Stability analysis of MWCNT with distilled water as a base fluids.
Though the nanofluids exhibit good thermal conductivity and they do not long last for real time applications due to settling of particles. Therefore, the stability of the nanofluid suspension is a crucial issue for both scientific research and practical applications to provide better cooling. To consider and evaluate stability of nanoparticles inside the base fluid, sedimentation velocity calculation of small spherical particles is found by using Stokes law. Stokes law "Equation (1)" includes the effective parameters for stability of nanofluids.
v = 2[r.sup.2]/9[mu]([[rho].sub.p]-[[rho].sub.f])g (1)
Where, 'v' is the sedimentation velocity, 'r' the radius of particles, 'p' is viscosity of liquid; '[rho]' is the density while 'p' and 'f subscripts are the particles and liquid, respectively. Finally, 'g' is the gravity acceleration, which is the main reason of sedimentation. There are three forces acting on suspended particle such as buoyancy force, drag force and body force. Their balance makes the nanoparticle stable. Buoyancy and drag forces are acting upward and resisting against body force acting downwards resulting from gravitational attraction Hiemenz and Dekker . Therefore, lower particle size, lower viscosity, lower temperature difference are the stability parameters. Addition of surfactant, pH control and Ultrasonic agitation (vibration) are the three common techniques for making stable nanofluids. Addition of surfactant and pH control is the two techniques to prevent clustering and agglomeration while ultrasonic vibration is applied to break down agglomeration. Zhu et al , Wang et al  and Pantzali et al  used all three techniques to improve the stability of nanofluid.
Surfactants can be defined as chemical compounds added to nanoparticles in order to lower surface tension of liquids and increase immersion of particles. Several literatures talk about adding surfactant to nanoparticles to avoid fast sedimentation, however, enough surfactant should be added to particle at any particular case. In researches, several types of surfactant had been utilized for different kinds of nanofluids. The most significant ones could be listed as a) Sodium dodecyl sulfate (SDS) Chandrasekhar et al , b) Salt and oleic acid, c) Cetyltri methyl ammonium bromide (CTAB), Jiang , d) Dodecyle trimethylammonium bromide (DTAB) and sodiumoctanoate (SOCT), Li et al , e) Hexadecyltri methyl ammonium bromide(HCTAB),Yu et al , f) Polyvinyl pyrrolidone' 1q (PVP), Pantzali et al , and g) Gum Arabic, Madni et al . Xie et al  showed the stability of carbon nanotubes/water nanofluids by taking simple acid treatment. This was caused by a hydrophobic-to-hydrophilic conversion of the surface nature due to the generation of a hydroxyl group. As the pH value of the solution departs from the Iso Electric Point (IEP) of particles the colloidal particles get more stable and ultimately modify the thermal conductivity of the fluid. The disadvantage of adding surfactant at the high temperatures as above than 60oC leads to damage the bonding between surfactant and nanoparticles. Ghadimi , reviewed the stability of nanofluids, instruments and methods that can rank the relative stability of nanosuspension. The list includes UV-Vis spectrophotometer, zeta potential, sediment photograph capturing, TEM (Transmission Electron Microscopy) and SEM (Scanning Electron Microscopy), light scattering, three omega and sedimentation balance method.
Up to today, most of the researcher published data on the factors that influence the nanofluids' stability and thermal conductivity, mainly focused on the effects of nanoparticles concentration , dispersant (surfactant) , viscosity of base liquid [21,22] and pH value [23,24]. However, no other studies have been found to directly point out the effects of the dispersion method on thermal conductivity and stability of nanofluids.
In this investigation, the MWCNT nano fluids have been prepared with Distilled water as base fluids at the concentration of 0.1, 0.3, and 0.5%. The MWCNT nanofluid have been prepared with two step method and the UV -Vis spectrometer, pH values, and sediment photograph methods have been carried out to analyze the stability
MATERIALS AND METHODS
Details of MWCNT nanostructures and base fluids:
Synthesis of MWCNT nanofluids
Preparation of Nano fluids is the first foot-step to the experimental studies of Nano fluids. The two primary methods to prepare Nano fluids are single-step preparation process and the two -step preparation process.. The one-step technique simultaneously makes and disperses the nanoparticles directly into a base fluid. This technique ensures stable dispersion and no agglomeration. In the two-step technique nanoparticles are produced by one of the physical or chemical synthesis techniques and proceed to disperse them into a base fluid. In this investigation nanofluids were prepared by using Multi walled Carbon Nano Tube (MWCNT) with Distilled water as base fluids at the volume concentration of 0.1, 0.3, and 0.5% and with adding Sodium dodecyle butane sulfate stabilizing agent. Two-step preparation process was used to prepare above nanofluids. In this study required amount of base fluid was first poured into 1000-ml glass beakers and mixed with MWCNT of 0.1, 0.3, and 0.5% volume concentration and the suspensions were dispersed using a magnetic stirrer. The homogeneous solutions were obtained after magnetic stirring as shown in Fig.2.
The magnetic stirrer employs a rotating magnetic field which stirs the magnetic pellet immersed in a fluid thus allowing it to spin very quickly which in turn enabling the even dispersion of the particles and ultrasonic vibrator (Toshiba, India) generating ultrasonic pulses of 100W at 36 [+ or -] 3 kHz also ensures dispersion of particles in the fluid.
Sonication is a process in which sound waves are used to agitate particles in solution. Such disruptions can be used to mix solutions, speed the dissolution of a solid into a liquid (like sugar into water), and remove dissolved gas from liquids. To get a uniform dispersion and stable suspension which determine the final properties of nanofluids, the nanofluids are kept under ultrasonic bath (Fig.3) continuously for 3 hour with maintained 400 C.
RESULTS AND DISCUSSIONS
A. Stability Inspection With Uv--Vis Spectrophotometer:
The UV-Vis spectrophotometer, measure of pH values, and sedimentation techniques has been used for stability analysis in this investigation by keeping the nanofluids under static condition period of 30 days. Sedimentation method is the most elementary method for evaluation of Nano fluids. An external force field is applied to start the sedimentation of nanoparticles in the Nano fluids. The weight of sediment or the volume of sediment indicates the stability of Nano fluids. Nano fluids are generally considered to be stable if the concentration of the supernatant particles remains constant with time. Sedimentation method was used to measure the stability of graphite suspension. Use of camera has proven to be a suitable aid to capture sedimentation photographs for observing the stability of Nano fluids. The sedimentation was recorded using photographs of samples after several days of preparation. Spectral analysis via UV- Vis spectrophotometer is another useful way to evaluate stability of Nano fluids. The UV-spectroscopy gives quantitative results corresponding to concentration of Nano fluids.
The UV Visible Spectroscopy of nanofluids Ultra Violet- Visible spectrophotometer (UV-Vis) measurements have been used to quantitatively characterize the stability of nanoparticles dispersed in base fluids. The UV-Vis spectrophotometer exploits the fact that the intensity of the light becomes different by absorption and scattering of light passing through a fluid. Jiang et al  were the first who proposed nanofluid sedimentation estimation by using UV-Vis spectrophotometer. Further, this method was used by Hwang et al , and Lee et al  have used the same method. In this investigation, the UV-Vis. spectrophotometer, Lambda 35 model, Perkin Elemer make, absorption range of 190 nm to 1100nm was used to study the stability of nanofluid. The inspection range is from 230nm to 600nm.
The MWCNT nanofluids with different base fluids have been characterized with sample of just after preparation and after 30 days. The UV- visible spectroscopy of sample after 30 days is given in Figs.4-6.
Fig.4 UV- visible spectroscopy of 0.1% volume concentration of nanofluids.
Fig.4 UV- visible spectroscopy of 0.1% volume concentration of nanofluid
Fig.5 UV- visible spectroscopy of 0.3% volume concentration of nanofluid
Fig.6 UV visible spectroscopy of 0.5% volume concentration of nanofluid
B. Stability Inspection by measuring ph value:
When dispersing MWCNT nanoparticles into any base fluid, the particle surface can acquire an electric charge by absorbing or desorbing at the particle/liquid interface, especially when the base fluid is a polar medium like distilled water Hunter . This absorbing and desorbing mechanism form two layers that surround the particle surface. The inner region is the Stern layer, where the ions are strongly attached to the particle surface. The diffuse layer, which is the outer layer, contains ions that are not firmly bound. The potential at this electrical double layer (EDL) boundary is known as the zeta potential ([xi]). The magnitude of Z represents the strength of the electrostatic energy barrier between particles. A greater [xi] increases the inter particle repulsion in a nanofluid of similar nanoparticles. Hence, less aggregation will occur and the nanofluids will be more stable. The [xi] and the thickness of the EDL are strongly dependent on the pH value. Once the pH value exceeds a certain limit, the ions cause significant shrinkage of the EDL, and the nanofluid is no longer be stable Hunter . Xie et al , , and Lee et al  measured the thermal conductivity of nanofluids with water, ethylene glycol, and pump oil as base fluid. They have reported significant decrease in thermal conductivity enhancement with increasing pH values. They have related the Iso -Electric Point (IEP) of MWCNT nanoparticles and pH value which causes mobility of nanoparticles. The (IEP) is the point at which there is no either positive or negative electrical charge of particles at certain pH value. Wang et al  suggested the pH value affects the thermal conductivity and stability of nanofluids. Zeta potential and associated suspension stability are: 0 mV - little or no stability, 15Mv- some stability but settling lightly, 30mV-moderate stability, 45mV-good stability, and 60mV- very good stability. Generally, a suspension with a measured zetapotential above 30 mV (absolute value) is considered to have good stability. Therefore, measuring the pH value corresponding to the IEP is one of the most common methods among the researchers to determine the stability. Therefore, it is essential to measure the pH value of nanofluid to ensure the optimum value for attaining the maximum thermal conductivity and stability before applying nanofluid in any thermal systems.
In this investigation, the pH (Hydrogen potential) meter (Deep vision, 0.01 accuracy, in the range of 1-14, working temperature range 0-100[degrees]C was used to measure the pH value of MWCNT/water nanofluid. The pH meter was calibrated by using a single point calibration technique, with a standard buffer solution of pH 7.00. Vincenzo Bianco  et al (2015) proposed the pH value corresponding to isoelectric point of MWCNT//water nanofluid is 7.2.
The pH value of the conventional and nanofluids were measured using pH meter (Fig.7) and reported in the table.6. In the measurement of pH value for both conventional and Nano fluids shows its acidic nature by its value in pH scale.
From table.2, it is found that the 0.5% volume concentration MWCNT/Distilled water nanofluid has poor stability and 0.3% volume concentration MWCNT/ Distilled water nanofluid has good stability of static condition. This is because the pH value of 0.1% & 0.5% MWCNT/distilled water nanofluids is away from the pH value correspond to the Iso electric point of MWCNT nanofluid and the 0.3% MWCNT/ distilled water nanofluids pH corresponding to the Iso electric point is nearly close to the value.
c. Stability Inspection with photograph capturing technique:
The photographs of test tubes with nanofluids were taken by using Sony digital camera of 16.1 Mega Pixel, W Series, 5x Optical Zoom Cyber-shot (Black).Sedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained, and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration or electromagnetism. The sedimentation of Nanofluids, after sonication were recorded using digital camera and shown in the fig.8. The sedimentation photograph after 15 days and 30 days were shown in fig.9 and fig. 10 respectively. In distilled water based nanofluid the MWCNT setteled down at the bottom after 30 days as shown in fig.10.
From figures 8-10, it is found that the 0.5% MWCNT/Distilled water nanofluid has poor stability and 0.3% MWCNT/ Distilled water nanofluid has good stability after 30 days of static condition.
In this experimental investigation, the 0.1, 0.3, 0.5% concentration MWCNT nanofluids have been prepared with Distilled water as the base fluids and with the two step methods to study the stability of MWCNT nanofluids. The prepared nanofluids were characterized by Atomic force microcopy, UV-Vis Spectrophotometer, with pH values, and Photograph capturing techniques for analyzing the stability of nanofluids. The samples are put under static condition. It is found from the three stability analysis techniques that the stability of nanofluids are in the increasing order of 0.1% MWCNT/Distilled water nanofluid, 0.5% MWCNT/Distilled water nanofluid, 0.3% MWCNT/Distilled water nanofluid. The 0.5% MWCNT/Distilled water nanofluid has poor stability and 0.3% MWCNT/ Distilled water nanofluid has good stability.
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(1) M. Muruganandam and (2) P. C. Mukesh Kumar
(1) Department of mechanical engineering, St.Joseph's College of Engineering and Technology, Thanjavur. 614403. Tamil nadu. India.
(2) Department of mechanical engineering, University College of Engineering, Dindigul.624622. India.
Received 28 February 2017; Accepted 22 May 2017; Available online 6 June 2017
Address For Correspondence: M. Muruganandam, Department of mechanical engineering, StJoseph's College of Engineering and Technology, Thanjavur.614403. T amil nadu. India. E-mail: Muthumuruga26@gmail.com
Caption: Fig. 1: SEM micrograph of MWCNT
Caption: Fig. 2: Magnetic stirrer
Caption: Fig. 3: Ultrasonic bath
Caption: Fig. 4, 5 & 6: shows the stability of 0.1%, 0.3% and 0.5% nanofluids after 30 days from preparation by using ultrasonic agitation. It is shown that all three nanofluids light absorption strength (broadband of Full Width at Half Maximum (FWHM)) is wider in range of 200nm to 290nm. The range of suspended nanoparticles absorption is 2.1-3.9.It is seen that the absorption strength of 0.1% nanofluid is lower. This is because the 0.1% nanofluids leaves more 'particle free region' in base fluids. The 0.3% and 0.5% nanofluids absorption strength are relatively higher than 0.1% nanofluids. The 0.1%, 0.3% and 0.5% nanofluids are stable after 30 days from preparation
Caption: Fig. 7: pH measurement of fluids
Caption: Fig. 8: Sedimentation samples of nanofluids just after Sonication
Caption: Fig. 9: Sedimentation samples of nanofluids after 15 days
Caption: Fig. 10: Sedimentation samples of nanofluids after 30 days
Table 1: Details of MWCNT Property Values Outer diameter 50-80nm inner diameter 5-15nm True density 2.1 g/[cm.sup.3] Bulk density 0.18 g/[cm.sup.3] Length 10-20pm Supplier Nanostructured & Amorphous Materials, Inc. Houston, TEXAS, and USA. Base fluids Distilled water Table 2: pH Values of MWCNT Nanofluids S.NO (MWCNT) % volume pH values for concentration with surfactant 1 0.1 6.52 2 0.3 6.33 3 0.5 6.53
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|Author:||Muruganandam, M.; Kumar, P.C. Mukesh|
|Publication:||Advances in Natural and Applied Sciences|
|Date:||Jun 1, 2017|
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