiling heat transfer flows [14]. A lower value of surface tension is considered preferable for efficient boiling heat transfer. Estelle et al. [13] described the importance of surface tension in boiling heat transfer, flow boiling in microchannels, critical heat flux, external flow convection, thermosiphons and heat pipes. Heat transfer characteristics of these applications, especially boiling heat transfer coefficient and critical heat flux [15,16], are highly dependent on the dimensionless numbers involving surface tension of the fluid. These dimensionless paraments involve Bond number [17], Weber number [18], Capillary number [19] and Kutateladze number [20]. Surface tension of fluids can be measured in different types of tensiometers, such as Wilhelmy plate, maximum bubble pressure, spinning drop, Du Nouy ring, Capillary rise and Pendant drop [13]. In a recent study by Wanic et al. [21], the surface tension of ethylene glycol-based nanofluids was investigated using Du Nouy ring and pendant drop methods. It was reported that both methods did not influence the surface tension measurements nanofluids containing different types of nitrides. Few recent experimental studies have been conducted on the surface tension of nanofluids with air. Berrada et al. [15] stated that the surface tension is a function of particle loading, nature, shape, size, base-fluid type and the surfactant characteristics. They investigated surface tension of MWCNT dispersions in water- and water-PG (60:40)based nanofluids using the pendant drop method at different temperatures and nanotubes loadings. It was found that the surface tension decreased with temperature. It was also found that the surface tension decreased in water-based nanofluids. However, the opposite trend was observed in water-PG based nanofluids. Chinnim et al. [22] introduced Al2O3, ZnO, TiO2 and SiO2 nanoparticles in PG/Water (60:40) nanofluids and developed a correlation for the surface tension of nanofluids based on nanoparticle concentration, temperature and nanoparticle size. It was concluded that the reduction of surface tension primarily depends on temperature compared to other two parameters. Bhuiyan et al. [23] found an enhancement in surface tension for different waterbased nanofluids. Radiom et al. [24] reported a decreasing trend in surface tension at higher nanoparticle loadings. However, Moosavi et al. [25] concluded the opposite trend. Tanvir and Qiao [26] studied surface tension at ambient temperature for a wide range of MWCNT loadings. It was concluded that water-based nanofluids showed an increasing trend of surface tension. However, ethanol-based nanofluid
2. Experimental procedure 2.1. Materials Graphene nanoplatelets of surface area 750 m2/g, an average thickness of 10 nm and a molecular weight of 12.01 is acquired from 2
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Fig. 1. Transmission electron microscopy analysis of graphene nanoplatelets.
Sigma-Aldrich Malaysia. The average size of the nanoplatelet is < 2 μm and the relative gravity is 2–2.25 g/cm3. Sodium chloride NaCl (99.5 %) powder and sodium dodecyl sulfate SDS (≥ 99 %) are also acquired from Sigma-Aldrich Malaysia. De-ionized water is used in preparing saline media, which is taken from the laboratory. The transmission electron microscopy (TEM) analysis is performed using Zeiss Libra 200FE to study nanomaterial morphology using high-resolution imaging, presented in Fig. 1. Graphene nanoplatelets with minute concentration is dispersed in de-ionized water using probe-type ultrasonication for one hour to obtain homogenous dispersion. Then, the undispersed big agglomerates are allowed to settle down and the sample drop is taken from the non-aggregated zone. The sample is dropped on the copper grid and dried prior to TEM analysis.
2.3. Surface tension measurement The surface tension of nanofluids is experimentally measured by the tensiometer IFT 700 by Vinci Technologies, France. The equipment is capable of measuring surface tension, interfacial tension and contact angle at a different range of temperatures and pressures. However, the present research focuses on the surface tension of nanofluids with air medium at ambient pressure. The nanofluid droplet at different temperatures (25−65 °C) is exposed to air using the pendant drop method and the surface tension is measured. The equipment can measure a wide range (0.1–72 mN/m) of surface tension measurement at an operating temperature range of ambient-180 °C. The temperature control has an accuracy of ± 0.1 °C. In the pendant drop method, the threshold scroll enables the selection of the drop shape detection according to the surrounding light and the threshold value is used to determine drop border line and compute different parameters such as diameter, volume, Bond Number and surface tension. The horizontal limit (red line) is set at the end of the droplet needle and is kept similar for all measurements. A stainless-steel capillary needle with external diameter 4.2 mm and 0.4 mm material thickness is used for all samples. The drop analysis system software (provided by the manufacturer) allows fast computation of surface. A schematic model of IFT 700 is shown in Fig. 2(a). The equipment consists of a viewing cell with electrical heaters. Two manual pumps (equipped with temperature sensors) are used bulk fluid and drop fluid). A calibrated capillary needle is used to create a drop of nanofluid into bulk fluid. A similar needle is used for all measurements to avoid any ambiguity in results. In this research, the bulk fluid is referred to air at ambient conditions and the drop fluid is referred to the base fluid or nanofluids. The validation of the presented research is performed by measuring surface tension of de-ionized water at varying temperatures and comparison of results with the literature [45] are presented in Fig. 2(b). All measurements for water, saline fluid and nanofluids at varying temperatures are repeated ten times and an average value is presented with maximum deviations at each point (y-error bars). The maximum uncertainty for surface tension of water is found to be ± 0.6 mN/m. Similarly, a maximum error (based on standard deviation) of ± 0.71 mN/ m is observed for the nanofluid samples. The validation results from Fig. 2(b) exhibits that the measured surface tension of water is in agreement with the international tables given by Vargaftik et al. [45].
2.2. Characterizations and sample preparations The nanomaterials are subjected to different characterizations. Xray diffraction (XRD) of graphene nanoplatelets is performed in Bruker, D8 Advance to identify the crystalline phase. The XRD spectrum is measured at Kα radiation λ of 1.542 Å at 45 kV and 40 mA. The data is recorded in 2θ scan range of 15°-70° and a step size of 0.02. Infrared (IR) spectroscopy is performed using Perkin Elmer - Spectrum one Fourier transform infrared (FTIR) analysis in the range of 400-4000 cm−1. A saline media typically contains 30–50 PPT of NaCl salt. Therefore, the saline solution is prepared at 30 PPT by dissolving NaCl in de-ionized water using a magnetic stirrer for 1 h. Nanofluids are prepared using the two-step method by dispersing graphene nanoplatelets in saline media. A total of six mass concentrations are prepared; 0, 0.05 wt %, 0.1 wt%, 0.15 wt%, 0.2 wt% and 0.25 wt%. Four sets of samples are prepared to study the effect of surfactant (SDS) addition on the stability of nanofluids. Surfactant concentration is an important parameter to inspect and stabilize nanofluids [44]. SDS concentration in nanofluids are varied in different ratios to the graphene nanoparticle concentration i.e., 0:1 (no surfactant), 1:1, 1.5:1 and 2:1. The nanofluid components are mixed in the order of saline media-SDS surfactant-graphene nanoplatelets. A magnetic stirrer is used for 30 min at 500 rpm to break bigger agglomerates into smaller agglomerates. Then, the nanosuspension is subjected to ultrasonic homogenization (Biologics Inc. 150 V/T, 20 kHz) for 30 min. A pule of 70 % and a power of 80 % is applied for optimum operation of ultrasonication for all samples. The temperature of the ultrasonic homogenizer is controlled by a water circulation bath (Hahnshin, HS 3005 N). Visual sedimentation method is used to evaluate the stability of nanofluids with different concentrations. Dynamic light scattering (DLS) technique is utilized to quantify the particle size distribution in the stabilized sample using Malvern Zetasizer Nano s90.
3. Results and discussion 3.1. Nanoparticle characterizations Graphene nanoplatelets are subjected to different surface characterizations. This type of nanomaterial has naturally occurring carboxylic or hydroxylic functional group at the edges of platelets and may 3
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Fig. 2. (a) Experimental setup for pendant drop IFT700; (b) Experimental comparison of the present study with Vargaftik et al. [45] for the surface tension of water.
peak at 3420 cm−1 attributes to OeH stretching vibration [47]. The stretching vibration at peak 1580 cm−1 corresponds to C]C functional group, hence confirming the presence of graphite nanomaterial. Few additional stretching vibrations are also observed at 1722 cm−1 and 1080 cm−1 peaks, which are attributed to the C]O functional group. This group may be present due to the exposure of nanomaterial in the atmosphere, since the analytical equipment operates in open-air. Similar vibrations were also reported in the literature [48] for different graphite-based nanomaterials 3.2. Dispersion characteristics The dispersion characteristics or the stability of nanofluids have significance towards the actual application of nanofluids. The stability of nanofluids does not only depend on the nanomaterial and base fluid properties, but the temperature, preparation method, the nature of mechanical mixing and chemical techniques may have an important role. In the past decade, not much attention is given to the stability of nanosuspensions. However, it is a crucial factor and must be given primary importance prior to properties measurements. A combination of mechanical mixing and chemical technique is applied in this research to stabilize graphene nanoplatelets in saline media. Four sets of nanofluids are prepared with varying SDS loadings. The aim of this study is to utilize a minimum amount of surfactant in nanofluids because of few critical factors involved with surfactants such as surfactant disintegration at high temperature operations, foaming during turbulent flows, and the economic aspects. Sedimentation analysis is performed for all concentrations of graphene nanoplatelets-based saline nanofluids, shown in Fig. 5. Most of the studies in literature have studied dispersibility of different nanofluids for several hours to a few days [49–50], and some studies specified on 72 h [51–53]. Therefore, the dispersion behaviour of nanofluids are investigated for 72 h in this research at room temperature by following the methodology in literature [51–53]. It is a notable fact that the stability of nanofluids is degraded with the passage of time and the elevation in temperature. The well-dispersed nanomaterials in the freshly prepared nanofluids may re-agglomerate with the passage of time and the stability tends to diminution [54]. Four sets of nanofluids correspond to varying surfactant and nanomaterial (GNP) ratio. Set A with no surfactant completely loses stability within a few hours. However, Set B, C and D with 1:1, 1:1.5 and 1:2 (nanomaterial: surfactant) are exhibiting better stability than Set A. It attributes that the anionic surfactant is suitable to stabilize pristine graphene nanoplatelets in saline media. A surfactant ratio of 1.5 (to GNP) is chosen to achieve optimum stability and further investigate the effective surface tension. Low stability of Set B is visible after observation of 72 h. The maximum concentration (0.25 wt%) is found to be stable than all other concentrations. Similarly, some of the samples are showing bubble/
Fig. 3. XRD spectrum of graphene nanoplatelets.
react with the humidity in the atmosphere during exposure. Therefore, a minimum exposure of the nanomaterial in the environment has endeavoured. Fig. 3 describes XRD spectra of graphene nanoplatelets. A graphite characteristic sharp peak with high intensity is observed at 2θ = 26.4°. This peak is describing the layer-by-layer structure of graphene nanoplatelets. Similar trends for pristine graphene nanoplatelets were also reported in the literature [46]. IR Spectroscopy is performed using FTIR, shown in Fig. 4. The broad
Fig. 4. IR Spectroscopy of graphene nanoplatelets. 4
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Fig. 5. Dispersion analysis of different sets of graphene nanoplatelets-based saline nanofluid samples with varying SDS concentration (wt/wt).
measurement of base-fluid is rather lower than all nanofluid samples. It is also found that the surface tension decreases linearly with the rise in temperature ranging from 25 to 65 °C for all samples. Maximum surface tension of 40.5 mN/m is observed for the base fluid at 25 °C. However, a drastically decrease of 14 % is found in surface tension values from 25 to 65 °C. This decrement is observed to be lower in graphene nanoplatelets-based saline nanofluids. An average decrease of 2–2.7 % in surface tension values with temperature is recorded for fluids containing graphene nanoplatelets. This decrement in surface tension values with the rise in temperature is attributed to the weakening of interstratify forces between the molecules. Similar decreasing trends of surface tension with temperature were found in the literature in the case of Al2O3-based nanofluids [9,23,55] and TiO2-based nanofluids [56,57]. The addition of graphene nanoplatelets decreases surface tension of saline fluid. The impact of graphene nanoplatelets concentration on the surface tension characteristics of saline fluids is highlighted in Fig. 8. A percentage decrement in the surface tension of nanofluids is calculated using Eq. (1). Surface tension of saline nanofluids and saline base fluid are represented by σnf and σbf, respectively. The outcomes of this research clearly suggest that graphene nanoplatelets have a strong impact on the surface tension of saline media. A maximum decrease of 21.5 % is observed for the highest concentration of nanofluid at 25 °C. Ahammed et al. [28] exquisitely explained the facts for the changes in surface tension of fluids with the addition of nanomaterials. The wettability characteristics of nanomaterials are the most essential parameter in surface tension of nanofluids. The repulsion forces between the hydrophobic nanomaterial and the bulk saline fluid increases and nanomaterials lean towards the free surface zone. This phenomenon intensifies the intermolecular spacing at the interface zone and reduces surface tension. The addition of hydrophilic nanomaterials in water or
foam formation at the top of the container in Set D, whereas, Set C shows less foaming formation with similar dispersion characteristics. Therefore, Set C is found to be the optimum surfactant ratio and is proceeded with further investigation. The particle size distribution for the lowest concentration of saline-nanofluids from the chosen Set C is quantified using DLS technique after 72 h of visual observation. It is found that the average particle size does not reach up to the micron level and the nanofluid holds the stability. The average size of approximately 400 nm is found for the nanosuspension after 72 h displaying a better dispersion. The visual sedimentation study for nanofluids with 1:1.5 (nanomaterial: surfactant) surfactant concentration is extended for one week of preparation and it is found that the graphene nanoplatelets have settled down in saline media at the bottom of the container. Therefore, it is concluded that graphene nanoplatelets in saline media with SDS have achieved an ideal stability for 72 h. It is also evident from Fig. 5 (DLS) that the nanomaterial agglomerate is close to micron level and with the passage of time, it may cause sediment after one week of preparation. 3.3. Surface tension of saline nanofluids The surface tension of nanofluids is experimentally measured at varying temperatures. A sample visual reading of each concentration at 25 °C is shown in Fig. 6. It can be noticed that the nanofluid concentrations from 0.1 wt% to 0.25 wt% are showing completely black due to high concentration of graphene nanoplatelets. The experimental data for different concentrations of graphene-nanoplatelet-based saline nanofluids is plotted in Fig. 7. Each measurement is repeated ten times and the average values are reported. The uncertainty in the measurements based on the standard deviation is shown as y-error bars. It is observed from Fig. 6 that the average uncertainty in the repetitive 5
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Fig. 6. Visual comparison of sample reading for different concentrations of nanofluids at 25 °C using pendant drop method.
particle concentrations.
saline media tends towards the lingering of a majority of the nanomaterials in the bulk liquid. Thus, very few nanoparticles stay at the interfacial layer zone due to strong nanomaterial-fluid interaction. Hence, increasing the surface tension of the nanofluid. An interesting surface tension behavior is observed for nanofluids when graphene nanoplatelets concentration is increased from the lowest 0.05 wt% to the maximum 0.25 wt%. An insignificant increase from 21.25 % (0.05 wt%) to 21.5 % (0.25 wt%) is observed at 25 °C. All investigated saline nanofluid samples with varying graphene nanoplatelets loading at different temperatures exhibit similar phenomena. It is evident from Fig. 8 (a–e) that a nominal increase is found when the concentration of nanoplatelets are increased. This shows that the concertation of graphene nanoplatelets higher than 0.05 wt% does not correspond to a significant change in surface tension behavior. Therefore, surface tension of saline nanofluids in the studied range is primarily dependent on temperature only and least dependent on increasing nanoparticle concentrations. A recent study by Wanic et al. [21] showed a similar trend for ethylene glycol-based nanofluids containing nitride nanomaterials. A slight change in the surface tension behavior was observed within the uncertainty range for 1−5 wt%
σnf ⎞ σ Decrement (%) = ⎜⎛1 − ⎟ × 100 σbf ⎠ ⎝
(1)
The reported studies in literature [32,33] suggests that the addition of surfactant in the base fluid significantly decrease the interfacial tension of the fluid. For instance, Cabaleiro et al. [35] reported considerable diminution in interfacial tension between paraffin and water with the addition of SDS surfactant to the water. An addition of 0.25 wt % SDS in paraffin-water emulsions corresponded to a decrease in interfacial tension from 42 mN/m to 8.3 mN/m. The present research aims to investigate the combined effect of SDS and graphene nanoplatelets on the surface tension of saline water. The scope of the presented research is limited to the investigation of surface tension of nanofluidair system. However, the change in surface tension behaviour of the base-fluid by the addition of surfactants is of highly significance, and future studies should be performed with and without surfactants for better understanding.
Fig. 7. Surface tension of saline nanofluids containing graphene nanoplatelets at varying temperatures. 6
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Fig. 8. The decrement in surface tension of saline fluid and nanofluids at varying graphene nanoplatelets concentrations.
4. Conclusions
Declaration of Competing Interests
This research emphasizes the surface tension behavior of graphene nanoplatelets in saline medium. Pendant drop method is used, and the validation of the equipment is presented by testing surface tension of pure water, exhibiting a good agreement with the literature. Few surface characterizations are performed for the nanomaterial, showing the morphology, structure and highest purity. Graphene nanoplatelets are dispersed in 30 PPT saline media using SDS as a stabilizer agent and the ultrasonic homogenization. An optimum surfactant concentration is chosen using the visual stability examination. DLS study confirms that the agglomerate size of graphene nanoplatelets does not reach micron level. Surface tension of five different concentrations of graphene nanoplatelets dispersion in saline media is experimentally investigated for the first time in this study. The results show that surface tension of the saline fluid drastically decreases up to 14 % with temperature. Whereas, the nanofluids exhibit approximately only 2%. A maximum of 21.5 % decrement is observed in surface tension of saline fluid is perceived by the addition of graphene nanoplatelets at 25 °C. An intriguing outcome is observed when the increase in the concentration of nanomaterial from 0.05 wt% to 0.25 wt% in saline media shows an insignificant change in surface tension characteristics of saline fluid. It summarizes that surface tension is highly dependent on temperature compared to nanoparticle concentration. However, there remains an ambiguity in the available literature.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This research is supported by the Institute of hydrocarbon recovery at Universiti Teknologi PETRONAS. The financial assistance is provided by the Grant YUTP015LC0-101. References [1] S.U.S. Choi, Enhancing thermal conductivity of fluids with nanoparticles, Am. Soc. Mech. Eng. Fluids Eng. Div. FED (1995) 231. [2] B.A. Suleimanov, F.S. Ismailov, E.F. Veliyev, Nanofluid for enhanced oil recovery, J. Pet. Sci. Eng. 78 (2011) 431–437, https://doi.org/10.1016/j.petrol.2011.06.014. [3] O. Mahian, L. Kolsi, M. Amani, P. Estellé, G. Ahmadi, C. Kleinstreuer, J.S. Marshall, M. Siavashi, R.A. Taylor, H. Niazmand, S. Wongwises, T. Hayat, A. Kolanjiyil, A. Kasaeian, I. Pop, Recent advances in modeling and simulation of nanofluid flowsPart I: fundamentals and theory, Phys. Rep. 790 (2019) 1–48, https://doi.org/10. 1016/J.PHYSREP.2018.11.004. [4] M. Alhuyi Nazari, R. Ghasempour, M.H. Ahmadi, A review on using nanofluids in heat pipes, J. Therm. Anal. Calorim. 137 (2019) 1847–1855, https://doi.org/10. 1007/s10973-019-08094-y. [5] R. Taylor, S. Coulombe, T. Otanicar, P. Phelan, A. Gunawan, W. Lv, G. Rosengarten, R. Prasher, H. Tyagi, Small particles, big impacts: a review of the diverse applications of nanofluids, J. Appl. Phys. 113 (2013) 011301, , https://doi.org/10.1063/1. 4754271. [6] F. Mashali, E.M. Languri, J. Davidson, D. Kerns, W. Johnson, K. Nawaz, G. Cunningham, Thermo-physical properties of diamond nanofluids: a review, Int. J. Heat Mass Transf. 129 (2019) 1123–1135, https://doi.org/10.1016/J. IJHEATMASSTRANSFER.2018.10.033. [7] H.J. Lee, Design and development of anti-icing textile surfaces, J. Mater. Sci. 47 (2012) 5114–5120, https://doi.org/10.1007/s10853-012-6386-2. [8] M. Agista, K. Guo, Z. Yu, A state-of-the-Art review of nanoparticles application in petroleum with a focus on enhanced oil recovery, Appl. Sci. Basel (Basel) 8 (2018) 871, https://doi.org/10.3390/app8060871. [9] J. Chinnam, D.K. Das, R.S. Vajjha, J.R. Satti, Measurements of the surface tension of nanofluids and development of a new correlation, Int. J. Therm. Sci. 98 (2015) 68–80, https://doi.org/10.1016/J.IJTHERMALSCI.2015.07.008.
CRediT authorship contribution statement Suhaib Umer Ilyas: Conceptualization, Methodology, Writing original draft, Investigation. Syahrir Ridha: Project administration, Funding acquisition, Supervision, Resources, Writing - review & editing, Investigation. Firas Ayad Abdul Kareem: Visualization, Software, Investigation, Formal analysis, Validation, Investigation.
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