A Novel Experimental Study on the Rheological Properties and Thermal Conductivity of Halloysite Nanofluids

Nanofluids obtained from halloysite and de-ionized water (DI) were prepared by using surfactants and changing pH for heat-transfer applications. The halloysite nanotubes (HNTs) nanofluids were studied for several volume fractions (0.5, 1.0, and 1.5 vol%) and temperatures (20, 30, 40, 50, and 60 °C). The properties of HNTs were studied with a scanning electron microscope (SEM), energy-dispersive X-ray analysis (EDX), Fourier-transform infrared (FT-IR) spectroscopy, X-ray powder diffraction (XRD), Raman spectroscopy and thermogravimetry/differential thermal analysis (TG/DTA). The stability of the nanofluids was proven by zeta potentials measurements and visual observation. With surfactants, the HNT nanofluids had the highest thermal conductivity increment of 18.30% for 1.5 vol% concentration in comparison with the base fluid. The thermal conductivity enhancement of nanofluids containing surfactant was slightly higher than nanofluids with pH = 12. The prepared nanofluids were Newtonian. The viscosity enhancements of the nanofluid were 11% and 12.8% at 30 °C for 0.5% volume concentration with surfactants and at pH = 12, respectively. Empirical correlations of viscosity and thermal conductivity for these nanofluids were proposed for practical applications.


Introduction
Conventional heat-transfer fluids such as water, propylene glycol, ethylene glycol, and engine oil have been broadly utilized in many industrial applications. The heat-transfer enhancement of these fluids can reduce the material cost, energy, process time, and size, and increase the lifetime of the device [1,2].
In the heat-exchange systems, one of the problems is that the conventional heat-transfer fluids have low thermal conductivity. The thermal conductivity of these fluids can be enhanced by dispersing Halloysite, belonging to the kaolin group, is a low-cost nanotubular clay with the chemical formula Al 2 Si 2 O 5 (OH) 4 ·nH 2 O, where n = 0-2 [35]. The length of the halloysite is from 0.02 to 30 µm, the inner diameter is from 10 to 100 nm, and the outer diameter is approximately 30 to 190 nm [36,37]. The inner surface includes Al-OH groups, while the outer surface comprises inert Si-O-Si groups. Therefore, the reactivities of the outer and inner surfaces are different [38]. Because of these properties, there are many different applications of halloysite, such as solvent-free nanofluids [39,40], nanoreactors [41], drug delivery [42], energy storage devices, etc. [43]. However, the ability to apply halloysite as nanomaterials to prepare water-based nanofluids has rarely been investigated. Alberola et al. prepared the halloysite nanofluid and improved its stability by setting pH = 12. The studied temperature was from 40 to 80 • C. The thermal conductivity enhancement was 8% at 5% volume concentration and T = 80 • C, while the viscosity increased with halloysite content [30].
Usage of high pH for stabilization limits the applications of halloysite-based nanofluids. In this research, the halloysite-based nanofluid was investigated by dispersing halloysite into the water, and the stability was improved by different surfactants. As far as authors know, there are no studies on stabilizing halloysite nanofluids with surfactants and investigations on heat-transfer applications. In addition, the size of halloysite used in this research is smaller than in previous research. The halloysite nanotube (HNT) was first analyzed by scanning electron microscopy, Fourier-transform infrared spectroscopy, X-ray powder diffraction, energy-dispersive X-ray analysis, and thermogravimetric analysis. The concentrations of nanofluids were prepared from 0.5 to 1.5 vol%. The thermal conductivity and dynamic viscosity of these nanofluids were measured. The temperatures during the experiments are from 20 to 60 • C. In order to evaluate the measurement results, the nanofluids were prepared with pH = 12 and the same concentrations.

Materials
HNTs were supplied by the University of Pannonia. The surfactants including Tween, oleylamine, Gum Arabic (GA), hexadecyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS) and sodium carboxymethylcellulose (SCMC) were bought from Sigma-Aldrich (Saint Louis, MO, USA). De-ionized water (DI) was used as base fluids. DI and 1M sodium hydroxide (NaOH) solution were supplied by the Department of Inorganic and Analytical Chemistry laboratory, Budapest University of Technology and Economics (Budapest, Hungary).

Preparation of Nanofluids
Halloysite nanofluids were prepared by dispersing different amounts of halloysite nanoparticles in DI base fluid. The volume concentrations of halloysite content were 0.5%, 1.0%, and 1.5%. Then, surfactants or 1 M NaOH solution were added to the nanofluids in the appropriate amount. Halloysite nanofluids were sonicated at 130 W and 45 kHz using an ultrasonication instrument for 1h. Table 1. shows pure halloysite nanofluid sample specifications.

Characterization Techniques
The halloysite powder samples were used, so the morphological characterization of halloysite was performed by a LEO 1440 XB scanning electron microscope (LEOGmbH, Oberkochen, Germany) at 5 kV with a secondary electron detector in a high vacuum mode.
The halloysite's chemical components were investigated by using energy-dispersive X-ray analysis with a JEOL JSM-5500LV electron microscope (Tokyo, Japan). The crystal structure of the halloysite was studied by utilizing a X'PERT PRO MPD X-ray diffractometer (PANalytical, Almelo, Netherlands), with Cu K α irradiation. The measurement results were recorded at a resolution of 3 degrees/min for the 2θ range of 5 • to 65 • . Fourier transform infrared (FT-IR) spectra of halloysite were investigated by an Excalibur FTS 3000 BioRad FT-IR (Bio-Rad, Digilab, UK) in the 400-4000 cm −1 domain in transmittance mode, with a resolution of 4 cm −1 , and the number of scans was 64. Raman spectrum was obtained utilizing a Jobin Yvon Labram Raman spectrometer (Horiba, Kyoto, Japan) containing an Olympus BX41 microscope (Olympus, Tokyo, Japan) equipped with a green Nd-YAG laser. The measurement range was 72-1560 cm −1 .
The thermal properties of HNTs were investigated in the air using an STD 2960 thermogravimetry/differential thermal analysis (TA Instruments Inc., New Castle, DE, USA) device. The heating rate was 10 • C/min, and the temperature range was from room temperature to 800 • C.
A Brookhaven ZETAPALS device (Bookhaven Instruments, Holtsville, NY, USA) was utilized for measuring zeta potential values of halloysite nanofluids. The zeta potential (ζ) was determined from the electrophoretic mobility of HNTs utilizing the Henry equation by considering the Smoluchowski estimation [44]. Three repetitions of each sample were measured, then their average value was taken into consideration.
The rheological behavior of halloysite nanofluid was studied utilizing an Anton Paar Physica MCR 301 (Anton Paar, Ashland, VA, USA) rotation viscometer at various temperatures and shear rates. The number of shear rates per measurement was 10. The angular frequency range was 100 to 2000 s −1 , while the amplitude was 5%.
The thermal conductivity of halloysite nanofluids was obtained utilizing an SKZ1061C TPS Thermal Conductivity Tester (SKZ Industrial, Shandong, China), which is based on the modified transient plane source approach. All samples were measured at five different temperatures of 20, 30, 40, 50, and 60 • C. A temperature-controlled oven was utilized to keep up the temperature at the defined setpoint.  210) and (300) planes, respectively [45]. The presence of the (001) peak at 2θ of 12.0 • corresponded to a layer spacing of 0.73 nm. This can be ascribed to halloysite-7 angstrom. The dehydrated state was also confirmed by the (100) diffraction peak at 2θ of 20.1 • (0.44 nm). The layer distance of the hydrated halloysite is 10 Angstroms. After dehydration-which is an irreversible process-the layer distance collapses to 7 angstroms. This is characteristic of tubular halloysite [46,47].  Figure 2 shows the SEM image of the morphological structure of HNTs. From SEM studies, it can be seen that the sample used was uniform in content, containing nanotubes with infrequent particle agglomerates. Between HNTs, some platy particles were presented due to residual kaolinite. By treating the image of micrographs, the mean outer diameter and the mean length were determined. The diameter and length were 58 and 436 nm, respectively. The aspect ratio was calculated as ca. 7.5. Compared to the HNTs used by Alberola et al. [30], the halloysite in this study is smaller. This can give the advantage of higher thermal conductivity and greater stability. The examination of functional groups of the used nanomaterials supports the effort to make a proper dispersion. Figure 3 shows the FT-IR spectrum of HNTs. The two infrared active modes centered at around 3694 and 3617 cm −1 are assigned to the stretching vibration (O-H) bonds of halloysite [48][49][50][51]. The peak at around 1630 cm −1 confirms the typical bending vibration of absorbed water. This peak in halloysite is more intense and broader than in kaolinite [52]. The peak at 1110 cm     Figure 5 shows the thermal analysis of HNT samples. At around 150 • C and below, weight losses refer to the loss of absorbed water (2%) on surface and interlayer [49]. From 150 • C to 400 • C, the interlayer water is removed completely [56], while at 400-500 • C, the Al-OH groups of HNTs are dehydroxylated with a loss of approximately 9% and the "metahalloysite" (Al 2 O 3 ·2SiO 2 ) is formed [49]. Above 500 • C, alumina-rich phase and amorphous SiO 2 is formed distinctly. In the differential thermal analysis diagram, because of the removal of water, the process is endothermic, and then the structural rearrangement of the material-above 800 • C-is exothermic [57].

Zeta Potential Measurement
The zeta potential of 0.5% HNT nanofluids with different surfactants is shown in Table 3. Due to the improvement of the stability of HNT nanofluids, different surfactants are utilized, such as cetyltrimethylammonium bromide (CTAB), sodium dodecylbenzenesulfonate (SDBS), gum Arabic (GA), SCMC, oleylamine, and Triton X-100 (TX). Among the used surfactants, the SCMC gives the best result with −30.54 mV. According to the zeta potential values, the HNT nanofluids have acceptable stability with SDBS and SCMC. By visual observation, it can be confirmed that the SCMC is the best choice. Colloidal solutions with zeta potentials as low as -30 mV have acceptable stability [6,58]. Table 4 shows the zeta potential values of HNT nanofluids with different concentrations. With surfactants, the zeta potential of 0.5, 1.0 and 1.5 vol% HNT nanofluids was −30.42, −33.03 and −43.33 mV, respectively, while with pH = 12 medium, the zeta potentials are −33.40, −39.72 and −32.39 mV on the same order. These values confirm the stability of nanofluids. Also, visual observation verifies that these nanofluids are stable for several days.

Rheological Properties of Halloysite Nanofluid
The rheology and viscosity of the nanofluids are important parameters determining the heat transfer. The viscosity of the base fluid and HNT nanofluids at different shear rates is measured for three volume concentrations of 0.5, 1.0, and 1.5 at five temperatures: 20, 30, 40, 50, and 60 • C. Figure 6 shows the shear rate-shear stress diagram of 0.5 vol% HNT nanofluids with surfactant at different temperatures. Shear stress of HNT nanofluids falls with increasing temperature and rises with increasing concentration of nanofluids. The increase in temperature causes the Brownian movement and thermal motion of molecules to be higher, thus the viscosity of nanofluids decreases [59,60]. The shear rate of HNT nanofluid is almost linearly dependent on the shear rate. We conclude that the nanofluids are Newtonian.  Figure 7 shows the viscosity increment of prepared HNT nanofluids at different temperatures. Relative viscosity is obtained by dividing the viscosity of nanofluids by that of the base fluids. It can be seen that the viscosity is higher with nanoparticle content due to the clusters formed from nanoparticles [6]. Temperature plays an essential role in the relative viscosity. This means that temperature decreases the viscosity of base fluids more than that of the nanofluids. Compared to the HNT nanofluids at pH = 12, the relative viscosity of nanofluids containing the surfactant doesn't have a significant difference. With surfactant, the HNT nanofluids have the lowest relative viscosity of 1.09 for 0.5% volume concentration and the highest relative viscosity of 1.31 for 1.5 vol%. The viscosity of nanofluids increased from 9% to 31% compared to the base fluid containing the surfactants.

Thermal Conductivity of Halloysite Nanofluid
The thermal conductivity of HNT nanofluids at different temperatures is presented in Figure 8. The device is reliable within 0.6% error when the calibration measurement is verified for distilled water. The nanofluids show greater thermal conductivity than the base fluid at experimental temperatures. HNT nanofluids with surfactant give 4.48%, 6.03% and 7.93% thermal conductivity increment at 0.5 vol%, 1.0 vol%, and 1.5 vol% in comparison with the base fluid at 20 • C, respectively. By increasing the temperature, the thermal conductivity of nanofluids increases due to the augmentation in the Brownian motion of the solid.
It can be seen that when the nanoparticle content in nanofluids increases, the thermal conductivity also increases because of the higher number of nanoparticles presented in the nanofluid. The thermal conductivity enhancement of nanofluids containing surfactant is slightly higher than nanofluids with pH = 12. It is concluded that like changing pH, surfactant supports using HNT in the preparation of nanofluids. Compared to the results from Alberola et al. [30], the thermal conductivity of the nanofluids in this study is greater. This may be due to the smaller halloysite used in this research.

Regression Correlations
According to the results of Azmi et al. [61], the following correlations are proposed from the measured results: (2) where µ and k represent the viscosity and thermal conductivity; ϕ and T are volume concentration and temperature. The tabulation of viscosity and thermal conductivity from the experiment and the proposed correlations is shown in Figure 9a with pH = 12 and Figure 9b with the surfactant. The average and standard deviations are 0.68% and 0.86% for viscosity; and 0.43% and 0.74% for thermal conductivity. It is concluded that the proposed correlations are suitable for the experimental results.

Conclusions
In this research, HNT nanofluids were investigated at different concentrations and temperatures by using surfactants and changing pH. The results show the high purity, shape, and dimensions of the used HNT. The zeta potential measurements and visual observation proved the stability of the HNT nanofluids.
With surfactants, the HNT nanofluids have the highest thermal conductivity increment of 18.30% for 1.5 vol% concentration in comparison with the base fluid. The thermal conductivity enhancement of nanofluids containing surfactant is slightly higher than nanofluids with pH = 12. From the rheological measurements, it is shown that the nanofluids were Newtonian. The viscosity enhancements of the nanofluid were 11% and 12.8% at 30 • C for 0.5% volume concentration with surfactants and pH = 12, respectively. Instead of changing pH, the surfactants give good results for the preparation of the nanofluid. Novel equations of viscosity and thermal conductivity for these nanofluids were proposed. Funding: An NRDI K 124212 and an NRDI TNN_16 123631 grants are acknowledged. The research within project No. VEKOP-2.3.2-16-2017-00013 was supported by the European Union and the State of Hungary, co-financed by the European Regional Development Fund. The research reported in this paper was supported by the BME Nanotechnology and Materials Science TKP2020 IE grant of NKFIH Hungary (BME IE-NAT TKP2020) and Stipendium Hungaricum scholarship grant.