Electrostatic and Sterical Stabilization of Cuo Nanofluid Prepared By
Materials Transactions, Vol. 50, No. 8 (2009) pp. 2098 to 2103 #2009 The Japan Institute of Metals EXPRESS REGULAR ARTICLE
Electrostatic and Sterical Stabilization of CuO Nanofluid Prepared by Vacuum Arc Spray Nanofluid Synthesis System (ASNSS)
Ho Chang1;*1, Xin-Quan Chen1;*2, Ching-Song Jwo2 and Sih-Li Chen3
1Department of Mechanical Engineering, National Taipei University of Technology, Taipei 10608, Taiwan, R. O. China 2Department of Energy and Refrigerating Air-Conditioning Engineering, National Taipei University of Technology, Taipei 10608, Taiwan, R. O. China 3Department of Mechanical Engineering, National Taiwan University, Taipei 10617, Taiwan, R. O. China
Nanoparticles can be dispersed by different methods in order to completely utilize their unique material characteristics. This study uses a self-developed nanofluid synthesis system to produce a CuO nanofluid with good suspension stability and particle dispersion without additives. The prepared CuO nanofluid has an average particle size of 60 nm and a substantially improved size distribution. The effect of the electrical double layer on the stability of the prepared CuO nanofluid was experimentally assessed by studies using different concentrations and by rheological experiments. Furthermore, the relationships between the pH value and Zeta potential, hydrodynamic radius and Uv/Vis spectra of the prepared CuO nanofluids are also discussed. The experimental results show that the CuO nanofluid still maintains its Zeta potential at above 30 mV for longer than 6 months, indicating high suspension stability. [doi:10.2320/matertrans.M2009129]
(Received April 9, 2009; Accepted June 4, 2009; Published July 25, 2009) Keywords: CuO nanofluid, zeta potential, sterical stabilization, rheology
1. Introduction force and van der Waals attraction force in the suspension fluid. Finally, employing DLVO theory to calculate the total In colloidal science, the interactions between particles in a interacting energy Vtotal produced by double-layer repulsion suspension fluid include van der Waals force, electrical force and van der Waals attraction force, the study inves- double layer action and sterical action. These interactions tigates the relationship between potential energy and stability lead to the dispersion of colloids, maintain the suspension of CuO nanofluid. and dispersion for a long time, and do not flocculate sediments.1,2) If nanoparticles can stay suspended in fluid 2. Theory Analysis stably, they can be applied in many industries to exert special effects of nanomaterials. The most common method for The model used to describe the ion distribution function particles to maintain good dispersion and stable suspension in and potential in the region near the charged interface was fluid has been ultrasonic vibration, a physical method. Many developed independently by Gouy and Chapman by combin- related research papers have reported the use of ultrasonic ing the Poisson equation for the second derivative of the vibration to disperse and suspend nanoparticles in fluid, electric potential as a function of charge density with the and investigated the influence of particle size and surface Boltzmann equation for the charge density as a function of potential on suspension and dispersion.3) But currently most potential.8–10) The net volume charge density at the points of the physical dispersion methods are being replaced by where the potential is is thus expressed as: chemical methods, mainly because the addition of chemicals ze can more evenly disperse the particles in suspension fluid, ¼ 2zen sinh ð1Þ 0 kT and can maintain the suspension stability over a longer time. Therefore, many studies have investigated the addition of is related to in Possion’s equation, which for a flat double different acidic and saltic materials in nanofluids to enhance layer takes the form as follows: the suspension stability.4–6) d2 The study uses a self-developed arc spray nanofluid ¼ ð2Þ 2 synthesis system (ASNSS) to prepare a CuO nanofluid with dx " high suspension stability.7) This good suspension stability where " is the permittivity. does not depend on a physical or chemical method, but it can The combination of eqs. (1) and (2) is expressed as: keep CuO nanoparticles stably suspended for an extended d2 2zen ze period. This study investigates the influence of the various ¼ 0 sinh ð3Þ 2 forces in the prepared suspension fluid, observes the dx " kT suspension properties of CuO nanofluid as well as the The solution of this expression, with the boundary conditions morphological and surface potential changes of CuO nano- ( ¼ 0 when x ¼ 0; and ¼ 0, d =dx ¼ 0 when x ¼1) fluid over an extended period of time, and explores the taken into account, can be written as:11) influence of electrostatic repulsion force, steric stabilization 2kT 1 þ exp½ x ¼ ln ð4Þ ze 1 exp½ x *1Corresponding author, E-mail: [email protected] *2Graduate Student, National Taipei University of Technology where Electrostatic and Sterical Stabilization of CuO Nanofluid Prepared by Vacuum Arc Spray Nanofluid Synthesis System (ASNSS) 2099
In the calculation of , the solution ionic strength, I ¼ exp½ze 0=2kT 1 p ¼ ð5Þ ð c z 2Þ=2, is often used. For water at 25 C, ¼ 3:288 I, exp½ze =2kT þ1 i i 0 which is measured in nm 1 while both the ion concentration 3 and ci and the ionic strength I are measured in mol/L. In 10 M 1 1 1:1aqueous electrolyte solution 1= ¼ 9:6 nm and for the 2 2 2 2 2 8 e NA 1 2e n0z = ¼ I 2 ¼ system of interest in colloid science, 1 ranges from a 1000"kT "kT fraction of a nanometer to about 100 nm.12) 1 1 DLVO theory is named after its developers, Derjaguin, 2e2N cz2 2 2F2cz2 2 ¼ A ¼ ð6Þ Landau, Verwey and Overbeek. It describes the change of "kT "RT interacting energy occurring when an electrolyte is added to a where NA is Avogadro’s constant, I is the ionic strength, colloid solution and the colloidal particles approach to each c is the concentration of electrolyte, n0 is the corresponding other, and how this influences the overall stability of the bulk concentration of each ionic species, z is the valence, colloidal solution. All the interactions among particles can be e is the charge on electron (1:602 10 19 C), F is the force acquired through the summing of the interacting energy of between two charges, R is the molar gas constant, and electric charge and the interacting energy of van der Waals k is Boltzmann’s constant. In the above equation, denotes force. the Debye-Huckel reciprocal length parameter and has When two atoms or molecules approach each other, mutual the unit of (length) 1, 1= is the distance at which the attraction will be produced because of the difference between potential drops to 1/e of its value at the Stern plane, and positive and negative electric charges. This kind of attraction this distance is called the double layer thickness or Debye is called van der Waals force, with its action equation length. expressed as follows:13)
A y y x2 þ xy þ x V ¼ þ þ 2ln ð7Þ van der Waals 12 x2 þ xy þ x x2 þ xy þ x þ y x2 þ xy þ x þ y
In the equation, A denotes the Hamaker constant, x denotes H0=2a1, y denotes the a2=a1, a1 and a2 denote the particle radius, and H0 denotes the distance separating the surfaces. When two different kinds of particles are in a solution, the solution’s double-layer action equation is expressed as follows: " 2R 1 þ expð H Þ V ¼ g ð 2 þ 2 Þ ln 0 þ ln½1 expð 2 H Þ ð8Þ double layer 12 01 02 2 0 4 1 þ R 1 expð H0Þ
1 1 1 where g12 ¼ða1 þ a2 Þ and R ¼ 02= 01. absorption. Hence, we can judge the suspension stability The term g12 is a geometry factor dependent upon the radii, from the concentration change. a1 and a2, of the two particles under consideration. 01 and 02 are the surface potentials of the two particles at infinite 3. Experimental separation, and " is the dielectric constant of the solution. Suppose that there is only a single kind of particles in Using the basic principles of the gas condensation method, solution, then eqs. (7) and (8) can be simplified as: this study has developed the vacuum arc spray nanofluid Aa synthesis system (ASNSS) combined with ultrasonic vibra- V ¼ ð9Þ 14,15) van der Waals 12H2 tion. The experimental devices are mainly comprised 0 of an electrical power utility, a servo-positioning system, a 2 ½ þ ð Þ "a 0 ln 1 exp H0 vacuum chamber, a vacuum pump, a heating source, a Vdouble layer ¼ ð10Þ 2 cooling system, and a pressure control unit. The ultrasonic Finally, the summing of the two kinds of energy yields the system allows different settings of frequency and amplitude. total energy for the dispersion stability of particles in With the help of ultrasonic vibration, the disturbance of the solution, as shown in the following equation: dielectric liquid can be increased and the nanoparticles thus V ¼ VattðvanderwaalÞþ VrepðelectronstaticÞ produced can quickly come out of the fusion zone. In the meantime, the gasified metal can be quickly cooled down. "a 2 ln½1 þ expð H Þ ð11Þ ¼ Aa=12H2 þ 0 0 Ultrasonic vibration can effectively improve the state of 0 2 dispersion of nanofluids and obtain smaller nanoparticles. From this equation, the relationship among the total action After the analysis of the results of multiple experiments, a force V, van der Waals attraction force Vatt and electro- CuO nanofluid with high suspension stability is prepared. static repulsion force Vrep can be seen. Besides, as known The main process parameters are peak current 4A, on-time from Lambert-Beer theory, the absorption of solution is pulse duration 50 ms, and off-time pulse duration 50 ms. The positively proportional to the concentration of solution and weight concentration of the prepared CuO nanofluid is the length of beam going through. Therefore, if the lengths of 0.01%. For observation of different lengths of settling time, beams going through the nanofluid are the same, and the CuO nanofluids allowed to rest for 7 days, 30 days, 90 days analyzed materials are also the same, then the concentration and 180 days are taken out to test the average particle of this solution will be positively proportional to the diameter, Zeta potential and pH value of the nanofluid. The 2100 H. Chang, X.-Q. Chen, C.-S. Jwo and S.-L. Chen
CuO nanofluid prepared by the study is preserved by pouring the newly prepared nanofluid immediately into a glass bottle with cap. Then the bottle is placed at rest under room temperature for 180 days. From the SEM image, the morphological change of particles can be observed. In addition, UV-Vis spectrophotometer and viscosity analyzer are used to test the absorption intensity and rheology of the nanofluids of different concentrations produced at different process durations. Finally, DLVO theory is employed to prove the suspension stability of CuO nanofluid. The particle size and morphology of the prepared nanoparticle was examined by transmission electron microscopy (TEM) and field emission scanning electron microscope (FESEM). The crystal structural of nanoparticle was determined by X-ray (a) diffraction (XRD). The zeta potential of the nanoparticle suspension was measured using zeta potential analyzer. In addition, a Ultraviolet-Visible (UV-Vis) spectrophotometer is used to measure the optical property of the nanofluids. The Calibration Curve can also be used to calculate the concentration of nanofluid.
4. Results and Discussion
4.1 Influence of settle times on the morphology of particle Figure 1(a) shows the morphology of the newly prepared Cu particles, which are in narrow strips. The length of long axis is around 200 nm, and the length of short axis is around 50 nm. Seven days later, their morphology is shown in (b) Fig. 1(b), appearing to be in the shape of a bamboo leaf; and their particle size is around 60 nm. Furthermore, the test results of XRD show that after 7 days, the Cu particles are oxidized to become CuO. Figure 1(c) shows the SEM image of a CuO nanoparticle after resting for 180 days. The particle size of the CuO nanoparticle shown in this figure is around 70 nm, with particles well dispersed. Comparing Figs. 1(b) and (c), it is known that after the CuO nanoparticles prepared by the study have remained at rest for 180 days, the morphology of the particles changes from the original bamboo leaf shape to be irregular polygon shape, but no obvious change in particle size is found in average. There- fore, it is demonstrated that the CuO nanofluid prepared by this study can maintain particle dispersion for an extended period of time, and have a good consistency in the CuO (c) nanoparticle dimensions. Fig. 1 SEM images of the nanoparticles prepared after different lengths of settling time: (a) newly prepared Cu nanoparticles; (b) CuO nanoparticles 4.2 Influence of settling time on the Zeta potential of after resting for 7 days; and (c) CuO nanoparticles after resting for 180 nanofluid days. Figure 2(a) shows the change in Zeta potential of CuO nanofluid at different process durations after different lengths CuO nanofluid tends to decrease gradually. Furthermore, the of settling time. The figure shows that the Zeta potential of concentration of CuO nanoparticles in CuO nanofluid would CuO nanofluid at a process duration of 10 min falls from affect the Zeta potential of nanofluid. The study finds that the 45.3 mV when newly prepared, to 34 mV after it has been CuO nanofluid with process duration of 10 min has the best rested for 6 months. As for the CuO nanofluids with process suspension stability. It is also known from the literature that duration of 5 min and 15 min, after they remain at rest for 180 when the Zeta potential of a fluid is greater than 30 mV, the days, their Zeta potentials fall from the original 39 mV to nanofluid can maintain its suspension stability. The nano- 30 mV and from 44.1 mV to 31 mV respectively. As shown in particles in a fluid achieve a stable situation through the Fig. 2(a), before the prepared CuO nanofluid is oxidized to be electrostatic repulsion force. Thus, as seen from Fig. 2(a), the CuO, its Zeta potential falls rapidly. In addition, with the CuO nanofluid prepared by this study can keep its suspension increase of settling time, the Zeta potential of the prepared stability at least for six months. Furthermore, the newly Electrostatic and Sterical Stabilization of CuO Nanofluid Prepared by Vacuum Arc Spray Nanofluid Synthesis System (ASNSS) 2101
(a) (a) 46 2.6 10min 2.4 44 10 min 2.2 15min 15 min 2.0 42 5 min 1.8
/mV 40 1.6 P 1.4 38 1.2 1.0 36 0.8 Absorbance (a.u.) 5min 34 0.6 Zeta potential, 0.4 32 0.2 0.0 30 300 400 500 600 700 800 Wavelength (nm) 0 20 40 60 80 100 120 140 160 180 200 (b)
Time, t /d 1.6 (b) 1.4 60 1.2 50 1.0 40 0.8 10min 30 /mV
P 5min 0.6 20 15min Absorbance (a.u.) 10 0.4
0 0.2 Zeta potential, -10 0.0 300 400 500 600 700 800 -20 Wavelength (nm)
-30 468101214 Fig. 3 Uv-Vis absorption spectra of the CuO nanofluids produced from pH Value, pH different process durations after settling time of (a) 7 days; (b) 180 days.
Fig. 2 (a) Change of Zeta potential of CuO nanofluid with different lengths of settling time under different process durations; (b) Relationship flocculate slightly. Thus, when the Zeta potential gradually between pH value and Zeta potential of the newly prepared nanofluid with process duration of 10 min. reduces with the increase of settling time, leading to the flocculation and sedimentation of particles, and reducing suspended particles in the fluid. As a result, the absorbance prepared nanofluid with process duration of 10 min is value is reduced. Figure 3(b) shows the Uv-Vis absorption immediately blended with 0.05 M of HCl or NaOH to spectra of CuO nanofluid after static placement for 180 days. produce solutions with different pH values. The Zeta It can be seen that the prepared CuO nanofluids with process potential change of the prepared nanofluids with different durations of 5 min and 15 min after being settling times of pH values is tested, as shown in Fig. 2(b). As observed from 180 days, their absorbance values are almost equal. Compar- Fig. 2(b), the pH value of nanofluid at the isoelectric point is ing Figs. 3(a) and (b), after the CuO nanofluid with process 9.2. The pH value of the CuO nanofluid prepared by this duration of 10 min have remained at rest for 180 days, its study is about 6, which is much smaller than 9.2. Therefore, absorbance value falls from the original 2.4 to 1.43, with a it is proved that the CuO nanofluid prepared by this study falling rate of 40.4%. And after the CuO nanofluids with meets the basic condition of suspension stability. process durations of 5 min and 15 min have remained at rest for 180 days, their absorbance value falls from the original 4.3 Influence of process duration of CuO nanofluid on 1.55 to 1.28 and from 2.3 to 1.3, with falling rates of 17.4% absorption intensity and 43.5% respectively. In order to further confirm that the The Uv-Vis absorption spectra of CuO nanofluid with falling absorbance value of CuO nanofluid is related to the different process durations was tested, as shown in Figs. 3(a) concentration of CuO nanoparticles in fluid, the study and (b). Figure 3(a) shows that the absorption intensity of inspects the concentration change of the CuO nanofluids CuO nanofluid with process duration of 15 min is less than with process durations of 5 min, 10 min and 15 min respec- that with process duration of 10 min. This is because when tively after they have remained at rest for different lengths of the process duration is over 10 min, the particles of the time, as shown in Fig. 4. As shown in Fig. 4, with the prepared nanofluid begin to agglutinate and sediment for increase of settling time, the concentration of CuO nanofluid being saturated, thus leading to the low suspension disper- tends to decrease gradually. After the CuO nanofluid with sion. Therefore, the process duration for better suspension process duration of 5 min has remained at rest for 180 days, and dispersion stability of CuO nanofluid is 10 min. More- the decrease rate (16%) of its concentration is the lowest. over, the absorbance value of CuO nanofluid reduces with the And after the CuO nanofluids with process durations of increase of settling time because the particles begin to 10 min and 15 min have remained at rest for 180 days, the 2102 H. Chang, X.-Q. Chen, C.-S. Jwo and S.-L. Chen
5min 0.080 (a) 10min 5min 0.075 15min 4.5 10min 0.070 4.0 15min 0.065 Water 0.060 3.5 0.055 3.0
0.050 /Pa σ 2.5 0.045 Concentration (mM) 0.040 2.0 0.035 1.5 0.030 Shear stress, 0 20 40 60 80 100 120 140 160 180 200 1.0 Time, t /d 0.5 Fig. 4 Relationship between settling time and concentration of CuO nanofluids with different process durations. 50 100 150 200 250 300 350 400 450 500 -1 Shear rate, S /s decrease rates of their concentration are 38% and 44% (b) 5min 1.7 10min respectively. This experimental result conforms to the 15min decrease rate of absorbance value of the CuO nanofluid with 1.6 Water the same process duration as in Fig. 3 after being at rest for 1.5 180 days. Besides, as known from Fig. 2(a), the CuO nanofluids with different process durations have their Zeta 1.4 potentials reduced with the settling time. However, after the 1.3 newly prepared nanofluid with higher Zeta potential has 1.2
remained at rest for 180 days, it can still maintain a higher Viscosity (mPa*s) Zeta potential. Therefore, appropriate process duration can 1.1 give better suspension stability of CuO nanofluid. Speaking 1.0 of suspension stability, the most ideal process duration of 0.9 50 100 150 200 250 300 350 400 450 500 CuO nanofluid prepared by the study is 10 min. -1 Shear rate, S /s
4.4 Analysis of rheology Fig. 5 Rheology of the CuO nanofluids with different process durations: Figure 5(a) shows how different process durations of CuO (a) shear rate on the change of shear stress; (b) shear rate on the change of nanofluids are related to the shear rate and shear stress of viscosity. deionized water, indicating that CuO nanofluids appear to have the properties of a dilatant fluid. The fluid prepared over (1/m) after calculation of eq. (6). After the parameters are a longer period of time is found to have saturated particles, so sequentially substituted in eq. (10), the potential energy a slight flocculation is caused, and its shear stress is greater curve for the double-layer repulsion force of CuO particles than that of the CuO nanofluid prepared in a shorter period of can be acquired. Synthesizing the above two interaction time. The main reason for this is that when particles in the energies among the particles of fluid, total interaction energy fluid start to flocculate, their flowability is reduced. Hence, a curves can be acquired. When interaction energy curves are shear stress should be applied to produce the strain rate. With being drawn, the x-coordinates represents the distance the CuO nanofluid prepared in a shorter period of time, since between particles, with a range of 0–10 nm and the interval its particles are more dispersed, it is more similar to a between each points being 0.2 nm. Figure 6(a) shows the Newtonian fluid. Figure 5(b) shows how different process potential energy curve (VT ¼ VA þ Vdouble layer) of CuO durations of CuO nanofluids are related to the shear rate and nanofluid after being prepared in 7 days. As the minimum viscosity of deionized water. This figure shows a fluid separation between the particles increases to the range of prepared over a longer period of time has higher viscosity. double-layer thickness, the double-layer repulsion force is This is because after the process exceeds 10 min, the greater than van der Waals attraction force. Therefore, a concentration of the fluid is saturated, and the particles then potential energy barrier is caused between two particles to start to flocculate, producing larger particles. This enhances prevent the flocculation of particles. As found from Fig. 6(a), the nanofluid viscosity and its flowability is reduced. the potential energy of the entire dispersion system is Therefore, in addition to the original viscosity of the carrier 18.92 kT, which is greater than the potential energy (15 kT) fluid, the particle size and dispersion are actually the factors required for the stability of dispersion system. Therefore, it is most influencing the viscosity of fluid. confirmed that the dispersion mechanism of the prepared CuO nanofluid makes use of double-layer repulsion force to 4.5 Interacting forces in CuO nanofluid maintain the dispersion stability. After that, the change of The Hamaker constant of CuO nanofluid (A131)is potential energy with the increase of settle times of CuO 9:219 10 20 J.16–18) Having substituted this numerical nanofluid is tested. Since the Zeta potential of CuO nanofluid value in eq. (9), the potential energy curve for the van der is slightly reduced and the particles are slightly enlarged Waals attraction force of CuO particles in solution can be with the increase of settling time, the potential energy tends acquired. Furthermore, the Debye length is 11651995.94 to fall. After CuO nanofluid is allowed to rest for 1 month, its Electrostatic and Sterical Stabilization of CuO Nanofluid Prepared by Vacuum Arc Spray Nanofluid Synthesis System (ASNSS) 2103
(a) dispersion stability of the fluid is quite good. 24 V (2) After 7 days of preparation, the structure of Cu particles 22 double layer V 20 van der Waals is oxidized to become CuO, with particle size being V 18 total around 60 nm. After CuO allowed to rest for 180 days, 16 14 the particle size of CuO nanoparticle is around 70 nm, /kT
V 12 and the dispersion of particles is good. 10 (3) The CuO nanofluid with better suspension and dis- 8 6 persion stability needs a process duration of 10 min. 4 Furthermore, the absorbance value of CuO nanofluid is 2 slightly reduced with the increase of settling time. Potential energy, 0 -2 (4) CuO nanofluid appears to have the properties of a -4 dilatant fluid. Besides, the shear stress of the fluid -6 prepared over a longer period of time is greater than 0246810 that prepared in a shorter period of time. The viscosity Separation distance, D /mm of the fluid prepared in a longer period of time is (b) greater. 18 V 16 double layer (5) According to DLVO theory, this study investigates the V 14 van der Waals dispersion stability mechanism of CuO nanofluid, and V total 12 calculates the potential energy curve of CuO fluid. The 10
/kT potential energy barrier between CuO nanofluid is V 8 18.92 kT, which can effectively prevent the flocculation 6 of particles, and maintain their dispersion stability. 4 2 0 Acknowledgement Potential energy, -2 -4 This study was supported by the National Science Council -6 of Taiwan, Republic of China under the project grant: NSC 024681096-2221-E-027-093.
Separation distance, D /nm
Fig. 6 Potential energy curve of CuO nanofluid after preparation for (a) 7 REFERENCES days (b) 180 days. 1) S. G. J. Heijman and H. N. Stein: Langmuir 11 (l995) 422–427. 2) V. Ramakrishnan, Pradip and S. G. Malghan: Collo. Surf. A 133 (1998) 135–142. potential energy barrier is reduced to 15.2 kT, and after six 3) N. Mandzy, E. Grulke and T. Druffel: Powd. Tech. 160 (2005) 121– months its potential energy barrier is 11.18 kT (Fig. 6(b)). 126. As seen from the experimental results, as the settling time 4) B. Frederic, A. Rulmont, R. Cloots and R. Moreno: J. Euro. Cera. Soc. lengthens, and when the Zeta potential of CuO nanofluid is 25 (2005) 3195–3201. reduced, this causes a slight flocculation of the particles in 5) X. Wang, B. I. Lee and L. Mann: Collo. Surf. A 202 (2002) 71–80. 6) W. Z. Zhu and S. C. Deevi: Mater. Res. Bulle. 38 (2003) 1227–1233. fluid, and the influence of van der Waals attraction force of 7) C. S. Jwo, H. Chang, T. P. Teng, M. J. Kao and Y. T. Guo: J. Nanosci. CuO nanofluid is created, thus leading to the reduction of Nanotech. 7 (2007) 2161–2166. potential energy. 8) S. Ross and I. D. Morrison: Colloidal System and Interfaces, (New York: John Wiley & Sons, 1988) p. 232. 5. Conclusion 9) D. J. Shaw: Introduction to Colloid and Surface Chemistry, (Oxford: Butterworth-Heinemann, 2000) p. 179. 10) L. Besra and M. Liu: Prog. Mater. Sci. 52 (2007) 1–61. The study investigates the dispersion stability of the 11) H. R. Kruyt: Colloid Science, Volume 1 (Amsterdam: Elsevier particles of CuO nanofluid prepared by the self-developed arc Publishing Company, 1952) p. 131. spray nanofluid synthesis system (ASNSS). After observing 12) R. J. Hunter: Foundation of Colloid Science, Volume 1 (Oxford: the solution over a long time, the study uses DLVO theory Clarendon Press, 1989) p. 332. 13) A. Bleier and E. Matijevic: J. Collo. Interf. Sci. 55 (1976) 510–524. to verify the suspension and dispersion stability of CuO 14) H. Chang, T. T. Tsung, Y. C. Yang, L. C. Chen, H. M. Lin, C. K. Lin nanofluid, with the following conclusions: and C. S. Jwo: Int. J. Adv. Manuf. Technol. 26 (2005) 552–558. (1) The prepared CuO nanofluid has 39 mV of Zeta 15) H. Chang and M. K. Liu: J. Cry. Grow. 304 (2007) 244–252. potential. Hence, without the addition of any dispersant, 16) J. N. Israelachvili: Intermolecular and Surface Forces (London: a stable suspension can be maintained, and an ex- Academic Press, 1991) p. 186. 17) J. N. Israelachvili: Intermolecular and Surface Forces (London: tremely good effect of sterical stabilization can be Academic Press, 1991) p. 200. achieved. After observation for a long time, its Zeta 18) C. Y. Cheng: Master Thesis, (National Tsing Hua University, Taiwan, potential can still remain above 30 mV. Therefore, the 2007) p. 16.