Devising Carbon Nanotube, Green Tea, and Polyaniline Based Nanocomposite plus Investigating Its Rheological together with Bactericidal Efficacies.

Subsequently, engines are designed to operate at low viscosity engine oils. Low viscosity oils take less power from engines, bring down the internal drag, cut the fuel consumption, and ultimately improve the engine's efficiency. Considering this focus, an approach has been made to formulate a multiwalled carbon nanotube based green tea and polyaniline nanocomposite, that is, GT/MWCNT/PANI, and incorporate it in engine oil (base fluid). The objective was to reduce the viscosity of engine oil by examining the effects of the constant shear rate and varying shear rates on the viscosity of Castrol class 15W-40 engine oil. The investigation was performed at a constant temperature of 25 °C for a fixed volume fraction of 0.1% GT/MWCNT/PANI in engine oil on the experimental setup rheometer from Anton Paar Series. Primordial findings revealed that, at a constant shear rate of 100 s-1, engine oil viscosity was lowered from 0.221000 to 0.001402 Pa·s, that is, 99% reduction in viscosity of the engine oil, after incorporating the GT/MWCNT/PANI nanocomposite. Furthermore, a new correlation has been proposed considering the experimental and theoretical models with an average percentage error of 0.040%. Also, at varying shear rates, up to 90 s-1, the shear viscosity of nanofluid decreases significantly, leading to shear-thinning behavior of the nanofluid, while at a shear rate of >90 s-1, it shows Newtonian behavior. Besides, the ternary nanocomposite with 0.2 wt % GT/MWCNT/PANI also showed significant bactericidal effects with the zones of inhibition of 19, 18, and 15 mm against Gram-negative (Pseudomonas aeruginosa, Escherichia coli) and Gram-positive (Staphylococcus aureus) bacteria, respectively, as measured using the well diffusion method.

Introduction

The past decade has recorded a rapid development of nanofluids (NF) science in many facets. Innumerable studies are focusing on the thermal conductivities of these fluids. NF viscosity is a less researched area and deserves the same attention as thermal conductivity. Since engines are designed to operate at low viscosity, reducing the NF viscosity of engine oils becomes one of the most required areas of study to reduce frictional losses in engines and to cut the fuel cost. A survey suggested that the market share of lower viscosity oils (10W-30 oils) will grow from about 6 to roughly 20% by 2022.[1]

Asadi et al. investigated NFs for the first time as a suspension of nanosized particles in conventional fluids: water, ethylene glycol, propylene glycol, oil, and so on.[2] To the best of our knowledge collected, examples of the nanoparticles (NPs) used in preparing nanofluids until now are pure metals (Au, Ag, Cu, and Fe), oxides of metals (ZnO, TiO2, Mg(OH)2, Fe3O4, Al2O3, SiO2, CuO), and different types of allotropes of carbon (graphite, diamond, carbon nanotubes, graphene).[1−5] The small size of NPs has solved issues like corrosion of fluids, impurity, and pressure drop and ultimately modified the properties of the base fluids exhibiting high thermal conductivity, tunable surface tension, and improved rheology.[6,7]

Over the last decade, rheology of carbon nanotube (CNT) suspension has been under in-depth study.[1−3,8] The first steady shear experiment on a CNT/polymer nanocomposite was reported by Safadi et al.,[9] where MWNTs were dispersed in toluene and the resulting suspensions were mixed with toluene solutions of 30 wt % polystyrene. The viscosities of the mixtures with various MWNT contents were measured at room temperature over a range of shear rates. Around the same time, Kinloch et al. reported the experimental rheology results for concentrated aqueous CNT dispersions, covering the steady shear, linear viscoelasticity (LVE), and recovery after pre-shear behavior of these dispersions. Bagheri and Nadooshan investigated the thermal conductivity coefficient of the composite ZnO/MWCNT into water/ethylene glycol nanofluids.[10] The thermal conductivity of the NF showed a 30% increase over the base fluid (bf). Pötschke et al. reported their work on rheological properties of CNT/thermoplastic systems from the Leibniz Institute of Polymer Research Dresden, Germany.[11]

Rheological studies of CNT based NFs demand a homogeneous suspension. The preparation of a homogeneous CNT suspension still appears to be a complicated thing.[12,13] CNTs have the property of agglomeration due to van der Waals forces, and mechanically mixing CNTs into a liquid does not produce a homogeneous suspension.[16] Several studies have been done regarding this issue by Sinija and Mishra,[16] Shinde and Kher,[17] and Chen et al.[18] The formation of a homogeneous suspension of CNT includes separating individual CNTs from the bundles, stabilizing the DE bundled CNTs to prevent reaggregation. High-shear mechanical mixing and sonication are two of the most common methods for dispersing CNTs into a suspending medium having a relatively low viscosity.[12−14] Dispersion by sonication relies on cavitation (the implosion of small bubbles generated in the suspending medium) occurring within the sample.[14] The influence of prolonged sonication on the length of CNTs has been reported in the literature.[19] The critical point is that, for fixed sonication conditions, there exists a limiting length beyond which the CNTs will not be further shortened. Huang and Terentjev[14] constructed a simple model to calculate the limiting length; theoretically, there should be an optimal sonication condition (in terms of sonication power, frequency, and time) that would allow the dispersion of CNTs without shortening their lengths significantly. The literature reports convey that dispersion state, aspect ratio, suspension medium concentration, and chemical treatments are some of the parameters that affect the rheology of CNT composite suspensions.[15]

Despite that, only recently, scientists have found green tea (GT) to be famous in the production of nanocomposites. GT is majorly composed of a polyphenolic compound catechin, which is regarded as an active reducing agent due to its ability to donate an electron or hydrogen atom easily. The four primary catechins found in green tea are (−)-epicatechin (EC), (−)-epicatechin-3-gallate (ECG), (−)-epigallocatechin (EGC), and (−)-epigallocatechin-3-gallate (EGCG).[16] Also, polyaniline (PANI), a conducting polymer, possesses fascinating properties. It has a flexible −NH– group in the polymeric chain hanged on both sides of the phenylene ring responsible for all the physical and chemical properties of PANI.[17] These properties like conductivity, magnetic, optical, and other features of PANI change depending on its oxidation state and degree of protonation and doping. Preparing composites with PANI is a smart way to improve the conductivity of conducting polymers. Much work has been already done on the synthesis and characterization of CNT/PANI based nanocomposites showing applications in the environmental and biological fields.[17,18]

In the present study, a GT/MWCNT/PANI is prepared and incorporated in engine oil (bf) with the aim to reduce the viscosity of engine oil. Low viscosity oils take less power from engines, bring down the friction, cut the fuel consumption, and ultimately improve the engine’s efficiency. By examining the effects of the constant shear rate (100 s–1) and varying shear rates (0–500 s–1) on the viscosity of Castrol class 15W-40 engine oil, the viscosity is found to diminish by 99% (i.e., from 0.221000 to 0.001402 Pa·s) on incorporating the GT/MWCNT/PANI nanocomposite into the engine oil, which is a much higher reduction percentile as compared to the already reported nanocomposites as presented in Table A new correlation has been proposed considering the experimental and theoretical models with an average percentage error of 0.040%. Also, at varying shear rates, up to 90 s–1, the shear viscosity of NF is found to decrease significantly, leading to shear-thinning behavior of the NF, while at a shear rate of >90 s–1, it showed Newtonian behavior. Additionally, the ternary nanocomposite with 0.2 wt % GT/MWCNT/PANI also showed significant bactericidal effects with the zones of inhibition of 19, 18, and 15 mm against Gram-negative (P. aeruginosa, E. coli) and Gram-positive (S. aureus) bacteria, respectively, as measured using the well diffusion method.

Table 1

Summary of the Recently Published Literature on Rheological Properties of Oil-Based Nanofluids

nanofluid	temp. (°C)	shear rate (s–1)	rheological behavior	% viscosity change	base fluid	ref
MWCNT(COOH)MgO	25–50	670–8700	non-Newtonian	75% drop	SAE 50	(40)
TiO2/MWCNT	50	0–700	non-Newtonian	42%	5W50	(41)
MWCNT/TiO2	25 and 50		non-Newtonian		SAE 50	(42)
ZnO-MWCNTs	5–55	666.5–13330	Newtonian		SAE 10W40	43
MWCNT and ZnO nanoparticles	5–55	6000	non-Newtonian		5W50	43
MWCNT-MgO	25–50	0–1200	non-Newtonian		SAE 40	44

Results and Discussion

A GT/MWCNT/PANI ternary nanocomposite was successfully synthesized using the sol–gel technique. An ultrasonic processor (Figure S1, Supporting Information) was used for the proper dispersion of MWCNT, a challenging step.

Morphological Accountancy Using Scanning Electron Microscopy

Figure a–c illustrates the SEM micrographs of the GT extract at different resolutions (Table S2, Supporting Information). Figure a,b shows irregular morphology of GT NPs having a range of 3–20 μm. They have no distinct shapes, and somewhat entangled surface structures of GT fibers are visible in Figure a.[19,20]Figure c reveals the open stomata structure of GT NPs with diameters of 1–10 μm, similar to the case reported in the literature.[19]Figure d–f illustrates the surface structure of the GT/MWCNT/PANI nanocomposite where MWCNT surfaces are coated with GT NPs in the PANI matrix.[21,22] From the figure, it is clear that GT NPs were found to have adhered to the surface of the nanotubes. Since GT contains numerous polyphenols, noncovalent interaction between MWCNTs and polyphenols by π–π dispersive interactions is believed to cause separation of the nanotube bundles through sonication and adhesion of GT NPs on the tube surfaces of MWCNT.[22] Besides, aniline is polymerized and aggregated in between the wedges of MWCNTs as well as on the tube surfaces. The groovy tubular structures of MWNTs are dispersed in the PANI matrix. PANI macromolecules can also be absorbed in the surface of MWNTs, forming a sandwiched structure of the nanocomposite. It should be observed from Figure S3 (Supporting Information) that, after functionalization of MWCNT, smooth surfaces became rough and reactive though no structural damage or shortening length was reported. This may be due to the mild condition of an aqueous acid solution.[23] SEM images at different magnifications also confirm the amorphous morphology of the composite.[24]

Figure 1

(a–c) SEM micrographs of pure green tea (GT) at 10, 5, and 10 μm and (d–f) SEM micrographs of GT/MWCNT/PANI nanocomposite at 10 μm.

Fourier Transform Infrared Spectroscopic Studies

The spectral studies of the GT/MWCNT/PANI composite (heated at 140 °C for 20 min) shown in Figure indicate the appearance of the band at around 3221 cm–1 representing the −NH stretching mode of aromatic amines[24] and the band at 3054 cm–1 attributed to the carboxylic acid (−OH stretching) vibration band.[25] Peaks in the region of 2925–2853 cm–1 belong to asymmetric/symmetric (methylene stretching) bands, which explains that the surfaces of MWCNT are coated with GT NPs in the matrix of PANI.[24] One weak peak appearing around 1690 cm–1 is ascribed to the presence of the C=O stretching mode of the −COOH group. This particular peak appears after functionalization through acid reflux only.[23] Also, characteristic absorption bands at around 1568 and 1495 cm–1 are attributed to quinonoid and benzenoid rings of PANI, respectively.[24] Moreover, peaks around 1302 and 1240 cm–1 represent the −C=N stretching modes of secondary aromatic amines. The sharp peak at around 1145 cm–1 is the characteristic band of the protonated form of PANI, which is a measure of delocalized of π electrons.[21,22] The wavenumbers at around 879 and 1042 cm–1 correspond to −C–H alkene and −C–O alcohols representing the presence of catechin.[26,27] The weak peaks appearing at 617 and 691 cm–1 are −N–H wags due to the presence of primary and secondary amines.

Figure 2

FTIR graph of GT/MWCNT/PANI.

Thermal Behavior

The pyrolysis curves of GT and GT/MWCNT/PANI nanocomposite are shown in Figure . The GT underwent one-step thermal degradation, which starts at a temperature of >200 °C with a significant weight loss of 74%.[26,28]

Figure 3

TGA of green tea (GT) and GT/MWCNT/PANI composite.

On the contrary, a four-stage weight loss was observed from the TGA curve of the GT/MWCNT/PANI nanocomposite. The first minor weight loss occurred in the temperature range of 70–190 °C, which may be due to the vaporization of moisture from PANI.[28] The second and third weight losses of 12% each were observed at temperatures of 220 and 373–500 °C, respectively. These weight losses were found to be associated with the degradation of GT. This result indicates a higher number of reactive sites for oxidation in the GT/MWCNT/PANI nanocomposite as compared to the nonconjugated CNTs due to the presence of GT elements.[29] The fourth weight loss that occurred in the temperature range of 500–684 °C was due to the thermal degradation of the nanocomposite.[30] These observations indicated the higher stability of the GT/MWCNT/PANI nanocomposite over the parental counterparts, GT and PANI. This higher stability is maybe because of the absorption of free radicals (generated during the decomposition of GT and PANI) on the surfaces of MWCNT.

Study of Rheological Responses

The effects of the constant shear rate (100 s–1) and varying shear rates (0–500 s–1) on the viscosity of NFs are discussed in this section. The viscosity of Castrol class 15W-40 engine oil, which did not contain the nanocomposite as measured using a rheometer from Anton Paar Series, was 0.221 Pa·s. Also, the viscosity of the GT/MWCNT/PANI in bf engine oil was measured at constant and varying shear rates. Figure shows the variation of viscosity in the NF with time at a constant shear rate of 100 s–1. The values indicate that there is a decrease in viscosity up to 50 s, and then later, it becomes constant. An attempt was made to develop a theoretical model of viscosity variation with time as given in eq where A1 = 0.00008, A2 = 12.02141, η is viscosity (Pa·s), and T is time (s).

Figure 4

Viscosity variation of GT/MWCNT/PANI in 15W-40 engine oil (base fluid) with time at a constant shear rate of 100 s–1.

The comparative account of experimental and theoretical models is presented in Figure , representing the average percentage error calculated to be 0.04041%. Figure shows the effect of the shear rate on viscosity of the NF. We observe from the plot that, on the addition of GT/MWCNT/PANI into the engine oil, there is an abrupt decrease in viscosity with the increase in shear rate (up to 90 s–1). This decreasing trend of viscosity as a function of increasing shear rates is due to the shear-thinning behavior of the NF. From the figure, we can see that, at low shear rate values, that is, up to 90 s–1, the NF is showing shear-thinning behavior.[31,32] This significant behavior may be shown to be due to the interconnection of CNT aggregates within their suspensions. The liquid near these aggregates is found to be less mobile. As a result, the nanofluid becomes more viscous. As the shear rate increases, these aggregates break into smaller or primary structures. A certain amount of immobilized liquid will be released, causing a decrease in viscosity leading to shear-thinning behavior in NFs. At shear rates of >90 s–1, there is no change in viscosity of the NF; it may be due to the complete removal of solvated layers at a higher shear rate value of 90 s–1.[8] Since the variations of the viscosity with respect to the shear rate are negligible, it can be concluded that the studied NF becomes Newtonian fluid at shear rates of >90 s–1.[8,33]

Figure 5

Comparison between the experimental and theoretical values of GT/MWCNT/PANI viscosities at a constant shear rate.

Figure 6

Effect of shear rate on viscosity of GT/MWCNT/PANI nanocomposite dispersed in 15W-40 engine oil.

Bactericidal Efficacy

The bactericidal efficacy of the nanocomposite was evaluated against Gram-positive (S. aureus) and Gram-negative (P. aeruginosa, E. coli) bacteria by the agar well diffusion method. Briefly, the discs were placed on the agar plates, and various concentrations of GT/MWCNT/PANI samples suspended in 25% DMSO (used as a negative control) were pipetted onto filter discs. Figure shows typical results of antibacterial percent present in the GT and GT/MWCNT/PANI on S. aureus, E. coli, and P. aeruginosa (ESBL). The antibacterial activity of the synthesized composites was evaluated by measuring the zone of growth inhibition against the test microbes. ZOI details are mentioned in Table . The nanocomposite (0.25wt % GT/MWCNT/PANI) showed bacterial efficacies of 19, 18, and 15 mm with Gram-positive (S. aureus) and Gram-negative (P. aeruginosa and E. coli), respectively. Figure shows the 3D bar charts representing the antibacterial potential of samples against bacterial isolates belonging to genera Gram-positive (S. aureus) and Gram-negative (P. aeruginosa and E. coli) as measured by the well diffusion test.[34,35]

Figure 7

Well diffusion assay of (a) S. aureus, (b) E. coli, and (c) P. aeruginosa showing zones of inhibition in the presence of samples (01, 02, and 03) (GT, 0.1 wt % GT/MWCNT/PANI, and 0.2 wt % GT/MWCNT/PANI).

Table 2

Zone of Inhibition (mm) of Bacterial Isolates against Test Compounds

	zone of inhibition (mm)
bacterial strain	DMSO	GT	GT/MWCNT/PANI (0.1 wt %)	GT/MWCNT/PANI (0.2 wt %)
S. aureus	0	14	19	19
P. aeruginosa	0	15	15	18
E. coli	0	14	14	15

Figure 8

3D bar charts representing the antibacterial potential of GT and 0.1% and 0.2% GT/MWCNT/PANI against bacterial isolates belonging to genera Gram-positive (S. aureus) and Gram-negative (P. aeruginosa and E. coli) as measured by well diffusion test.

Conclusions

A ternary nanocomposite GT/MWCNT/PANI was synthesized using a sol–gel approach. The effect of changes on the rheological behavior of hybrid nanofluid (NF) GT/MWCNT/PANI-15W-40 was investigated. The effect of constant and varying shear rates was studied. Results revealed that the shear rate, as an independent parameter, plays a vital role in the prediction of viscosity. At a constant shear rate of 100 s–1, viscosity was reduced by 99% from 0.2210 to 0.001402 Pa·s. At varying shear rates, up to 90 s–1, the shear viscosity of NF decreases significantly, leading to shear-thinning behavior of the NF, while at a shear rate of >90 s–1, it shows Newtonian behavior. Besides, the ternary nanocomposite with 0.2 wt % GT/MWCNT/PANI also showed significant bactericidal effects with ZOIs of 19, 18, and 15 mm against Gram-negative (P. aeruginosa, E. coli) and Gram-positive (S. aureus) bacteria, respectively, as measured using the well diffusion method.

Experimental Section

Materials Used

Merck supplied the monomer, aniline, and KPS (potassium persulfate) used in the study. Carbon nanotubes (MWCNT) were purchased from Sigma-Aldrich. These MWCNTs had a length of 20 μm, an outer diameter of 10–20 nm, and an inner diameter of 2–6 nm, and an aspect ratio of 1000. Green tea (brand name Lipton) was purchased from a market in India. The experimental material for rheological activity was Castrol engine oil class 15W-40. All the solutions were made using double distilled water during the synthesis.

Modification of MWCNT

Functionalization was done with 3:1 solution of 1 M H2SO4 and 1 M HNO3, which was then ultrasonicated for 15 min, filtered using a centrifuge, and dried for 24 h at 60 °C giving a black powdered functionalized MWCNT.[36,37]

GT Sample Preparation and Composite Formation

GT leaves were crushed under a mortar, and 2 g of crushed powder was added in 100 mL of DMW. It was kept for boiling.[38,39] The filtrate of GT was then kept for further steps. Then 0.15 g of MWCNT was added to 100 mL of GT filtrate and kept for ultrasonication for 30 min. As a result of sonication, a GT/MWCNT composite with 0.15 g of MWCNT was synthesized. Similarly, 0.25 g of MWCNT was added to another 100 mL of GT filtrate and sonicated for 30 min yielding the MWCNT/GT composite with 0.25 g of MWCNT.[28]

Polymerization

1 mL of aniline was added dropwise to the GT/MWCNT suspension/binary composite, which was further sonicated for 20 min, and then 50 mL of potassium persulfate (KPS) solution prepared in 1 M aqueous HCl was added dropwise to the suspension, stirred at 10 to −5 °C in an ice bath (2 h), then filtered using a centrifuge at 4500 rpm for 15 min, and washed several times with double-distilled ethanol. The sample was dried for 24 h at 60 °C leading to the formation of the required ternary nanocomposite (i.e., GT/MWCNT/PANI).[24]

Characterization

SEM, FTIR, and TGA were used to analyze the surface morphology, functional groups, and thermal studies of the GT/MWCNT/PANI nanocomposites. Scanning electron microscopy (SEM) analysis was performed on LEO 435-VF to identify the surface morphology of the synthesized nanocomposite. The Fourier transform infrared (FTIR) spectrum was recorded on a PerkinElmer Spectrum 2 Version 10.4.00 with KBr pellets in the range of 400–4000 cm–1 to identify the functional groups present in the synthesized GT/MWCNT/PANI nanocomposite. A Shimadzu TGA Model 60 H recorded the thermal analysis of the synthesized nanocomposite.

Preparation of Nanofluid (NF) for Rheological Investigation

The above-synthesized GT/MWCNT/PANI nanocomposite powder was dispersed in engine oil followed by 30 min of sonication using an ultrasonicator (Figure S1, Supporting Information) yielding NF. Rheological investigation was done on the prepared samples to analyze the viscosity of the NF system with 0.1% volume fraction in engine oil, first at a constant shear rate and then at varying shear rates by using a modular compact rheometer from Anton Paar series (MCR 102 SN 81270415 FW3.70) equipped with conical shaped geometry having a diameter of 40 mm and an angle of 1°, at a fixed temperature of 25 °C as shown in Figure S4 (Supporting Information). A volume (0.15 mL) of the prepared solution was placed on the measuring plate, and then the viscosity of the sample was measured. The interaction between the base fluid (bf) and nanocomposite plays a vital role in determining the rheological behavior of NF.

Bactericidal Activity

The antibacterial activity of the synthesized GT/MWCNT/PANI nanocomposite was tested against P. aeruginosa, E. coli, and S. aureus by the disc diffusion method.[34] A single colony of the composite strain was inoculated in Luria Beltane’s (LP) broth under sterile conditions and incubated at 37 °C overnight. Turbidity produced against the negative control confirmed growth. A 0.1 mL (1 × 107 cells/mL) fraction from culture was uniformly spread on Luria agar plates. The zone of growth inhibition was determined on 4 mm diameter wells cut on other plates.