Effect of an Amphoteric Surfactant Concentration on Absorbance, Contact Angle, Surfactant, and Thermal Conductivity of CNT Nanofluids

Abstract

In this work, the effects of carbon nanotubes and an amphoteric surfactant, namely lauryl betaine, on the absorbance, contact angle, surface tension, and thermal conductivity of DW were experimentally investigated. The concentration of the carbon nanotubes was 0.5 wt% and that of lauryl betaine was 100, 500, and 1000 ppm in distilled water. From the absorbance measurement results, the addition of lauryl betaine could increase the absorbance in the wavelength range of UV and visible rays (200~1000 nm). In addition, the higher the surfactant concentration, the higher the dispersibility. The contact angle of the distilled water showed a monotonic decreasing trend with an increase in the surfactant blending ratio, while there were no significant changes in that of the carbon nanotube nanofluid. Analogous behaviors were observed in the surface tension measurements. The surface tension of the distilled water dramatically decreased with an increase in the surfactant blending ratio. The highest decrement was 46.05% at the surfactant concentration of 1000 ppm. In contrast, there were no significant changes in the case of the carbon nanotube nanofluid. Adding 0.5 wt% of the carbon nanotubes to distilled water could substantially enhance the thermal conductivity up to approximately 3%. The degradation effect of the amphoteric surfactant on the thermal conductivity of the fluids was observed in both distilled water and nanofluids.

1. Introduction

Carbon nanotubes (CNTs) have been widely used in various research fields, such as transportation [1,2], nuclear cooling systems [3], solar energy systems [4,5], oil recovery [6,7], and so on. The greatest advantage is that CNTs have excellent thermophysical properties in the aforementioned fields. For instance, the thermal conductivity, a representative thermophysical property, of single- and multi-walled CNTs is approximately ~6000 and ~3000 W/m·K [8,9], respectively. One of the most simple and useful ways to utilize the advantages of CNTs is to mix them with base fluids, resulting in CNT nanofluids [10,11,12].

CNT nanofluids have been used particularly as heat transfer media since a current trend in mechanical engineering is to minimize the geometrical size in components. In this development trend, for CNT nanofluids, the greatest benefit is that equivalent heat transfer intensification can be obtained by decreasing the pumping power. In addition, they have high thermal conductivity as compared to conventional fluids.

However, there is great concern about a performance degradation due to the sedimentation and agglomeration of the CNTs in the base fluid. Owing to the high aspect ratio and Van der Waals forces of the CNT, it deteriorates the dispersion characteristics of the CNT nanofluid [13,14]. As a result, this can decrease the thermophysical properties, such as the thermal conductivity, wettability, surface tension, and so on. Hence, it is required to keep the CNT nanofluid stable and ensure its dispersion characteristics over a long time period.

In order to find a solution to the aforementioned problems, various methods have been conducted in the mechanical and chemical research fields. The representative method is to add chemical additives for the functionalization of the CNT in the chemical field. According to Lee et al. [15], who prepared a CNT nanofluid with alkalization and oxidation treatment, the absorbance of the CNT nanofluids was higher than that of a raw CNT nanofluid when they were functionalized by alkaline treatment (K₂S₂O₈) and oxidation treatment (HCl, H₂SO₄, and HNO₃). This means that the higher the absorbance, the higher the dispersibility in nanofluids. Furthermore, the enhanced dispersion characteristic provided greater stability than in the raw CNT nanofluid.

Tserengombo et al. [16], who investigated the mechanical and alkaline synthesis methods in a CNT/alumina nanocomposite, reported that both synthesis methods could increase the absorbance of the CNT/alumina nanofluid. In particular, chemical treatment could provide the CNT/alumina nanofluid with better dispersibility than mechanical treatment, attributed to planetary ball milling. However, the dispersion stability result was completely different from that of the dispersibility. According to the result of measuring the zeta potential of the CNT/alumina nanofluid, the mechanically treated CNT nanofluid kept a positive charge with an increased value. On the contrary, the zeta potential charge of the chemically treated CNT nanofluid was reversed from positive to negative. In particular, the intensity of the chemically treated CNT/alumina nanofluid dramatically decreased due to the relationship between the functional groups on the surfaces of the CNT and alumina nanoparticles, which had negative charges and positive charges, respectively.

In summary, it is necessary to introduce chemical treatment to improve the dispersion characteristics of nanofluids, although this can increase the absorbance. Moreover, chemical treatment entails complicated processes and requires considerable time to complete the chemical reactions. For these reasons, there has been a demand for methods to easily improve the dispersibility of nanofluids.

One of the simplest methods to increase the dispersibility of nanofluids is to add surfactants, known as surface-active agents. The addition of surfactants plays a role in stabilizing the nanofluids, as well as changing their wettability to hydrophilic characteristics in general. Surfactants can be classified into four types based on the polarity of the hydrophilic head: non-ionic, anionic, cationic, and amphoteric. Xia et al. [17] reported that the thermal conductivity of Al₂O₃/deionized water nanofluids was influenced by the type and concentration of surfactants. A suspension with a non-ionic surfactant (polyvinylpyrrolidone) showed better dispersion performance than one with an anionic surfactant (sodium dodecyl sulphate, SDS). Harikrishnan et al. [18], who measured the surface tension of aqueous surfactant solutions with an increase in the concentration of surfactants, reported that SDS showed higher surface tension than dodecyl trimethylammonium bromide, which is a cationic surfactant. Therefore, care should be taken to select and use the appropriate type of surfactant, because it can affect the properties of the nanofluid [19,20,21,22,23,24,25,26]. Many studies have been conducted on the effect of the addition of surfactants to nanofluids. To the best of our knowledge, a comprehensive comparative study related to the effect of an amphoteric surfactant on the thermophysical properties of CNT nanofluids remains to be performed. In this work, the effect of an amphoteric surfactant on the dispersion and thermophysical properties of CNT nanofluids was investigated.

2. Materials and Measurements

2.1. Preparation of Nanofluids

Distilled water (DW), maintaining the maximum distillate conductivity of 3.7 µs/cm at 22 °C, was used as a base fluid in this work. MWCNTs (Carbon Nano-Material Technology Co., Ltd., Pohang, Republic of Korea), which were produced with a diameter of ~20 nm and length of ~5 μm, were added to the base fluid. This is called the base nanofluid (DW + CNT) in this work. According to the literature [27], the peak heat flux increase, as well as the cooling rate, when the CNT concentration increases up to 0.5 wt% during the quenching of CNT nanofluids. However, it has been confirmed that both the peak heat flux and cooling rate are degraded when the CNT concentration exceeds 0.5 wt%. Additionally, the contact angle and viscosity decrease more than in the base fluid. In reference to these findings, the suitable concentration of the CNT nanofluid was determined as 0.5 wt%.

To investigate the effect of an amphoteric surfactant, lauryl betaine (LB, C₁₆H₃₃NO₂, Avention Co., Ltd., Siheung, Republic of Korea) with a molecular weight of 271.44, the surfactant was arranged and mixed with the base fluid and base nanofluid by stirring with a glass rod. After the mixing process, the LB concentration of the mixtures was adjusted to 100, 500, and 1000 ppm. Then, the mixtures were ultrasonicated for 3 h using an ultrasonicator (SK5200GT, LABOTEC Co., Ltd., Seoul, Republic of Korea) at 33 kHz and 180 W. It is well-known that ultrasonication can break nanoparticles. Thus, the nanoclusters become more uniform. Said et al. [28], who studied the effect of the ultrasonication duration, reported that ultrasonication can provide CNT nanofluids with better dispersibility and increased thermal conductivity. Consequently, the thermal conductivity of 0.3 vol.% of CNT nanofluids was improved in the case of an ultrasonication duration of 200 min. The prepared solutions are shown in Figure 1.

2.2. Analysis of Dispersibility, Contact Angle, Surface Tension, and Thermal Conductivity

The dispersion, contact angle, surface tension, and thermal conductivity characteristics of the DW and 0.5 wt% of the CNT nanofluids with or without various ratios of the LB were investigated. One of the most intuitive parameters to assess the dispersion characteristic is the absorbance in the wavelength of ultraviolet to visible rays. This method measures the absorption ratio of a certain amount of solution employing the Beer–Lambert law and makes it possible to compare the dispersion characteristics of the fluids in terms of the absorption intensity, especially for the same type of fluid. The traditional form of the Beer–Lambert law can be expressed by Equation (1).

$$\mathrm{A}\equiv \ln \left\{\frac{I\left(O\right)}{I\left(X\right)}\right\}=\epsilon cX$$

(1)

Here, A is the absorbance, and $I\left(O\right)$ and $I\left(X\right)$ are the intensity of monochromatic light entering the solution perpendicularly to one face and light exiting the solution through the opposite face, respectively. $\epsilon$ is the molar absorption coefficient, c is the molar concentration, and X is the optical path length.

When the nanoparticles dispersed in DW are not agglomerated, the suspension absorbs more rays than the poorly dispersed suspension. Due to this, many researchers have introduced the absorbance to assess and compare nanofluids’ dispersion characteristics [29,30,31]. In this work, the absorbance was measured in 9 different wavelengths (200, 300, 400, 500, 600, 700, 800, 900, and 1000 nm) by a UV/Vis spectrophotometer (X-ma-3100, Human Corporation Co., Ltd., Seoul, Republic of Korea, optical quartz cell size: 10 mm × 10 mm × 45 mm) at a room temperature of 21 ± 1 °C. This instrument provides some information about the absorbance throughout wavelengths from 190 to 1100 nm. Considering the instability of the CNT nanofluids during each measurement, the total volume of the subjected sample was 3.1 mL for each measurement; namely, 0.1 mL of the subjected sample was diluted with 3.0 mL of DW. In addition, the measurements were conducted after zero calibration based on DW.

The contact angle and surface tension of the DW and CNT nanofluid with or without the LB were measured using a software-controlled optical tensiometer (FEMTOBIOMED Inc., Seongnam-si, Republic of Korea), which had an automatic droplet dispensing system and high-speed focusing optics. As can be seen in Figure 2, the contact angle measurement of both fluids was based on a sessile drop or the static contact angle. In this measurement, a static liquid droplet is placed on the substrate, resulting in the formation of an equilibrium contact angle (${\theta }_E$) among the solid, liquid, and gas phases. This measurement method, the sessile drop method, is based on Equation (2).

$${\gamma }_{SV}-{\gamma }_{SL}={\gamma }_{LV}\cos {\theta }_E$$

(2)

where ${\gamma }_{SV}$, ${\gamma }_{SL}$, and ${\gamma }_{LV}$ are the surface tension of the liquid, the interfacial tension between the solid and liquid, and the surface tension of the solid, respectively.

Hence, several assumptions and ideal features are required for the surface of the substrate: the surface is chemically homogeneous and non-reactive with the liquid, and the surface is smooth and clean.

The measurement was carried out 3 times for each type of fluid, to reduce errors caused by the surface conditions, microdistinction of the droplet volume, etc., at a room temperature of 21 ± 1 °C. The standard deviation of the DW and DW with LB at 100, 500, and 1000 ppm was 2.02°, 0.37°, 0.21°, and 0.51°, respectively, while those of the 0.5 wt% of CNT nanofluid were 4.45°, 3.82°, 1.33°, and 3.15°, respectively.

Surface tension measurement was carried out with the pendant drop technique. This is widely used to measure the surface tension of fluids and obeys Equation (3). The traditional form of the equation explains the pressure difference in the interface between the liquid and gas by introducing the principal radii of curvature R₁ and R₂ (Figure 3). The pressure difference, namely the Laplace pressure, can be defined as follows:

$$∆\mathrm{P}=\mathsf{\gamma }\left(\frac{1}{R_1}+\frac{1}{R_2}\right)$$

(3)

The pressure difference ($∆\mathrm{P}$) can be rewritten as Equation (3) when gravity is the only additional force.

$$∆\mathrm{P}\approx ∆{\mathrm{P}}_0-\left(∆\rho \right)\mathrm{g}z$$

(4)

where $∆\rho$ is the difference in density in two bulk fluids (${\rho }_L-{\rho }_G$) in Figure 3.

The measurement was carried out 5 times, to reduce or eliminate errors caused by the vaporization of the droplet, vibration from the surroundings, etc. The volume of the droplet was automatically made up to a pre-set value by a software-controlled piston slider equipped with a 21-gauge flat-tip cannula. Surface tension measurements were carried out for each sample at a room temperature of 21 ± 1 °C. The standard deviation of the DW and DW with LB at 100, 500, and 1000 ppm was 0.71 mN/m, 1.69 mN/m, 1.26 mN/m, and 0.52 mN/m, respectively, while those of the 0.5 wt% CNT nanofluid were 0.72 mN/m, 0.38 mN/m, 0.60 mN/m, and 0.76 mN/m, respectively.

In order to measure the thermal conductivity, a transient hot wire thermal conductivity measurement system (LAMBDA system, F5 Technologie, Wunstorf, Germany) was used. This measurement system consists of three main parts: a heating and cooling bath equipped with a thermostat, a software-controlled microprocessor console, and a thermally insulated double jacket with a measurement sample container. A platinum wire (with a diameter of 0.10 mm and greater than 99.99% purity) that served as a detector of temperature changes was used as a hot wire. The total volume of the cylindrical fluid container was 40 mL and it interlocked with the measuring head. The hot wire was fixed in the vertical direction and located in the middle of the space. Interestingly, a time-dependent and homogenous temperature field can be generated as the platinum hot wire is gradually heated in the sample container. The specific linear principle of the correlation between the resistance and the temperature is based on Equation (5).

$$k=\frac{q}{4\pi \left(T_2-T_1\right)}\ln \frac{t_2}{t_1}$$

(5)

where k is the thermal conductivity of the fluid, q is the heat generation per unit length, and T₁ and T₂ are the temperatures at time t₁ and t₂, respectively. The measurement was carried out 3 times to reduce measurement errors at 20 ± 0.1 °C. The standard deviation of the DW and DW with LB at 100, 500, 1000 ppm was 1.19 mW/m·K, 0.73 mW/m·K, 0.67 mW/m·K, and 3.78, respectively, while those of the 0.5 wt% CNT nanofluid were 1.81 mW/m·K, 2.33 mW/m·K, 3.63 mW/m·K, and 2.33 mW/m·K, respectively.

3. Results and Discussion

3.1. Effect of LB on Dispersion Characteristics of CNT Nanofluids

The effect of an amphoteric surfactant on the dispersion characteristics of nanofluids was assessed at the wavelength range of UV to visible rays in terms of absorbance. The result of absorbance in the 0.5 wt% CNT nanofluid with or without LB is shown in Figure 4. As shown in Figure 4, the absorbance of the 0.5 wt% CNT nanofluid without LB had the lowest values, regardless of the wavelength. Furthermore, interestingly, it was observed that the absorbance of the 0.5 wt% CNT nanofluid increased with an increase in the concentration of the LB. In particular, the largest absorbance difference was shown between the 0.5 wt% CNT and that with LB at 1000 ppm at a wavelength of 800 nm. Specifically, the absorbance increased from 0.27 to 0.61, corresponding to a 225.9% increment. Conversely, the difference was the smallest at the wavelength of 300 nm; the absorbance increased from 0.23 to 0.26, corresponding to a 13.0% increment. As a result, it can be interpreted that the addition of LB into the 0.5 wt% CNT nanofluid provided the fluid with better dispersion characteristics throughout the wavelength from UV to visible rays, and this effect depended on the concentration of the LB up to 1000 ppm. This enhancement throughout the wavelength range can be effective in absorbing solar energy. In other words, CNT nanofluids with LB can absorb more solar energy than those without LB in direct absorption solar collectors, PV panels, or hybrid solar collectors.

Similar enhancement effects can be found in the literature. Kim et al. [32], who measured the absorbance of a carbon-based nanomaterial with LB and various blending ratios, found that the addition of LB into graphene aqueous nanofluids and MWCNT nanofluids increased the absorbance of the mixtures. In the case of a 0.1 wt% graphene nanofluid, the absorbance increased with an increase in the concentration of LB up to a 1:3 ratio (graphene:LB). Similarly, in the case of a 0.1 wt% MWCNT nanofluid, the absorbance increased with an increase in the concentration of the LB.

Interestingly, there is a contradictory study that is related to the degradation effect of LB on the dispersion characteristics of nanofluids. According to the experimental results [33], the absorbance of 0.5 wt% of an Al₂O₃/water nanofluid decreased throughout the wavelength range of 700 to 1000 nm. However, it is worth noting that the absorbance of the 0.5 wt% Al₂O₃/water nanofluid increased with an increase in the concentration of LB, although that without LB had the highest value. In contrast, it was observed that the absorbance of the 0.5 wt% Al₂O₃/water nanofluid increased in the case of using anionic surfactants such as SDS and SDBS.

Regarding the aforementioned results, the contradictory effect of LB on the dispersion characteristics of nanofluids is related to the electrostatic attraction and repulsion between the surfactant and nanomaterial [33].

3.2. Effect of LB on Contact Angle of DW and CNT Nanofluids

Figure 5 shows the results of the contact angle measurement of the DW and CNT nanofluids with or without three different concentrations of LB on a clean glass surface. In the case of DW, the observed behavior was monotonic, showing a continued decrease in contact angle upon adding the amphoteric surfactant. In other words, it can be clearly seen that the contact angle of the DW was reduced when adding LB, and it decreased, moreover, with an increase in the concentration of LB. As shown in

Table 1. Contact angle and standard deviation of DW and 0.5 wt% of CNT nanofluid with or without LB on a clean glass surface.

Fluid	DW				CNT Nanofluid
ppm	0	100	500	1000	0	100	500	1000
Contact angle	99.13	93.54	82.53	75.20	93.97	94.60	92.33	93.67
Standard deviation	2.02	0.37	0.21	0.51	4.45	3.82	1.33	3.15

, the contact angle was reduced from 99.13° to 93.54°, 82.53°, and 75.20°, with LB in order of concentration.

On the contrary, in the case of CNT nanofluids, there was no significant change in the contact angle as the amphoteric surfactant was added. As can be seen in Table 1, the contact angle was 93.97°, 94.60°, 92.33°, and 93.67°, with LB in order of concentration. This means that the addition of LB to the 0.5 wt% CNT nanofluid up to 1000 ppm did not affect the contact angle. This is in contrast to the results of Karthikeyan et al. [34], who also investigated the contact angle of a CNT nanofluid. They measured the contact angle of 50 ppm of a CNT nanofluid with or without 50 and 500 ppm of an anionic surfactant, namely SDS. Based on their results, the contact angle of the 50 ppm CNT nanofluid decreased from 30.85° to 11.18° when adding 50 and 500 ppm of SDS, and this corresponded to a 63.76% decrement. The greatest difference between the literature studies and our experiment was the type of surfactant.

This phenomenon can be found in different nanomaterials. According to Hunter et al. [35], the contact angle of silica nanofluids was decreased with an increase in the concentration of non-ionic surfactant, namely TX-100.

This implies that the type of surfactant and nanomaterial leads to different droplet behavior in terms of the contact angle.

3.3. Effect of LB on Surface Tension of DW and CNT Nanofluids

As shown in Figure 6, the surface tension of the DW and 0.5 wt% of CNT nanofluid with or without the surfactant was measured. Based on the results shown in Figure 6, the observed behavior was monotonic, showing a continued decrease in surface tension upon adding the amphoteric surfactant in the case of DW. Adding the LB decreased the surface tension of the DW. In particular, the surface tension was dramatically decreased when the concentration of the LB was over 100 ppm. After this, it decreased gradually. More specifically, the surface tension of the DW was degraded from 72.20 mN/m to 67.97 mN/m, 42.34 mN/m, and 38.95 mN/m, with LB, corresponding to 5.86%, 41.36%, and 46.05%, respectively. This phenomenon can be explained by the concept of the critical micelle concentration (CMC) [33]. The CMC is the specific concentration of the surfactant in the base fluid at which the formation of micelles begins. The surface tension of fluids decreases continuously with the addition of surfactants. When the concentration of the surfactant exceeds the CMC, there is little change in the surface tension of the fluid.

In contrast to the case of DW, adding LB to the 0.5 wt% CNT nanofluid did not change significantly the surface tension. As can be seen in

Table 2. Surface tension and standard deviation of DW and 0.5 wt% of CNT nanofluid with or without LB.

Fluid	DW				CNT Nanofluid
ppm	0	100	500	1000	0	100	500	1000
Surface tension	72.20	67.97	42.34	38.95	71.82	70.69	69.75	70.21
Standard deviation	0.71	1.69	1.26	0.52	0.72	0.38	0.60	0.76

, the surface tension of the 0.5 wt% CNT nanofluid was 71.82 mN/m, 70.69 mN/m, 69.75 mN/m, and 70.21 mN/m in order of surfactant concentration. This characteristic is consistent with the results of the contact angle in Section 3.2. Furthermore, in the case of adding 0.5 wt% of CNT into the DW, it was observed that this also did not change the surface tension of the DW. An analogous result was reported by Karthikeyan et al. [34]. They added functionalized MWCNTs into water at concentrations of 10~120 ppm. Based on their results, it was observed that the addition of functionalized MWCNTs was not able to increase or decrease the surface tension of the base fluid.

On the other hand, there are converse results regarding to the use of nanomaterials. Baek et al. [33], who measured the surface tension of Al₂O₃ nanofluids, reported that the addition of 0.5 wt% of Al₂O₃ nanopowder to DW resulted in a decrease in the surface tension of the DW of up to 6.71%.

3.4. Effect of LB on Thermal Conductivity of DW and CNT Nanofluids

The thermal conductivity of the DW and 0.5 wt% of CNT nanofluid with or without surfactants was measured. The effect of adding the CNT and the amphoteric surfactant into the DW is shown in Figure 7. Before the measurements, the measurement instrument was calibrated using DW at 20 °C. From the calibration data, a 0.02% error was obtained; this result was in great agreement with that in [36].

As can be seen in Figure 7, the thermal conductivity of DW decreased with an increase in the concentration of LB. As shown in

Table 3. Thermal conductivity and standard deviation of DW and 0.5 wt% of CNT nanofluid with or without LB.

Fluid	DW				CNT Nanofluid
ppm	0	100	500	1000	0	100	500	1000
Thermal conductivity	597.96	597.19	595.44	593.92	617.34	614.58	611.18	604.71
Standard deviation	1.19	0.73	0.67	3.78	1.81	2.33	3.63	2.33

, it decreased from 597.96 mW/m·K to 597.19 mW/m·K, 595.44 mW/m·K, and 593.92 mW/m·K in order of the surfactant concentration. This corresponds to a 0.13%, 0.42%, and 0.68% decrement. However, in the case of adding 0.5 wt% of CNTs into the DW, the thermal conductivity of the DW increased from 597.96 mW/m·K to 617.34 mW/m·K, corresponding to a 3.24% increment. This result shows reasonable agreement with the literature [32]. Based on their result, adding 0.1 wt% of CNTs to DW was able to increase the thermal conductivity of the DW by approximately 0.25% at 25 °C. This is based on the two main mechanisms of conducting heat. The stronger the lattice vibrations, the more heat is transported through the medium, and the flow of free electrons raises the electrical conductivity of the solids. As a result, the increased thermal conductivity of the fluid can improve the thermal performance of thermal systems, such as heat recovery or heat management systems.

In the case of the CNT nanofluid, it was clearly observed that the thermal conductivity of the 0.5 wt% CNT nanofluid showed a monotonic decreasing trend as the concentration of the amphoteric surfactant increased. As shown in Table 3, the thermal conductivity of the 0.5 wt% CNT nanofluid was 517.34 mW/m·K, 614.58, 611.18, and 604.71 in order of the surfactant concentration. This corresponds to a 0.45%, 1.00%, and 2.05% decrement. Overall, the decrement characteristic of the 0.5 wt% CNT was observed to be greater than that of DW in terms of adding the surfactant.

4. Conclusions

The effect of the amphoteric surfactant on the dispersion, contact angle, surface tension, and thermal conductivity characteristics of the DW and CNT nanofluids was experimentally confirmed. The major findings of this work are summarized below.

Adding an amphoteric surfactant, namely LB, can increase the absorbance of 0.5 wt% CNT nanofluids throughout the wavelength range of UV and visible rays. A 225.9% increment in absorbance was observed between the 0.5 wt% CNT nanofluid and that with LB at 1000 ppm at the wavelength of 800 nm. Similarly, in the case of 0.1 wt% of MWCNT nanofluid, the absorbance increased with an increase in the concentration of the LB.

In the case of contact angle measurement, a monotonic decrease was observed for the DW with the increase in the concentration of the LB, while there was no significant change in the 0.5 wt% CNT nanofluid with an increase in the concentration of LB.

A similar trend as in the results of the contact angle measurement was observed in the case of the surface tension measurement. The surface tension of the DW sharply decreased by up to 46.05% when adding LB at 1000 ppm. However, that of the 0.5 wt% CNT nanofluid was not changed meaningfully.

The effect of the degradation in the thermal conductivity of the DW and 0.5 wt% CNT nanofluid was observed as the concentration of LB increased. The maximum decrement for the DW and 0.5 wt% CNT nanofluid was 0.68% and 2.05%, respectively. Furthermore, adding 0.5 wt% of CNTs could substantially enhance the thermal conductivity of the DW by 3.24%.