NANOFLUID STABILITY OPTIMIZATION BASED ON UV-VIS SPECTROPHOTOMETER MEASUREMENT

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1 Journal of Engineering Science and Technology Special Issue on SOMCHE 2014 & RSCE 2014 Conference, January (2015) School of Engineering, Taylor s University NANOFLUID STABILITY OPTIMIZATION BASED ON UV-VIS SPECTROPHOTOMETER MEASUREMENT A. GHADIMI 1, *, H. S. C.METSELAAR 2, B. LOTFIZADEHDEHKORDI 3 1 School of Engineering, Taylor s University, Taylor's Lakeside Campus, No. 1 Jalan Taylor's, 47500, Subang Jaya, Selangor DE, Malaysia 2 Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur, Malaysia 3 Department of Mechanical Engineering, Faculty of Engineering, 302 E Buchtel Ave, Akron, Ohio, United States *Corresponding Author: Azadeh.ghadimi@taylors.edu.my Abstract As a newly engineered colloidal suspension, nanofluid needs to be stable in order to take part in future industrial applications. A decade ago, the UV-Vis Spectrophotometer was introduced as a new quantitative method for stability measurement of nanofluids. In this research, the stability of aqueous Titania nanofluid of 0.1wt. % was measured by UV-Vis Spectrophotometer at the elapsed time intervals of one day, two days, one week and one month after preparation. The results showed this monitoring instrument is suitable to detect the stability of nanofluid for two days after preparation onwards. Keywords: Nanofluid, Stability, UV-Vis Spectrophotometer, Optimisation, RSM. 1. Introduction Nanofluid, which is a new class of cooling medium, was introduced by Choi in 1995[1]. These contemporary fluids show enhanced thermal conductivity which is very promising for industrial applications [2-4]. Homogeneous dispersion and stability of nanofluids are matters of concern among scientists and industries [5-7]. Therefore, various stabilization methods such as ultrasonication, ph control, and adding surfactant had been considered independently in preparation of nanofluids. Previously, the methods to inspect stability were limited to sedimentation, photo capturing, TEM (transmission electron microscopy), SEM (scanning electron microscopy), however, in 2003, a new method for stability measurement by UV-Vis Spectrophotometer was introduced by Jiang et al. [8]. In 32

2 Nanofluid Stability Optimization Based on UV-Vis Spectrometer Measurement 33 Abbreviations CCD CNT DOE IEP RSM SDS SEM TEM Central composite design Carbon nanotube design of experiment Iezo electric point response surface methodology sodium dodecyl sulphate Scanning Electron Microscopy Transmission Electron Microscopy this technique, the supernatant concentration of a CNT suspension was measured quantitatively versus the sedimentation time. Although this method was at first only applicable to ceramic suspensions, Jiang et al. (2003) proved the validity of this technique in nanofluid stability measurements as well [8]. Since there is limited attention to nanofluid stability and optimizing preparation method by UV-Vis spectrophotometer and all the discrepancy regarding to nanofluid thermal and electrical conductivity results are due to the lack of standardization in preparation method, stability inspection is considered as a critical necessity to conduct some experiments. In this paper, based on the preliminary studies and reviewed literature [14,15] the effect of ph values (between 10 and 12) and SDS (sodium dodecyl sulphate) surfactant concentration loading from one-tenth to twice the amount of nanoparticle concentration (ranging wt.%) has been monitored on the stability measurement of 0.1 wt.% Titania (TiO 2 ) nanofluid. The absorbance measurements using UV-Vis spectrophotometer were taken as the stability responses in the periods of one day, two days, one week (168 hrs) and one month (720 hrs) after preparation. The objective of this work is to optimize the stability based on two methods of ph control and surfactant addition. In addition results obtained from the nanofluid, were imported to DOE for optimizing these two factors separately and simultaneously by each factor and their interactions. In addition, the appropriate measurement time for a clear detection of the stability of nanofluid by the UV-Vis spectrophotometer is discussed. Furthermore, the optimum stable combination of SDS wt. % and ph value is determined. 2. Methodology 2.1. Preparation of nanofluid The experiments were conducted using 0.1wt.% Titania nanoparticles with all the specification reported in our previous work [9, 10, 15]. In this study, nanofluid was prepared by two-step method. The TiO 2 absorbance was measured from 200 nm to 800 nm wavelengths and the peak absorbance was found to be 320 nm. The resulting absorbance and calibration graph of concentration of the nanofluid in this wavelength is depicted appendix. The reason for focusing on these two variables is due to conflicting optimum values reported by previous studies. In fact, the ph range ( ) is the Isoelectric point (IEP) of TiO 2 from which the prepared Titania nanosuspension should be far away to be stable [14]. In addition,

3 34 A. Ghadimi et al. in literature review, there is no specific ph value or an optimum ratio of surfactant to nanoparticle concentration has been found in the literature Optimization study The Design-Expert software (version 8.0) was used for the statistical design of experiments and data analysis. The Response Surface Methodology was chosen to model the imported data and the UV absorbance after four different intervals as four stability responses accordingly. The experimental conditions were optimized using Central Composite Design (CCD). For each of the studied variables, high (coded value: +1) and low (coded value: 1) set points were selected as shown in Table 1. In addition, Table 2 shows the factors in coded and real values based on the CCD methodology in DOE software. This table demonstrate. Table 1. Variables and values used for CCD. Variable Name A SDS (wt. %) B ph Data and statistical analysis The design of experiment aimed to achieve the optimum range of SDS dosage and ph value to have the best stability based on the UV-Vis Spectrophotometer measurement for short term and long term application of 0.1 wt. % Titania nanofluid. The relationship between the two variables and the four stability responses of the UV-Vis Spectrophotometer absorbance measurement (after one day, two days, one week, and one month) was analysed using RSM. The quadratic equation model for predicting the optimal conditions is given by Eq. (1). k k 2 k k Y = β 0 + i= 1βi. X i + i= 1βii. X i + i j jβij. X i. X j e (1) where βi is the linear coefficient, βii is the quadratic coefficient, β ij is the regression coefficient, k is the number of factors studied and optimized in the experiment, e the random error and Y the predicted response. The response surface plots were obtained by a statistical process in which the relationships between the parameters and the responses were graphically illustrated by RSM. Meanwhile, the analysis of variance (ANOVA) and least square techniques were used to evaluate the statistical specification of the constructed models (for further information see [11-13]). Three-dimensional plots and their respective contours were obtained for stability measurement by the UV-Vis Spectrophotometer concentration based on the effects of SDS dosage and ph at five levels. Furthermore, the optimum region was identified based on the main parameters in the multiple response method, called desirability, which is simply a mathematical method to find the optimum.

4 Nanofluid Stability Optimization Based on UV-Vis Spectrometer Measurement Results and Discussion A total number of eleven experiments including three replicates at the centre were carried out. The tested variable combinations and experimental results are tabulated in Table 2. The effect of SDS concentration and ph value on UV absorbance was evaluated using a regression model based on the equations shown in Table 3. These equations show the importance of each factor based on the respected coefficient. Higher coefficient implies more influence on the response. Run Table 2. The obtained experimental results based on the CCD method. SDS wt.% ph coded value A coded value B UV after 1 M UV after 1 W UV after 2 D UV after 1 D (0) (-0.5) (+1) (-1) a (0) (0) (+1) (+1) (-1) (-1) (+0.5) (0) a (0) (0) (-0.5) (0) (-1) (+1) (0) (+0.5) a (0) (0) a: indicates 3 repeats at center point, M:month, W:week, D:day. Model 1 1 day Table 3. The developed models and ANOVA results. UV response after preparation 2 2 days Final equation in terms of coded factors E-003A B B B A B AB A B A 2 B 3 1 week A B AB 4 1 month A B AB A 2 Analytic plots such as the predicted versus actual values (Fig. 1) help us to judge the model satisfactoriness. The plots of predicted versus actual value responses with respect to the factors are presented in Fig. 1 based on the statistical analysis provided by DOE software Graph analysis Referring to Fig. 2 as three-dimensional (surface) models of the responses, the blue areas indicate the minimum absorbance while the red strips show the

5 36 A. Ghadimi et al. maximum amounts of the responses. The four sub-diagrams in this figure demonstrate the absorbance measurements at one day, two days, one week and one month after nanofluid preparation. It can be seen in these figures that the absorbance after one day had not changed by altering ph value. Consequently in comparison with the rest of the graphs, it doesn t show a proper variation. High absorbance after two days came out with high ph values in the low surfactant concentration region. Another noticeable finding is the small area of medium concentration samples which is about to grow for the ph value of 10. As ph control and surfactant addition are two main methods to control stability [14], results after one week show that wider range of ph control and SDS concentration could make the nanofluid stable after one week. This response conveys that high ph values should be accompanied with low concentrations of SDS to reach high stability after one week. Similarly, in order to have low ph values, high SDS concentration should be maintained. These graphs also show that probably a more stable nanofluid could be reached without surfactant at a ph value of around 9.3. However, this value has not been reported in the literature. Finally, after one month the long term stable nanofluid appeared at ph values of between and surfactant weight concentration of equal to twice the amount of nanoparticle loading. Fig. 1. Predicted vs. actual values plot for UV-Vis spectrophotometer concentration measurement: (a) after one day, (b) after two days, (c) after one week, and (d) after one month.

Optimization If all the responses showed high relative stability at the same weight simultaneously (excluding the first day response) the optimum condition could be achieved.

6 Nanofluid Stability Optimization Based on UV-Vis Spectrometer Measurement 37 Fig. 2. Response surface plot for the UV-Vis spectrophotometer concentration measurement; (a) after one month (b) after one week, (c) after two days, and (d) after one day Optimization If all the responses showed high relative stability at the same weight simultaneously (excluding the first day response) the optimum condition could be achieved. In this case, the maximum desirability was calculated of located at ph of 10 with SDS weight percentage of 0.09 % (shown in Fig. 3) can be claimed to have the same loading of nanoparticle. After two days, the measured absorbance showed low SDS concentration as acceptable stability at an area with a high ph controlled nanofluid. In addition, we observed a measurable area of absorbance at a wider range of surfactant weight percentage of about However, for lower concentrations of surfactant, ph would be confined to a range of 10 to 12. In the case of higher SDS addition, we detected that the stable area occurred only at ph of 10. The stability measurement after one month showed that for a long-term application of this suspension, SDS weight percentage of were accompanied by low ph values. This result showed that other than this area, our prepared nanofluid would not be applicable after one month. Fig. 3. Three dimensional surface desirability of responses for optimum stability area of Titania nanofluid without the first response.

7 38 A. Ghadimi et al Point prediction evaluation To assess the accuracy of the second order equations, three-additional UV absorption experiments were performed using the SDS concentration and ph value that were prepared under the RSM-predicted optimum conditions. The maximum values of the repeated experiments and predicted results are presented in Table 4. As observed in this table, the deviations errors between the experimental and predicted values of UV measurement at experimental intervals of 2 days, one week and one month after the preparation were 1.27%, 1.1% and 0.73%, respectively. The good agreement between the experimental values and those predicted from the CCD model suggested that the developed model is suitable to simulate variables within 5% of accuracy. The optimization results presented in this paper provide background information for detailed process improvement in future researches. Table 4. Evaluation of experimental and predicted values at optimum condition. Responses of Concentration Predicted values Experimental values* Error (%) After 2 days After one week After one month Conclusions The current study revealed the results about the stability of aqueous titania nanofluid with weight concentration of 0.1 wt.%. UV-Vis spectrophotometer would fulfil the measurement criteria of low concentration nanofluid stability, except for the UV measurement after one day. This validation shows that ph value of 10 with almost the same ratio of surfactant to nanoparticle loading would be a proper combination to homogenize nanofluid in order to keep its stability even after one month with the accuracy of 5%. Design of experiment software is a great tool in order to analyse the stability of the nanofluid as this new engineered medium has a long way to be implemented in industry. Acknowledgements The authors wish to acknowledge the Ministry of Higher Education of Malaysia, Kuala Lumpur, Malaysia, for the financial support under UM.C/HIR/MOHE/ ENG/21 (D ) and express their gratitude to University of Malaya for the financial support grant provided with the reference number of (PPP-PV B). References 1. Choi, S.U.S.; and Eastman, J.A. (1995). Enhancing thermal conductivity of fluids with nanoparticles. International Mechanical Engineering Congress

8 Nanofluid Stability Optimization Based on UV-Vis Spectrometer Measurement 39 and Exhibition, San Francisco CA (United States), Nov, ASME: San Francisco Godson, L.; Raja, B.; Lal D.M.; and Wongwises, S. (2010). Enhancement of heat transfer using nanofluids--an overview. Renewable and Sustainable Energy Reviews, 14(2), Yu, W.; France, D.M.; Routbort, J.L.; Stephen, U.; and Choi, S. (2008). Review and comparison of nanofluid thermal conductivity and heat transfer enhancements. Heat Transfer Engineering, 29(5), Trisaksri, V.; and Wongwises, S. (2007) Critical review of heat transfer characteristics of the nanofluids. Renewable and Sustainable Energy Reviews, 11, Hwang, Y.; Lee, J.K.; Lee, C.H.; Jung, Y.M.; Cheong, S.I.; Lee, C.G.; Ku, B.C.; and Jang, S.P. (2007). Stability and thermal conductivity characteristics of nanofluids. Thermochimica Acta, 455, (1-2), Meibodi, M.E.; Sefti, M.V.; Rashidi, A.M.; Amrollahi, A.; Tabasi, M.; and Kalal H.S. (2010). The role of different parameters on the stability and thermal conductivity of carbon nanotube/water nanofluids. International Communications in Heat and Mass Transfer, 37(3), Nasiri, M.; Etemad, S.G.; and Bagheri, R Experimental heat transfer of nanofluid through an annular duct. International Communications in Heat and Mass Transfer, 38(7), Jiang, L.; Gao, L.; and Sun, J. (2003). Production of aqueous colloidal dispersions of carbon nanotubes. Journal of Colloid and Interface Science, 260(1), Beck, M.P. (2008) Thermal conductivity of metal oxide nanofluids, in School of Chemical and Biomolecular Engineering. Georgia Institute of Technology: Georgia. 10. Wang, X.-j.; Zhu, D.-s.; and Yang, S. (2009). Investigation of ph and SDBS on enhancement of thermal conductivity in nanofluids. Chemical Physics Letters, 470(1-3), Ghadimi, A.; and Metselaar, I.H. (2013). The influence of surfactant and ultrasonic processing on improvement of stability, thermal conductivity and viscosity of Titania nanofluid. Experimental Thermal and Fluid Science, 51, Nosrati, S.; Jayakumar, N.S.; and Hashim, M.A. (2011). Extraction performance of chromium (VI) with emulsion liquid membrane by Cyanex 923 as carrier using response surface methodology. Desalination, 266(1-3), Ghadimi, A.; and Metselaar, H.S.C. (2013). Clustering theory evaluation for thermal conductivity enhancement of Titania nanofluid. Material science and Technology, Transactions, Wiley-American Ceramic Society: Pitssburgh.

9 40 A. Ghadimi et al. Appendix Fig. A1. UV vis spectrophotometer for TiO 2 nanofluid with 25 nm average diameter directly after preparation.

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