PCCP PAPER. Temperature-dependent effect of percolation and Brownian motion on the thermal conductivity of TiO 2 ethanol nanofluids. 1.

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1 PAPER Cite this: Phys. Chem. Chem. Phys., 216, 18, Temperature-dependent effect of percolation and Brownian motion on the thermal conductivity of TiO 2 ethanol nanofluids Chien-Cheng Li, a Nga Yu Hau, a Yuechen Wang, a Ai Kah Soh* b and Shien-Ping Feng* a Received 23rd January 216, Accepted 6th May 216 DOI: 1.139/c6cp5d Ethanol-based nanofluids have attracted much attention due to the enhancement in heat transfer and their potential applications in nanofluid-type fuels and thermal storage. Most research has been conducted on ethanol-based nanofluids containing various nanoparticles in low mass fraction; however, to-date such studies based on high weight fraction of nanoparticles are limited due to the poor stability problem. In addition, very little existing work has considered the inevitable water content in ethanol for the change of thermal conductivity. In this paper, the highly stable and well-dispersed TiO 2 ethanol nanofluids of high weight fraction of up to 3 wt% can be fabricated by stirred bead milling, which enables the studies of thermal conductivity of TiO 2 ethanol nanofluids over a wide range of operating temperatures. Our results provide evidence that the enhanced thermal conductivity is mainly contributed by the percolation network of nanoparticles at low temperatures, while it is in combination with both Brownian motion and local percolation of nanoparticle clustering at high temperatures. 1. Introduction Nanofluids, in which nano-sized particles are suspended in liquids, have emerged as a potential candidate in the design of heat transfer fluids. The experimental observations of nanofluids have shown much higher thermal conductivity than the predictions of Maxwell effective medium theory. Potential mechanisms, such as local Brownian motion, percolation of nanoparticle clustering, and liquid layering, have been proposed to explain this enhancement. 1 3 Recently, ethanol-based nanofluids have attracted increasing interest due to their potential applications in fuels, thermal convection and thermal storage. 4 7 However, there are contradictory data regarding the thermal conductivity of ethanol-based nanofluids, such as temperature dependency, particle size, and mass fraction. 1,3,8 In reviewing the previously reported experiments on ethanol-based nanofluids, we summarize four points which are the possible causes of the inconsistencies in thermal conductivity data reported by different groups. Firstly, nanofluids are usually made by a two-step process, in which the nanoparticles are pre-synthesized and then added to the a Department of Mechanical Engineering, The University of Hong Kong, Hong Kong, China. hpfeng@hku.hk; Fax: ; Tel: b School of Engineering, Monash University Malaysia, Malaysia. soh.ai.kah@monash.edu; Fax: ; Tel: Electronic supplementary information (ESI) available: Additional details about the home-made thermal insulation cell and experimental set-up for thermal conductivity measurements. See DOI: 1.139/c6cp5d base fluids. Since nanoparticles have high surface energy, they readily aggregate together when added to the solution, leading to a large variation of particle size distribution The aggregation and sedimentation of nanoparticles cause a stability problem in the nanofluids, particularly in high mass fractions and at high temperatures. This is the reason why most research work on nanofluids is focused on low mass fractions (.1 1 wt%), 5,13,14 but very little work has been conducted on high mass fractions. 2,15,16 Secondly, the surfactant or additives are usually used to prevent nanoparticle aggregation, but their effects are not considered. Thirdly, the accuracy of thermal conductivity is highly dependent on the measured environment (temperature, vibration, and noise). Lastly, very little existing studies have considered the effect of inevitable water content in ethanol, which influences the thermal conductivity of base fluids versus temperature. In this paper, stirred bead milling was employed to fabricate TiO 2 ethanol nanofluids without the need for adding surfactants/ additives, 17,18 which produced a uniform particle distribution, high mass fraction and excellent long-term and thermal stability. TiO 2 nanoparticles were selected because of their low material cost, good electrocatalytic activity, and excellent thermal/chemical stability, enabling them to serve as potential candidates in nanofluids for combustion systems and compact reactor-heat exchanger. 19,2 A home-made thermal insulation cell was used to provide a non-disturbing environment for thermal conductivity measurements. By using this stable suspension, we investigated the heat transport behaviors of TiO 2 ethanol nanofluids over a broad range This journal is the Owner Societies 216 Phys. Chem. Chem. Phys., 216, 18,

2 of mass fractions (.5 5 wt%) and temperatures (1 5 1C). We also considered the water content in ethanol, which causes the temperature-dependent transition for the thermal conductivity of base fluids. And the same method has also been used for the investigation of graphene aqueous nanofluids by our group now. circulating water bath. Thermal conductivity was recorded automatically every 15 min for 6 h. The reported values represent the average of at least 12 measured data. Electrochemical impedance spectroscopy was performed using an AutoLab-PGSTAT32N workstation. 2. Experiment 2.1 The preparation of TiO 2 ethanol nanofluids The TiO 2 nanoparticles used for dispersions were purchased from Evonik Degussa (Aeroxide s P9, Germany) with a BET surface area of 7 1 m 2 g 1 and anatase content greater than 9%. The average diameter of TiO 2 nanoparticles was about 21 nm. Ethanol (96% BP, Guangdong Guanghua Chemical Factory Co., Ltd, China) was used as a base fluid. Ethanol (99.5% GC, Merck, Germany) and ethanol/di water mixtures were used for the calibration of thermal conductivity. TiO 2 ethanol nanofluids were prepared by dispersing TiO 2 nanoparticles in ethanol (96%) through stirred bead milling (JBM-B35, JUSTNANO, Taiwan) for 6 h. The ZrO 2 ball loading and stirrer speed were maintained at 6gand2rpm,respectively.TheparticlesizeoftheZrO 2 ball is about.5 mm. The nanoparticles were mechanically dispersed in ethanol at various mass fractions (.5 wt% (.92 vol%), 1 wt% (.185 vol%), 3 wt% (.566 vol%), 5 wt% (.959 vol%)). No additional dispersants/additives were added into the solutions. 2.2 Material characterization The morphology and microstructure of the TiO 2 nanoparticles were determined by HRTEM (Tecnai, G22S-Twin). The TEM samples were prepared by dropping 2 ml ofupperwell-suspended solutions on the copper grid and then dried in the ambient environment for 1 min. The particle size distributions and zeta potentials were measured using a dynamic light scattering (DLS) analyzer (Microtrac, Nanotrac Wave, USA) at room temperature. Here the particle size determined by the scattered light intensity in DLS is the size of nanoparticle clusters in the suspension (not the size of the individual TiO 2 nanoparticle). Fourier transform infrared (FTIR) spectra were obtained using a Bruker Tensor 27 spectrometer. 2.3 Measurement of nanofluids The electrical conductivity, viscosity, and thermal conductivity of the nanofluids were measured at the temperatures of 1 1C, 2 1C, 3 1C, 4 1C, and 5 1C. The viscosity of the nanofluids was measured using a rotating viscometer (NDJ-9S, Shanghai Pingxuan Instrument Co., Ltd, China) at the spinning rate of 2 rpm. Thermal conductivity was measured by the transient hot-wire method using a thermal conductivity meter (KD2 pro, Decagon Devices, USA). The probe, 1.3 mm in diameter and 6 mm in length, was vertically immersed in the center of nanofluids. Calibration of the probe was done by measuring the thermal conductivity of DI water, ethanol (99.5%), and ethanol (96%). The dimensions of the home-made thermal insulation cell were 3 mm diameter and 7 mm length (Fig. S1, ESI ). The probe is fixed to the cap of the cell. The entire unit was immersed in a 3. Results and discussion Ultrasonic dispersion is the most common method for preparing nanofluids. 1,21,22 In this paper, we successfully produced a uniform and stable suspension of TiO 2 nanoparticles in ethanol using stirred bead-milling methods, 11,23 where the particle particle interaction between the nanoparticles can be controlled appropriately during the process. Fig. 1 shows the TEM images of TiO 2 nanoparticles before and after stirred bead milling. As shown in Fig. 1a, there are more aggregated nanoparticles in TiO 2 ethanol nanofluids by using ultrasonic dispersion as compared to other samples using stirred-bead milling (Fig. 1b e). The diffraction patterns of TiO 2 nanoparticles in the inset images show that all the samples are in anatase phase, indicating that the crystallinity of TiO 2 NPs does not change after the stirred bead-milling process. A photograph of TiO 2 ethanol nanofluids with mass fractions of.5 wt%, 1 wt%, 3 wt%, and 5 wt% after 6 days of preparation is shown in Fig. 1f, which shows excellent long-term stability without obvious sedimentation for.5 wt%, 1 wt%, and 3 wt% TiO 2 nanofluids (Note: the change in thermal conductivity is maintained within 1%after6days).For5wt%TiO 2 nanofluids, it can be seen that some sediment particles appeared at the bottom of the vial after 6 days. Fig. 2 shows the particles size distribution of TiO 2 ethanol nanofluids for the as-prepared sample (black curve) and the sample after 6 days (red curve) at room temperature, as well as for the sample at 5 1C (blue curve). After 6 days, the mean diameters of TiO 2 nanoparticles in nanofluids for the mass fractions of.5 wt%, 1 wt%, and 3 wt% can be still maintained at 3 nm, which are similar to those of the as-prepared samples (Fig. 2a c); while the mean diameter became smaller (B1 nm) in the 5 wt% TiO 2 ethanol nanofluids because only the small-sized nanoparticles remained in suspension after the sedimentation of big nanoparticle aggregates (Fig. 2d). As shown by the blue curves in Fig. 2a c, the particle size in the TiO 2 ethanol nanofluids slightly increased after heating up to 5 1C and became well-dispersed TiO 2 nanoparticle clusters in ethanol without sedimentation. This indicates that local clustering of nanoparticles occurred with the increase of temperature. These nanoparticle clusters would re-disperse in the base fluid and thus return back to their original individual particles after cooling to room temperature. In comparison, 3 wt% TiO 2 ethanol nanofluids prepared by the ultrasonic dispersion method have a wide range of nanoparticle distribution, as shown in the green curve of Fig. 2c. Table 1 shows the values of zeta potential for TiO 2 ethanol nanofluids with.5, 1, and 3 wt% at different time intervals of 1, 15, 3, and 6 days and at 5 1C. All the measured samples show non-degradation of zeta potentials even if heating up to 5 1C or after 6 days. The high zeta potential comes from the stable repulsive forces between Phys. Chem. Chem. Phys., 216, 18, This journal is the Owner Societies 216

3 Fig. 1 TEM images of TiO2 nanoparticles before (a) and after bead-milling for 6 h at various concentrations of (b).5 wt%, (c) 1 wt%, (d) 3 wt%, and (e) 5 wt% (insets: the corresponding SAED pattern of TiO2 nanoparticles). (f) Photograph of TiO2 ethanol nanofluids with mass fractions of.5, 1, 3, and 5 wt% after 6 days of preparation. Fig. 3 FT-IR spectra of ethanol and TiO2 ethanol nanofluids at the concentration of 3 wt%. Fig. 2 Particle size distribution of (a).5 wt%, (b) 1 wt%, (c) 3 wt% and (d) 5 wt% mass fractions of TiO2 nanoparticles in ethanol-based nanofluids for the as-prepared sample (black curve), sample after 6 days (red curve) at room temperature, and sample at 5 1C (blue curve) and sample prepared by an ultrasonic method (green curve). Table 1 Zeta potentials for TiO2 ethanol nanofluids with.5, 1, and 3 wt% concentrations at different time intervals of 1, 15, 3, and 6 days and the samples after heating up to 5 1C TiO2 nanofluids Zeta As-prepared potential After 15 days (mv) After 3 days After 6 days Heated up to 5 1C wt%.5 wt% 1 wt% 3 wt% 5 wt% the TiO2 nanoparticles in ethanol. Fig. 3 presents the FTIR spectra of ethanol and TiO2 ethanol nanofluids, where the This journal is the Owner Societies 216 characteristic peaks at 155, 2981 and 3391 cm 1 correspond to the C H (as shown in Fig. 3), C H and O H stretching vibrations, respectively (three values corresponding to 3 terms). The additional peak in the spectra of TiO2 nanofluids in the transmittance band at 1659 cm 1 corresponds to the stretching vibration of Ti OH. It is believed that the abundant and uniform Ti OH functional group can lead to the well dispersion of TiO2 nanoparticles in ethanol by electrostatic repulsive forces.24 Thus, the stirred-bead milling produces TiO2 ethanol nanofluids without the need for adding surfactants/additives, which have a uniform particle distribution, high mass fraction and excellent long-term and thermal stability that enable the investigation of thermal conductivity over a broad range of mass fractions and temperatures in TiO2 ethanol nanofluids. Before measuring the thermal conductivity of TiO2 ethanol nanofluids, the thermal conductivity of ethanol (base fluid) was measured in the temperature range of 1 5 1C. Fig. 4a shows the thermal conductivity versus temperature for ethanol with different volume concentrations of water. The measured value Phys. Chem. Chem. Phys., 216, 18,

4 Fig. 4 (a) Thermal conductivity of ethanol in concentrations of 99.5%, 96%, 9% and 85% vs. temperature. (b) Thermal conductivity of 96% ethanol and.5, 1, 3 and 5 wt% TiO 2 ethanol nanofluids vs. temperature. (c) Thermal conductivity ratio of.5, 1, 3 and 5 wt% TiO 2 ethanol nanofluids compared to 96% ethanol vs. temperature. (d) Viscosity of 96% ethanol and.5, 1, 3 and 5 wt% TiO 2 ethanol nanofluids vs. temperature. The corresponding volume fractions (vol%) are.92%,.185%,.566%, and.959% for.5, 1, 3 and 5 wt% TiO 2 ethanol nanofluids. of 99.5% ethanol at 3 1C is W mk 1, compared with the theoretical data for W mk 1 at 3 1C for pure ethanol (1%). This difference should come from the content of water in ethanol, which is inevitable due to azeotropy. There is a V-shaped transition of thermal conductivity as a function of temperature because the thermal conductivity of ethanol decreases with increasing temperature while that of water increases with increasing temperature. With increasing water content, the temperature dependence of thermal conductivity becomes steeper. Fig. 4b and c shows the thermal conductivity of TiO 2 ethanol nanofluids at various temperatures and mass fractions of TiO 2 nanoparticles (.5 5 wt%). Basically, the V-shaped curve of thermal conductivity as a function of temperature follows the performance of base fluids, and the incorporation of TiO 2 nanoparticles enhances the thermal conductivity. The dashed lines in Fig. 4c are the thermal conductivities versus temperatures based on the predictions of the modified Maxwell s model. 25 The discrepancy between the measured and calculated thermal conductivities suggests a lack of explanation for the temperature-dependent effect on thermal conductivities of TiO 2 ethanol nanofluids. As seen in 3 wt% TiO 2 ethanol sample, the enhancement in thermal conductivity is about 8% at 2 1C and about 11% at 5 1C. Over the past decade, the mechanisms of thermal conductivity enhancement in nanofluids were intensely debated, such as the Brownian motion, percolation, nanoparticle clustering, ballistic transport and interfacial layering. 2,3,26 28 According to the previously reported percolation threshold of.5.1 vol% for TiO 2 -based nanofluids, the particle concentrations of vol% (.5 5 wt%) in our systems should be higher than the percolation threshold. 19,29 In our experiment, when temperature was low (o2 1C), the thermal conductivities of TiO 2 ethanol nanofluids increased with increasing TiO 2 mass fraction but were not affected much by temperature. Note that when Brownian motion exists, the thermal conductivity should increase with increasing temperature. Therefore, one may infer that the contribution of Brownian motion is only nominal at this low temperature range. Our previous research has found that the percolation network of nanoparticles was the key contributor to this thermal conductivity enhancement. 2,3 When the temperature was over 3 1C, the thermal conductivity of TiO 2 ethanol nanofluids increased with increasing temperature, which indicated that the enhanced thermal conductivity was related to microconvection caused by Brownian motion of the nanoparticles. 3,28 Although the role of Brownian motion is debatable, it may be an important factor when the viscosity of nanofluids significantly decreases with increasing temperature, which is the possible mechanism in our experimental observation (Fig. 4d). The shear thinning effect is negligible in the range of viscosity measurement. 31 It has been reported that Brownian motion and nanoparticle clustering were related, and not completely independent of each other. 27,28,32 As mentioned above, the rapid clustering of nanoparticles took place to form nanoparticle aggregates when the temperature was increased. This nanoparticle clustering would decrease the Brownian motion due to the increase of the mass of the aggregates; whereas, the thermal conductivity would be increased due to the local percolation behavior as the nanoparticles came in contact with each other within the aggregates. The individual aggregates would have a higher thermal conductivity, which can be considered as new particles with larger effective radii. As mentioned in Fig. 2, the size of nanoparticles in our system was slightly increased after heating up to 5 1C, which indicates that the well-dispersed TiO 2 nanoparticles would rapidly and locally aggregate as well-dispersed nanoparticle clusters in ethanol while increasing the temperature and thus leading to a local percolation effect. Meanwhile, the decrease of fluid viscosity would compensate the mass effect of large-size nanoparticle clusters, leading to the microconvection caused by the enhancement of Brownian motion. Therefore, one may infer that the combination of nanoparticle clustering (local percolation behavior) and Brownian motion (microconvection) would cause the increase of thermal conductivity while increasing the temperature. Note that the thermal conductivity enhancement decreased with continuous agglomeration of clusters to make a much bigger size, as in the case of 5 wt% TiO 2 ethanol nanofluids. Therefore, we propose a model that the enhancement of thermal conductivity is dominated by the percolation network formed by nanoparticle clustering at low temperatures; however, at high temperatures it is governed by a combination of the local percolation behavior of nanoparticle clustering and the microconvection caused by Brownian motion. By taking advantage of the V-shape transition shown in Fig. 4, the thermal conductivities of ethanol were similar at 2 1C and 5 1C, but those of TiO 2 ethanol nanofluids were different at 2 1C and 5 1C. In the present study, AC impedance is employed to study the impact of structural transformation on the transport properties of 96% ethanol (base fluid) and 3 wt% Phys. Chem. Chem. Phys., 216, 18, This journal is the Owner Societies 216

5 Fig. 5 Electrochemical impedance spectroscopy (EIS) characterization of 96% ethanol and 3 wt% TiO 2 ethanol nanofluids at 2 1C and 5 1C respectively. (a) Nyquist plots and (b) Bode plots. Bode plots of the (c) real and (d) imaginary parts of the complex capacitance. TiO 2 ethanol nanofluids at 2 and 5 1C. Nyquist and Bode plots, as presented in Fig. 5a and b respectively, show that the impedances of ethanol at 2 and 5 1C are similar, but that of 3 wt% TiO 2 ethanol nanofluids at 5 1C is smaller than that at 2 1C. This indicates different heat transport behaviors of the nanoparticles at low and high temperatures because of the constant mass fractions of nanoparticles and similar thermal conductivities of base fluids. The large semicircle in the Nyquist plot can be modeled as the intra- and intercluster impedance responses by two RC parallel circuits in series. 2,33,34 The RC unit with higher characteristic frequency represents the intracluster impedance response, while the RC unit with lower characteristic frequency represents the intercluster impedance. The results show that the intracluster resistance decreases to 26% and the intercluster resistance decreases to 13% with the increase of temperature from 2 1C to 5 1C. The real and imaginary parts of the complex capacitance in Fig. 5c and d show an increase of capacitance at 5 1C as compared to that at 2 1C. When increasing the temperature, the viscosity and permittivity of ethanol decrease and its electrical conductivity does not change much, as shown in Fig. 4d and Fig. S2 (ESI ). The effective surface area would be reduced due to the effect of nanoparticle clustering at 5 1C. Therefore, the explanation for the increased capacitance at 5 1C should come from the reduction of EDL (electric double layer) effective thickness. 35 This result led us to infer that the microconvection induced by Brownian motion possibly plays a role in the reduction of EDL effective thickness when increasing the temperature. 4. Conclusions This paper presents a stirred bead-milling method, which does not need the addition of additives, to prepare TiO 2 ethanol nanofluids having uniform particle distribution, high mass fraction, and excellent stability. By using these nanofluids, the thermal conductivity behaviors of ethanol-based TiO 2 nanofluids were investigated over a broad range of concentrations (.5 5 wt%) and temperatures (1 5 1C). At low temperatures, the nanoparticle clustering formed a percolation network, which dominated the enhanced thermal conductivity. By increasing the temperature, the well dispersed TiO 2 nanoparticles rapidly aggregated as well-dispersed nanoparticle clusters in ethanol, which gave rise to a local percolation behavior. Meanwhile, the decrease of fluid viscosity enhanced the Brownian motion of nanoparticles, which led to the microconvection effect. A combination of nanoparticle clustering and Brownian motion caused the enhancement of thermal conductivity at high temperatures. The impedance spectroscopy provided evidence that both the intra-/inter-cluster resistances decreased with increasing temperature from 2 1C to51cat constant mass fractions of TiO 2 nanoparticles and constant impedances of base fluids. Conflict of interest The authors declare no competing financial interest. Acknowledgements This work was supported by the General Research Fund of the Research Grants Council of Hong Kong Special Administrative Region, China: Award Number: HKU E (S. P. Feng). It was also partially supported by the FRGS Grant (Project no. FRGS/2/213/SG6/MUSM/1/1) provided by the Ministry of Higher Education (MOHE), Malaysia (A. K. Soh). References 1 S. Cingarapu, D. Singh, E. V. Timofeeva and M. R. Moravek, Int. J. Energy Res., 214, 38, R. Zheng, J. Gao, J. Wang, S.-P. Feng, H. Ohtani, J. Wang and G. Chen, Nano Lett., 211, 12, R. Prasher, P. Bhattacharya and P. E. Phelan, J. Heat Transfer, 26, 128, A. K. Singh and V. S. Raykar, Colloid Polym. Sci., 28, 286, S. S. Park and N. J. Kim, J. Renewable Sustainable Energy, 214, 6, L. Lu, L.-C. Lv and Z.-H. Liu, Thermochim. Acta, 211, 512, Y. Gan, Y. S. Lim and L. Qiao, Combust. Flame, 212, 159, R. Prasher, P. Bhattacharya and P. E. Phelan, Phys. Rev. Lett., 25, 94, B. T. Branson, P. S. Beauchamp, J. C. Beam, C. M. Lukehart and J. L. Davidson, ACS Nano, 213, 7, J.-H. Lee, S.-H. Lee and S. Pil Jang, Appl. Phys. Lett., 214, 14, This journal is the Owner Societies 216 Phys. Chem. Chem. Phys., 216, 18,

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