domain: sdv spi

text:
Agnès Drochon
email: agnes.drochon@u-bordeaux.fr

Olivier Fouquet
email: olfouquet@chu-angers.fr

Christophe Baufreton
email: chbaufreton@chu-angers.fr

Extracorporeal circulation during on-pump cardiac surgery: an evaluation of the Energy Equivalent Pressure (EEP) index based on waveforms decomposition in harmonics

Keywords: cardiac surgery, extra-corporeal circulation, Energy Equivalent Pressure, pulsatile flow

The use of pulsatile perfusion (PP) instead of Non-Pulsatile Perfusion (NP) during cardiopulmonary bypass (CPB) continues to be a source of debate. The disagreements among the conclusions of the published studies may be due to different factors: differences in the type of patients included in the studies, differences in the protocol of the studies and difficulty to quantify the pulsatility of the flow. In the present paper, we propose a quantitative evaluation of Shepard's EEP index, based on the harmonic decomposition of the physiological aortic pressure and flow rate signal. It is thus demonstrated that the surplus energy provided by pulsatile flow remains moderate (of order 10 mmHg), but that it can be improved by changing the relative shapes of the pressure and flow waves.

increase the risk of hemolysis and the risk of turbulence, especially in the near field of the canula. The maximum blood velocities and shear stresses in the vessels are also enhanced in the case of pulsatile perfusion [START_REF] Assmann | Pulsatile extracorporeal circulation during on-pump cardiac surgery enhances aortic wall shear stress[END_REF][START_REF] Benim | Simulation of blood flow in human aorta with emphasis on outlet boundary conditions[END_REF]. In addition, these pumps were expensive and difficult to sterilize, and they did not reproduce the exact physiological flow pattern. Consequently, as it is mentioned in the recent "2019 EACTS/EACTA/EBCP guidelines on cardiopulmonary bypass in adult cardiac surgery" [START_REF] Wahba | EACTS/EACTA/EBCP guidelines on cardiopulmonary bypass in adult cardiac surgery Eur[END_REF], all over the world, the majority of CPB procedures are run with a continuous flow (with roller or centrifugal pumps).

Moreover, some groups conclude that pulse is unnecessary, and that efforts to optimise pulsatility are not justified [START_REF] Voss | Cardiopulmonary bypass with physiological flow and pressure curves: pulse is unnecessary![END_REF]. Elbers et al. [START_REF] Elbers | Direct observation of the human microcirculation during cardiopulmonary bypass: effects of pulsatile perfusion[END_REF] used sidestream dark-field imaging to record video clips of the sublingual human microcirculation. Although the pulse pressure was higher during pulsatile perfusion, they found that the perfused vessel density did not differ between nonpulsatile and pulsatile perfusion. They concluded that there is no obvious relationship between pulse pressure and microcirculatory parameters.

The disagreements among the conclusions of all these studies may be due to different factors: differences in the type of patients included in the studies, differences in the protocol of the studies (for example, the duration of perfusion or the delay of observation ( 12)), influence of the components of the extra-corporeal circuit (other than the pump: oxygenator, cannulae and their position [START_REF] Ahn | Feasibility of a 3D printed patient-specific model system to determine hemodynamic energy delivery during extra-corporeal circulation[END_REF], tubing [START_REF] Durr | Hemodynamic energy during pulsatile extracorporeal circulation using flexible and rigid arterial tubing: a reassessment[END_REF], …), and difficulty to quantify the pulsatility of the flow. Some classical indexes used to describe the pressure or flow pulse are recalled in the paper of Voss et al. [START_REF] Voss | Cardiopulmonary bypass with physiological flow and pressure curves: pulse is unnecessary![END_REF].

Among these quantities, they quote the EEP (Energy Equivalent Pressure), proposed by Shepard et al. [START_REF] Shepard | Energy Equivalent Pressure[END_REF] in order to quantify and compare the energy of pulsatile and non-pulsatile blood flow. EEP can be obtained from pressure p(t) and flow q(t) waveforms and is defined as follows:

(Eq.1)

It represents the work done in one cycle of the pulsatile flow divided by the volume of fluid moved during that cycle. In pulsatile flow, EEP is supposed to be larger than the mean pressure but if there is no pulsatility, it will be equal to the mean pressure. The intuition of Shepard was that a pulsatile arterial wave dissipates significantly more utilizable energy to the tissues than does a non-pulsatile wave of the same mean pressure.

The hemodynamic parameter EEP has also been used recently to quantify the pulsatility of flow in some decellularization experiments [START_REF] Park | Effect of pulsatile flow perfusion on decellularization[END_REF].

In 1985, Wright and Furness [START_REF] Wright | What is pulsatile flow?[END_REF] suggested that it was necessary to define more precisely what is pulsatile flow. They explained that some types of pulsatile flows are more effective than others in preserving tissue function. In order to describe more precisely the morphology of the flow or pressure waveforms, they recommended to use the harmonic wave decomposition. This mathematical theory was established by Fourier and shows that a signal of any shape, s(t), can be transformed to a mean level plus a series of sine waves of differing amplitude, frequency and phase (the harmonics). The harmonic frequencies are multiples of the fundamental wave frequency. For a heart rate of 60 beats/min, the fundamental frequency is 1 Hz, the second harmonic is 2 Hz, and so forth. At zero frequency, impedance is equivalent to resistance in the steady-flow state.

The phase differences between pressure and flow are caused by compliance and inertance in the vascular system, and these compliance and inertance allow energy storage and transmission.

In order to go further, in the present paper, we propose a quantitative evaluation of Shepard's EEP index, based on the harmonic decomposition of q(t) and p(t).

MATERIALS and METHODS

Simultaneous data of pressure and flow from the ascending aorta in three patients undergoing cardiac surgery are available in Patel et al. [START_REF] Patel | Pressure-flow relationships in the ascending aorta and femoral artery of man[END_REF]. The Fourier series calculated from the aortic pressure and flow curves are recalled here, in Table 1. The three Patients are called: "CH", "DC", and "LF". The series are limited to the ten first terms. This is actually enough, because the important signal information is contained in the first harmonics.

Let us write the flow rate q(t) as: (Eq.2) and the pressure p(t) as:

(Eq.3)
where q 0 and p 0 are the constant term (harmonic "zero", with frequency zero (no time dependency)) for the flow and pressure, q k and p i are the amplitude of the harmonic "k" or "i",  k and  i are the phase angles, and f denotes the cardiac frequency. Note that q 0 and p 0 are also the mean values of the flow and pressure signals over each cardiac cycle.

Multiplying term by term q(t) by p(t), and integrating this quantity, EEP turns out to be:

(Eq.4)

The data of Patel et al. [START_REF] Patel | Pressure-flow relationships in the ascending aorta and femoral artery of man[END_REF] that we have used for our calculation are re-called in Table 1.

RESULTS

The flow rates obtained using Eq. ( 2) and the numerical data given in Table 1 are shown in Fig. 1, for each Patient and for 3 cardiac cycles. The same type of curves are shown in Fig. 2 for the pressures (obtained using Eq.( 3)). They are quite similar to the instantaneous pressure and flow contours recorded from the ascending aorta and reproduced in the paper of Patel et al. [START_REF] Patel | Pressure-flow relationships in the ascending aorta and femoral artery of man[END_REF]. These data can thus be used through Eq. ( 4) for some reliable calculations of the EEP index.

The values obtained for EEP, along with the mean values of pressure over a cardiac cycle, and the relative percentage of increase are gathered in Table 2. These results demonstrate that the energy surplus due to the pulsatility remains lower than 17% in any case.

DISCUSSION and CONCLUSION

Our EEP calculations show that the surplus energy provided by pulsatile flow remains moderate (of order 10 mmHg). This result is in agreement with the study of Durr et al. [START_REF] Durr | Hemodynamic energy during pulsatile extracorporeal circulation using flexible and rigid arterial tubing: a reassessment[END_REF] , who compared different tubings (more or less compliant) and different aortic models with physiological and sub-physiological compliance. They found that the surplus of energy is better when the tubings or vessels are less compliant, but in any case, it remains lower than 14% of mean arterial pressure. An order of magnitude of 10% is also given by Wright [START_REF] Wright | Hemodynamic analysis could resolve the pulsatile blood flow controversy[END_REF]. The question is then: could this amount be sufficient to propel pulsatility beyond the arterioles?

The increase in EEP values found in the case of pulsatile flow has been related by several groups to some improvement in the microcirculation (better flow of lymph, interstitial fluid, better flow of blood in the capillaries, less microvessels shunting or collapse, …). A reduction in peripheral vascular resistance may thus be associated with pulsatile perfusion. One of the future direction of our work will be to test the sensitivity of the EEP index to the value of the peripheral resistance through a classical circulatory model. Indeed, the "2019 Guidelines on CBP" (9) recommend to use arterial vasodilators if the patient's mean arterial pressure is higher than 80 mmHg (or vasoconstrictors if mean pressure < 50 mmHg). Vasodilators will decrease the peripheral resistance and, on the contrary, vasoconstrictors will increase it. This confirms the importance of this parameter, and of the mechanisms of mecanotransduction: conversion of the mechanical forces (shear forces exerted on the walls of the vessels, and radial deformations due to the propagation of the pressure and flow pulses) into biological signals [START_REF] Pinaud | In vitro protection of vascular function from oxidative stress and inflammation by pulsatility in resistance arteries. The Jour[END_REF]. Theoretical studies of sinusoidal blood flow in deformable vessels prove that the radial deformation of the vessel is directly related to the flow rate [START_REF] Drochon | Sinusoïdal flow of blood in a cylindrical deformable vessel exposed to an external magnetic field[END_REF][START_REF] Womersley | Oscillatory motion of a viscous liquid in a thin-walled elastic tube: I. The linear approximation for long waves[END_REF].

In addition, it is important to mention that the pulsatile power outputs of some "pulsatile pumps" are small compared with those of the human heart. It should be interesting to capture the flow and pressure curves just at the exit of the pump and calculate EEP in order to compare it with EEP obtained with physiological data. Equation 4 demonstrates that the EEP value depends on : i) the moduli of pressure harmonics (p i ), that remain rather small, ii) the ratios (q k /q 0 ) of the moduli of the flow rate harmonics to the mean flow rate , iii) the phase lag between pressure wave and flow wave (  j - j ). Consequently, a possible way to increase the value of EEP at the exit of an artificial flow device could be to adjust conveniently the ratios (q k /q 0 ), and /or the phase lag ( j - j ).

In the particular context of extra-corporeal circulation, one has to consider also the damping effect of several components (oxygenator, aortic cannula, arterial filters, …) and the difficulty to quantify the amount of pulsatile energy lost in these components [START_REF] Voss | Cardiopulmonary bypass with physiological flow and pressure curves: pulse is unnecessary![END_REF]. A study from Lim et al. [START_REF] Lim | Optimizing the circuit of a pulsatile extracorporeal life support system in terms of energy equivalent pressure and surplus hemodynamic energy[END_REF] indicates that up to 80% of the total hemodynamic energy generated by a pulsatile pump is absorbed by the components of the extra-corporeal circuit and only a small proportion of the pulsatile energy is delivered to the patient. The influence of the aortic compliance and the possible mismatch between cannula and aorta would have to be more carefully studied. This point was already mentioned in the Editorial of Wright and Furness [START_REF] Wright | What is pulsatile flow?[END_REF]. They explained that the arterial line and the cannula can considerably modify the pulse. As demonstrated in several publications from the group of A. Undar [START_REF] Undar | Defining pulsatile perfusion: quantification in terms of energy equivalent pressure[END_REF], reducing the energy lost in the circuit requires to reduce all the pressure drops (linear or singular). Patel et (18) used for our calculation.

TABLES TABLE 1. Data from

The flow rates q k are in ml/s, the pressures p i in mmHg , and the phase angles  k and  i in degrees.

Patient "CH"

Patient "DC" Patient "LF" f = 1.25 s -1 f = 1.47 s -1 f = 1.0 s -1 q 0 = 67 p 0 = 94 q 0 = 84 p 0 = 96 q 0 = 65 p 0 = 74 q 1 = 120 Aortic pressure (mmHg)

 1 = 9 p 1 = 14  1 = 311 q 1 = 137  1 = 22 p 1 = 14.4  1 = 319 q 1 = 135  1 = 6 p 1 = 15.2  1 = 305 q 2 = 88

 2 3  10 = 325 FIG. 2 -

 23103252 FIG.2-Pressure curves obtained using Equ.(3) and the numerical data given in Table1, for each Patient and for 3 cycles.

CONFLICT OF INTEREST

The authors declare that there is no conflict of interest, financial or otherwise.


Jolanta Sobczak

Javier P Vallejo

Julian Traciak

Samah Hamze

Jacek Fal

Patrice Estellé

Luis Lugo

G Żyla

Gaweł Żyła

Thermophysical profile of ethylene glycol based nanofluids containing two types of carbon black nanoparticles with different specific surface areas

Keywords: nanofluid, carbon black, viscosity, thermal conductivity, heat capacity, surface tension

Thermophysical profile of ethylene glycol based nanofluids containing two types of carbon black nanoparticles with different specific surface areas

Nomenclature

One of the most important factors determining the development of society, science and technology is the efficiency of energy transfer and storage. During last decades it is observed that the energy demand has increased while the impact of the energy sources based on fossil fuels on the natural environment is adverse. Because of that, the needs to exchange "traditional" for renewable energy systems has been emphasized. Among many solutions there is solar energy and well-known collectors. However there is a problem with heat losses and poor heat transfer so this solution required improvements. At a certain point, the idea of replacing the previously used heat transfer fluids with nanofluids was born in order to increase the efficiency of these systems.

Nanofluid is a liquid-solid mixture in which there are dispersed one or various types nanometer-sized particles. One of the first experimental studies on this subject was conducted by Masuda et al. [START_REF] Masuda | Alteration of thermal conductivity and viscosity of liquid by dispersing ultra-fine particles. dispersion of al2o3, sio2 and tio2 ultra-fine particles[END_REF]. They analysed thermal properties of system containing Al 2 O 3 , SiO 2 and TiO 2 particles in water. The result of their research was the improvement in thermal conductivity for Al 2 O 3 -water and TiO 2 -water, however no increase was observed for SiO 2 -water [START_REF] Masuda | Alteration of thermal conductivity and viscosity of liquid by dispersing ultra-fine particles. dispersion of al2o3, sio2 and tio2 ultra-fine particles[END_REF]. In 1995 Choi and Eastman presented results of their study where the word "nanofluid" appears for the first time [START_REF] Choi | Enhancing thermal conductivity of fluids with nanoparticles[END_REF]. They showed that thermal conductivity of fluid increases when nanoparticles are added to it and suggested many potential benefits for the scientific world in the future. Since then research work in order to find out more about the properties has risen. The current stage of knowledge on the thermo-physical properties of nanofluids can be found in recent review papers [START_REF] Qiu | A review of recent advances in thermophysical properties at the nanoscale: From solid state to colloids[END_REF][START_REF] Estellé | Current trends in surface tension and wetting behavior of nanofluids[END_REF][START_REF] Sharaf | On the colloidal and chemical stability of solar nanofluids: From nanoscale interactions to recent advances[END_REF][START_REF] Okonkwo | An updated review of nanofluids in various heat transfer devices[END_REF][START_REF] Fal | Nanofluids in the service of high voltage transformers: breakdown properties of transformer oils with nanoparticles, a review[END_REF][START_REF] Minea | A review on electrical conductivity of nanoparticle-enhanced fluids[END_REF][START_REF] Geng | A comprehensive presentation on nanoparticles electrical conductivity of nanofluids: Statistical study concerned effects of temperature, nanoparticles type and solid volume concentration[END_REF][START_REF] Khodadadi | A comprehensive review on rheological behavior of mono and hybrid nanofluids: effective parameters and predictive correlations[END_REF][START_REF] Jóźwiak | Rheology of ionanofluids-a review[END_REF][START_REF] Sadeghi | On the natural convection of nanofluids in diverse shapes of enclosures: an exhaustive review[END_REF][START_REF] Murshed | Experimental research and development on the natural convection of suspensions of nanoparticles-a comprehensive review[END_REF]. Thermophysical properties and possible applications of nanofluids containing various types of carbon structures have been summarized in Refs. [START_REF] Tam | Carbon nanomaterial-based nanofluids for direct thermal solar absorption[END_REF][START_REF] Mashali | Thermo-physical properties of diamond nanofluids: A review[END_REF][START_REF] Sidik | A review on the use of carbon nanotubes nanofluid for energy harvesting system[END_REF]. The review of the papers on lubrication properties of nanofluids containing carbon structures was presented in Ref. [START_REF] Kotia | Carbon nanomaterials as friction modifiers in automotive engines: Recent progress and perspectives[END_REF].

Thermal conductivity of nanofluids with different types of nanoparticles are still the subject of experimental studies. Shamaeil et al. [START_REF] Shamaeil | The effects of temperature and volume fraction on the thermal conductivity of functionalized dwcnts/ethylene glycol nanofluid[END_REF] focused on the effects of temperature and volume fraction on the thermal conductivity for double-walled carbon nanotubes (DWCNTs)-ethylene glycol (EG) nanofluids. The results show that thermal conductivity ratio increases with increasing the volume fraction of particle in suspension and temperature. Analysed data also reveals that the maximum enhancement is 24.9% for 0.6% volume concentration at 325. [START_REF] Mashali | Thermo-physical properties of diamond nanofluids: A review[END_REF] On the other hand oscillatory measurements at 298.15 K carried out for ethylene glycol based graphite/dimaonds mixture nanofluids shows that it behaves like non-Newtonian fluid [START_REF] Żyła | The influence of ash content on thermophysical properties of ethylene glycol based graphite/diamonds mixture nanofluids, Diamond and Related A c c e p t e d M a n u s c r i p t[END_REF].

Similary the rheology results of EG-based nanofluid with single-walled carbon nanohorn presents that it has non-Newtonian shear thinning behaviour at higher nanohorn loading [START_REF] Selvam | Effective thermal conductivity and rheological characteristics of ethylene glycol-based nanofluids with single-walled carbon nanohorn inclusions[END_REF]. Recently

Żyła presented results of the experimental study on the rheological behavior of ethylene glycol based nanofluids containing low concentrations (up to 1 wt.%) of carbon black nanoparticles [START_REF] Żyła | Nanofluids containing low fraction of carbon black nanoparticles in ethylene glycol: An experimental study on their rheological properties[END_REF].

He showed that these materials exhibit non-Newtonian viscoelastic nature.

Among all of these properties heat capacity and mass density of EG based nanofluids are also the subject of experimental studies. However most of the papers concern oxides and nitrides [START_REF] Mariano | Thermal conductivity, rheological behaviour and density of non-newtonian ethylene glycol-based sno2 nanofluids[END_REF][START_REF] Pastoriza-Gallego | Thermophysical profile of ethylene glycol-based zno nanofluids[END_REF][START_REF] Mariano | Co3o4 ethylene glycol-based nanofluids: thermal conductivity, viscosity and high pressure density[END_REF][START_REF] Cabaleiro | Specific heat of metal oxide nanofluids at high concentrations for heat transfer[END_REF][START_REF] Żyła | Isobaric heat capacity and density of ethylene glycol based nanofluids containing various nitride nanoparticle types: An experimental study[END_REF], while results on the ethylene glycol based nanofluids containing carbon nanostructures are rare. The isobaric heat capacity and mass density of EG based nanofluids containing nanodiamonds with different purities were experimentally investigated by us previously and presented elsewhere [START_REF] Żyła | Nanodiamonds-ethylene glycol nanofluids: Experimental investigation of fundamental physical properties[END_REF]. We have presented that classical models described these properties with good accuracy.

Recent results of the studies on the surface tension of EG based nanofluids starts to earn interest [START_REF] Żyła | Thermophysical and dielectric profiles of ethylene glycol based titanium nitride (tin-eg) nanofluids with various size of particles[END_REF][START_REF] Moosavi | Fabrication, characterization, and measurement of some physicochemical properties of zno nanofluids[END_REF][START_REF] Wanic | Surface tension of ethylene glycol-based nanofluids containing various types of nitrides[END_REF]. According to the best authors knowledge the surface tension of nanofluids Despite the great interest in the thermophysical properties of nanofluids containing various types of carbon based nanoparticles, there are only few papers, which reports their electrical properties. Żyła et al. [START_REF] Żyła | Nanodiamonds-ethylene glycol nanofluids: Experimental investigation of fundamental physical properties[END_REF] investigated electrical conductivity of nanodiamonds ethylene glycol nanofluids. They prepared nanofluids with two types of nanodiamond nanoparticles with different level of purity (87% -ND87, 97% -ND97). Five samples show nonlinear increase in electrical conductivity with increasing volume fraction, and the maximum value was achieved at 0.0338 volume fraction. They also noticed that there is no visible difference between nanofluids containing nanodiamonds with various purities. Different types of nanofluid based on carbon based materials and ethylene glycol were also investigated by Żyła et al. [START_REF] Żyła | The influence of ash content on thermophysical properties of ethylene glycol based graphite/diamonds mixture nanofluids, Diamond and Related A c c e p t e d M a n u s c r i p t[END_REF]. This time, they used two types of graphite and diamond mixtures, with different ash content, to prepare ethylene glycol based nanofluids. The five samples with volume fraction between 0.004-0.023 were prepared for both type of nanoparticles. Obtained results show nonlinear increase in electrical conductivity with increasing volume fraction for both types of nanofluids. They also revealed that ash content has a significant impact on the electrical conductivity of such types of nanofluids. Nanofluids prepared with nanoparticles with a higher content of ash are less electrically conductive. The maximum electrical conductivity enhancement was noted for the nanofluid with lower ash content and it was almost 200 times greater than that for pure ethylene glycol and 10 times higher than that with higher ash content at the same volume fraction.

In this paper results of the experimental studies on thermal conductivity, heat capacity, viscosity, mass density, surface tension, refractive index and electrical conductivity of nanofluids containing two different types of carbon black nanoparticles are presented.

Materials and methods

In the following sections the description of materials and methods used in this study is presented. The information about the measuring equipment together with the uncertainty determination are also reported. The energy-dispersive X-ray spectroscopy microanalysis (EDS) was performed with Inca Energy 300 (Oxford Instruments, Oxford, UK) coupled to the previous described SEM. The EDS spectrum of the used nanopowders were presented in Fig. 2. Performed measurements confirm the chemical structure of carbon (C), as expected for these nanomaterials. The appearance of the silica (Si) peak can be explained by the physical support over which the analyzes were performed.

Nanoparticle characterization

where A is a constant depending on the reflection plane 0.9 and 1.84 for the 002 and 110 reflections, respectively,  is the XRD wavelength,  is the scattering angle (in radians), and B is the full width at half-maximum (fwhm). For both samples studied, a L is less than 1 nm in agreement with the board XRD features shown in Fig. 3 and the disordered structure expected for carbon black particles. The Brunauer-Emmet-Teller (BET) specific surface area of both carbon black nanopowders was determined from nitrogen physisorption with a Micromeritics Gemini VII 2390t instrument using a mixture of N 2 /He (30%/70%). To avoid the surface hydration, samples were previously outgassed under vacuum at 323.15 K.

The BET surface area of CB1 is experimentally evaluated to 1012  4 m 2 g 1  , which is in good agreement with manufacturer data as the deviation is 1.2%. For CB2, the measured BET value is 450  8 m 2 g 1  that is lower by 21% than the manufacturer data. These measurements clearly demonstrate that the carbon black nanopowders present distinct BET surface area.

Sample preparation

There are two methods in which a nanofluid could be prepared: (a) single step and (b) two-step preparation process. The single step method involves the direct production of nanoparticles in a base liquid and can be completed in various ways. For example by reducing the compound in the presence of another substance under microwave irradiation, a nanoparticle-source evaporation and deposition of the evaporant into a base fluid, and sonication and using the vacuum-SANSS (submerged arc nanoparticle synthesis system). Samples can also be prepared based on two-step method by dispersing the dry particles within the base fluid. Taking into account fact that preparing dry nanoparticles is often easy and cheap process, and nanoparticles are commercially available this method seems to be more attractive for potential application, which demands a large-scale production process. However two-step method has some drawback, e.g some types of nanoparticles easily aggregate and sediment because of strong van der Walls force between particles and gravity. In the present study two types of carbon black A c c e p t e d M a n u s c r i p t nanoparticles were used to prepare nanofluids with two-step method. Both nanopowders (with two different specific surface area, ash content and sizes) were purchased commercially and their specification is presented in the Table 1. In the first stage of sample preparation nanoparticles were weighed on an analytical balance. After that the base fluidethylene glycol (EG) produced by Fisher Chemical (Loughborough, UK) with the purity over 99%, was added in order to achieve desired concentration of the particles in the suspension. For the dispersion nanoparticles, mechanical mixing and ultrasonic bath were used. Sonication is well known method of dispersing nanoparticles, as it was described in recent review paper [START_REF] Asadi | Effect of sonication characteristics on stability, thermophysical properties, and heat transfer of nanofluids: A comprehensive review[END_REF]. No surfactants were used during sample preparation.

Stability characterization

After preparation of the samples, the stability of these type of nanofluids was evaluated.

The high light absorbance of nanofluids containing carbon black nanoparticles and its high viscosity make it impossible to precise the determination of the stability of samples containing the same fraction of particles that were used in the study of thermophysical properties. The stability evaluation of these nanofluids was performed on diluted samples containing 0.02 wt.% of particles.

One of the popular methods of the stability determination of nanofluids is measuring its Zeta potential as it was described in Ref. [START_REF] Yu | A review on nanofluids: Preparation, stability mechanisms, and applications[END_REF]. The Zeta potential of the CB-EG nanofluids was measured with Zetasizer Nano ZS device (Malvern Instruments Ltd, Malvern, UK) at 298.15 K.

The values obtained in this study were 57 6  mV and 110 4  mV for nanofluids containing CB1 and CB2, respectively. Nanofluids with absolute Zeta potential value higher than 60 mV are consider as very stable [START_REF] Yu | A review on nanofluids: Preparation, stability mechanisms, and applications[END_REF], however one must note that this method could not be fully reliable for ethylene glycol based nanofluids, therefore another type of stability examination was also performed.

The Dynamic Light Scattering (DLS) method allows the apparent size distribution of nanoparticles in suspension to be determined. This method was used with Zetasizer Nano ZS device (Malvern Instruments Ltd, Malvern, UK) for 0.02 wt.% nanofluids at 298.15 K and using a scattering angle value 173 o . Samples were analyzed just after preparation (0 h) and 2, 6, 24 and 48 h later under static conditions. It should be noted that the size value by DLS measurements is assumed as the diameter of a perfect sphere with a translational diffusion coefficient equivalent to A c c e p t e d M a n u s c r i p t that of the dispersed particle. Thus, these values are usually referred to hydrodynamic diameter or apparent size. Consequently, it should be taken into account that the stability analysis is focused on the temporal evolution of the DLS measurements rather than the values of the sizes themselves.

Figure 4 presents obtained results. One could note that the apparent size of CB1 nanoparticles is two times higher that CB2 nanoparticles and it corresponds with the manufacturer declaration and SEM pictures. Nanofluids containing CB2 nanoparticles tend to agglomerate after 48 hours, however dispersion for samples with CB1 nanoparticles is stable with time during measurement.

The apparent size distribution analysis presented in Fig. 4A) and Fig. 4B), show similar profiles for both samples, CB1 and CB2 nanofluids, along the observed time (2 days). Nevertheless results collected for CB1-EG sample show a low intensity peak around 0.5  m that appears 24 h after preparation and increases after 48 h. On the other hand, the CB2-EG nanofluid shows a similar small peak for the first time 48 h after preparation. These peaks reveal symptoms of agglomeration of nanoparticles that is occurring within the fluids, which is stronger and faster for the nanofluid containing CB1 nanoparticles.

Finally, the possible occurence of sedimentation was not being observed until several hours after preparation of the samples that were studied. Therefore, taking into account all these factors, the examined nanofluids were thermophysically studied for the few hours after samples preparation, in conditions of stable dipersions.

Methods

Thermal conductivities of the base fluidethylene glycol and the elaborated nanofluids were measured at 298.15 K by means of a THW-L2 thermal conductivity device (Thermtest Inc., Canada) based on the transient short hot wire (THW) technique, that was already used in examination of nanofluids [START_REF] Cabaleiro | Thermal and physical characterization of peg phase change materials enhanced by carbon based nanoparticles[END_REF][START_REF] Banisharif | Thermophysical properties of water ethylene glycol (weg) mixture-based fe3o4 nanofluids at low concentration and temperature[END_REF][START_REF] Zeroual | Ethylene glycol based silver nanoparticles synthesized by polyol process: Characterization and thermophysical profile[END_REF]. The THW-L2 apparatus allows obtaining direct thermal Experimental isobaric heat capacities for CB-EG nanofluids were determined by a quasi-isothermal temperature modulated differential scanning calorimetry technique. For this purpose, a differential scanning calorimeter, Q2000 (TA Instruments, New Castle, USA) was used at 298.15 K as measurement temperature. Samples are housed into T zero aluminum pans and weighted before and after performing the tests to validity the measurement. Sinusoidal temperature modulations around the studied temperatures were selected to obtain the isobaric heat capacities. More details about the p c determination by using this device were previously reported [START_REF] Cabaleiro | Specific heat of metal oxide nanofluids at high concentrations for heat transfer[END_REF][START_REF] Vallejo | Potential heat transfer enhancement of functionalized graphene nanoplatelet dispersions in a propylene glycol-water mixture. thermophysical profile[END_REF], the total relative uncertainty being lower than 3%.

The mass density of the investigated nanofluid was determined using an automatic oscillating U-tube densitymeter DMA 4100M (Anton Paar, Graz, Austria). The sample was inserted into a U-shaped borosilicate glass tube and constant temperature 298.15 K was maintained. Three series of measurements were carried out, ten measurements in each series. The uncertainty of the values obtained with this equipment was determined on the base of (a) the deviation between the experimental data and the corresponding value in the literature and (b) standard deviation of one hundred measuring points distilled water at 298.15 K. For these one hundred measurements, the obtained mass density of water was 0.9969 g cm 3  with standard deviation 0.0002 g cm 3  . According to the literature [START_REF] Steckel | Physical properties of heavy oxygen water. part 1. density and thermal expansion[END_REF], the water density is 0.997066  0.000001 g cm 3  . Considering the literature values and the obtained experimental results the relative uncertainty of mass density measurements could be declared as 0.1%.

Surface tension between air and nanofluid measurements were performed with Drop Shape Analyzer DSA25 (KRÜSS GmbH, Hamburg, Germany). This equipment uses the analysis of the In order to measure surface tensions of nanofluids a drop "hang" for 10 minutes to equalize nanofluids temperature with ambient air temperature. After such stabilization the surface tension of a droplet was measured. To obtain reliable results after each measurement a drop was dropped and another one was produced with the waiting time for temperature stabilization. The experimental study included three series of measurements that followed each other. Environmental chamber (KRÜSS GmbH, Hamburg, Germany) allows to perform measurements in strict controlled temperature, 298.15 K. Relative uncertainty of obtained surface tension values was estimated as 1% as presented elsewhere [START_REF] Żyła | Thermophysical and dielectric profiles of ethylene glycol based titanium nitride (tin-eg) nanofluids with various size of particles[END_REF][START_REF] Wanic | Surface tension of ethylene glycol-based nanofluids containing various types of nitrides[END_REF].

The equipment used to perform the rheological properties examination was a rheometer HAAKE MARS 2 (Thermo Electron Corporation, Karlsruhe, Germany). Two types of measurements were performed: (a) the rotational tests in control rate mode, and (b) the oscillatory tests at constant oscillation frequency 1.0 Hz. Taking into account fact that CB-EG nanofluids exhibit a non-Newtonian character, as presented in previous study [START_REF] Żyła | Nanofluids containing low fraction of carbon black nanoparticles in ethylene glycol: An experimental study on their rheological properties[END_REF], the cone-plate measuring geometry was used; the diameter of the rotor is 60 mm, and cone angle is 2 o . Constant temperature of the sample, 298.15 K, was maintained by the Peltier system coupled with a Phoenix 2 thermostat (Thermo Electron Corporation, Karlsruhe, Germany); this system allows to keep the temperature of the sample with the accuracy of 0.1 K. Rotational measurements were performed at shear rate range from 0.1 s 1  to 1000.0 s 1  and oscillatory tests in deformation range from 0.1% (0.001) to 1000.0% (10.000). The step of shear rate and deformation increases was planned on the logarithmic scale. This measuring system was previously used, therefore more detailed description of the uncertainty discussion could be find elsewhere [START_REF] Żyła | Nanodiamonds-ethylene glycol nanofluids: Experimental investigation of fundamental physical properties[END_REF][START_REF] Żyła | Experimental studies on viscosity, thermal and electrical conductivity of aluminum nitride-ethylene glycol (aln-eg) nanofluids[END_REF].

The refractive index was measured with the Refractometer Abbemat 550 (Anton Paar

OptoTec GmbH, Seelze-Letter, Germany). A 500  L sample was placed in the refractometer using the HTL Discovery Comfort DV1000 single channel pipette (HTL Lab Solutions, Warsaw, Poland) with an accuracy of volume of 0.5%. The sample temperature was stabilized at 298.15 K with an accuracy of 0.03 K for 5 minutes. The refractive index value was determined as the average of a series of three measurements performed for various samples. The relative uncertainty of measurement of refractive index performed at this station was determined at less than 1% of the measured value, as shown in Ref. [START_REF] Żyła | Ethylene glycol based silicon nitride nanofluids: An A c c e p t e d M a n u s c r i p t experimental study on their thermophysical, electrical and optical properties[END_REF]. TetraCone 925 (WTW GmbH, Weilheim, Germany). Measurement was performed for 10 minutes, at constant temperature 298.15 K with 0.2 K accuracy. The value of electrical conductivity was obtained as the average value. The temperature was stabilized at least 10 minutes before measurement. Relative uncertainty of this measuring setup was evaluated to 1% of the measured value as presented in Ref. [START_REF] Żyła | Experimental studies on viscosity, thermal and electrical conductivity of aluminum nitride-ethylene glycol (aln-eg) nanofluids[END_REF].

The electrical conductivity of carbon black ethylene glycol was measured with MultiLine

Results and discussion

In the following subsections the obtained results have been presented along with the discussion and graphic representation.

Thermal conductivity

The experimental value of atmospheric pressure thermal conductivity of ethylene glycol at 298.15 K, Table 2, agrees well with those of Lide [START_REF] Lide | CRC handbook of chemistry and physics[END_REF], with a deviation of 2.2  %, Akilu et al.

[45], with a deviation of 1  %, and Z y ł a et al. [START_REF] Żyła | Nanodiamonds-ethylene glycol nanofluids: Experimental investigation of fundamental physical properties[END_REF], with an agreement of 1.8  %. As it can be observed, these deviations are lower than uncertainty in thermal conductivity measurements.

Results of experimental study on the thermal conductivity, k , of CB1-EG and CB2-EG nanofluids were summarized in Table 2 and presented in Fig. 5, where it is shown how does the value of thermal conductivity change with increasing mass fraction of nanoparticles. The picture depicts that the highest enhancement of thermal conductivity (4.8%) was observed for nanofluids containing CB1 nanoparticles with the highest examined mass concentration 2 wt.%. Additionally it was presented that the dependence of the thermal conductivity on the mass fraction of these materials could be modeled with linear function:

= 1 2.42 , nf m bf k k   ( 2
)
where k is thermal conductivity,  is fraction, and subscripts nf , bf , m stands for nanofluid, base fluid and mass respectively. In the case of the fitting the linear function (2) to the experimental data the coefficient of determination 2 =1 R .

Thermal conductivity for the second type of investigated nanofluids is definitely lower than in case of CB1-EG nanofluids. The enhancement comparing with the pure ethylene glycol is  m 2 g 1  and 450 8  m 2 g 1  for CB1 and CB2 respectively. Furthermore the average particle size which is grater for the first type can contribute to better thermal conductivity for this nanofluid. The effect of particle size on the thermal conductivity was analyzed for alumina nanofluids where diameter in the range from 8 to 282 nm was taken [START_REF] Beck | The effect of particle size on the thermal conductivity of alumina nanofluids[END_REF]. Beck et al. came to the conclude that the thermal conductivity decreases as the particle size is below value of 50 nm. The improvement in the thermal conductivity due to the increasing specific surface area (SSA) is well recognized in the literature. From the results by studying carbon nanofluids [START_REF] Mehrali | Investigation of thermal conductivity and rheological properties of nanofluids containing graphene nanoplatelets[END_REF][START_REF] Mehrali | Effect of specific surface area on convective heat transfer of graphene nanoplatelet aqueous nanofluids[END_REF][START_REF] Lee | Enhanced thermal conductivity of nanofluids containing graphene nanoplatelets prepared by ultrasound irradiation[END_REF][START_REF] Timofeeva | Particle size and interfacial effects on thermo-physical and heat transfer characteristics of water-based  -sic nanofluids[END_REF][START_REF] Alawi | Thermal conductivity and viscosity models of metallic oxides nanofluids[END_REF], it was found as higher thermal conductivity belongs to carbon-based nanoadditives with higher specific surface area. Nowadays, it is extensively recognized that the presence of a nanolayer at the solid-liquid interface and nanoparticle aggregation entails the main way for thermal conductivity enhancement in nanofluids. The liquid molecules close to particle surfaces allow the formation of layered structures and act much like a solid [START_REF] Mehrali | Investigation of thermal conductivity and rheological properties of nanofluids containing graphene nanoplatelets[END_REF]. Therefore, the dimension (thickness) of the A c c e p t e d M a n u s c r i p t interfacial layer has a noticeable effect on the enhanced thermal conductivity of the carbon-based nanofluids [START_REF] Mehrali | Effect of specific surface area on convective heat transfer of graphene nanoplatelet aqueous nanofluids[END_REF]. Thus, nanoadditives with higher specific surface area lead to decreases on the contact resistance, and consequently superior thermal conductivity enhancements [START_REF] Mehrali | Investigation of thermal conductivity and rheological properties of nanofluids containing graphene nanoplatelets[END_REF][START_REF] Lee | Enhanced thermal conductivity of nanofluids containing graphene nanoplatelets prepared by ultrasound irradiation[END_REF]. In addition, it should be noted that all surfaces measured with BET measurement in equilibrated suspensions will acquire layers of fluid molecules that will behave as an interfacial phase [START_REF] Timofeeva | Particle size and interfacial effects on thermo-physical and heat transfer characteristics of water-based  -sic nanofluids[END_REF].

 nf k nf k / bf k nf k nf k / bf k - W m 1  K 1  - W m 1  K 1  - 0.
Suspended nanoparticles can form larger agglomerates through the solid/liquid/solid interface, but volume and contribution of the interfacial phase will always remain proportional to the total surface area of dry powders. Thus, the microscopic properties corresponding to the SSA values for the CB1 and CB2 nanopowders correlate to the macroscopic effect in the thermal conductivity enhancements of the nanofluids. Thermal conductivity is superior in nanofluids with larger nanoparticle surface area, CB1-EG.

Isobaric heat capacity

The experimental isobaric heat capacity value for EG at 298.15 K, presented in Table 3, is in good agreement with literature data [START_REF] Nikolaev | Heat capacity of ethylene glycol and ethylene deuteroglycol in the temperature range 80-300k[END_REF][START_REF] Sv | Behaviour of excess heat capacity of aqueous non-electrolytes[END_REF][START_REF] Stephens | Saturated liquid specific heats of ethylene glycol homologs[END_REF][START_REF] Zaripov | Experimental study of the isobaric heat capacity of liquid organic compounds with molecular weights of up to 4000[END_REF][START_REF] Żyła | Isobaric heat capacity and density of ethylene glycol based nanofluids containing various nitride nanoparticle types: An experimental study[END_REF]. Deviations of +0.1%, +0.6%, +0.8%, +0.4% and 1.4  % were obtained with respect to the values from Nikolaev and Rabinovich [52],

and Subrahmanya [START_REF] Sv | Behaviour of excess heat capacity of aqueous non-electrolytes[END_REF], Stephens and Tamplin [START_REF] Stephens | Saturated liquid specific heats of ethylene glycol homologs[END_REF], Zaripov [START_REF] Zaripov | Experimental study of the isobaric heat capacity of liquid organic compounds with molecular weights of up to 4000[END_REF], and Z y ł a et al. [29],

respectively. The experimental results for the different-loaded CB-EG nanofluids are presented in Table 3. It can be observed that the isobaric heat capacity decreases slightly with the increasing fraction of both CB nanoparticles. These decreases reach 5.0% and 4.6% for the nanofluids with the highest examined CB1 and CB2 concentration, respectively. The literature about nanofluids evidenced different effects in the isobaric heat capacity of the base fluid by the dispersion of nanoadditives. According to Riazi et al. [START_REF] Riazi | Specific heat control of nanofluids: A critical review[END_REF], when the base fluid is water or a glycol, it tends to decrease with the increasing nanoadditive loading. Nevertheless, it is usual to observe increases in the isobaric heat capacity of nanofluids based on molten salts or ionic liquids caused by the nanoparticle addition. Therefore, our reported experimental behaviour in Fig. 6 is in accordance with the described behaviours for glycol-based nanofluids.

The decreases caused by the addition of CB1 and CB2 are similar among them, 0.7% being the maximum deviation. Likewise, systematic lower values appear by the dispersion of CB1, the nanoadditive with higher specific surface area. The correlation from Xuan and Roetzel [START_REF] Xuan | Conceptions for heat transfer correlation of nanofluids[END_REF][START_REF] Żyła | Nanodiamonds-ethylene glycol nanofluids: Experimental investigation of fundamental physical properties[END_REF] describes the isobaric heat capacity of nanofluids on the basis of the mass fraction of nanoparticles in the dispersion:

, , , = (1 )
,

p nf m p p m p bf c c c   (4)
were p c is the specific heat capacity, m  is the nanoadditive mass fraction and subscripts nf , p and bf stand for nanofluid, nanoparticles and base fluid, respectively.

Experimental isobaric specific heat capacities of 1.017 kJ kg 1  K 1  and 1.148 kJ kg 1  K 1  were obtained for the CB1 and CB2 raw nanopowders at 298.15 K. The heat capacity of CB1 is slightly lower than that of CB2, which contributes to further reducing the heat capacity of the base fluid in which it is dispersed, as indicated above. Deviations between experimental and predicted values by using these experimental values here reported as , pp c are lower than 4%. Likewise, the , pp c is usually taken in the literature as the value of the heat capacity for the bulk material [START_REF] Żyła | Isobaric heat capacity and density of ethylene glycol based nanofluids containing various nitride nanoparticle types: An experimental study[END_REF].

Taking into account the reported p c value of 0.716 kJ kg Takahashi and Westrum [START_REF] Takahashi | Glassy carbon low-temperature thermodynamic properties[END_REF] or 0.770 kJ kg 1  K 1  for dense graphite by Lutcov et al. [START_REF] Lutcov | Thermal conductivity, electric resistivity and specific heat of dense graphites[END_REF],

deviations between experimental and predicted values for the 0.005 and 0.010 nanoadditive mass fractions lower than 2.2% are obtained. For the highest nanoadditive mass concentration the deviation increases up to 3.7%.

Mass density

Table 4 shows the obtained experimental results of mass density measurements for two different nanofluids at 298.15 K. The density measurement of the base fluid is highly consistent with our previous work [START_REF] Żyła | Nanodiamonds-ethylene glycol nanofluids: Experimental investigation of fundamental physical properties[END_REF] with a deviation less than 0.02% . In Fig. 7, a clear increase in the density of the nanosuspension can be observed as the mass fraction of the nanoparticles increases.

No significant differences between mass density values of CB1-EG and CB2-EG were noted.  , of the particles for the CB1-EG (○) and CB2-EG (•) nanofluids at 298.15 K.

Surface tension

Results of experimental study on the surface tension,  , of CB1-EG and CB2-EG based nanofluids were summarized in Table 5. The value of surface tension decreases for both nanofluids with increasing mass fraction of nanoparticles as it is presented on graph in Fig. 8.

A c c e p t e d

M a n u s c r i p t 5) and ( 6) respectively.

Based on these results it can be concluded, that the surface tension of the CB1-EG is decreasing more significantly that in case of CB2-EG. On the base of the results presented in Fig. 8 one might note that the surface tension of nanofluids with mass concentration 2 wt.% is 39.76 mN m 1  and 46.25 mN m 1  for CB1 and CB2 respectively. Experimental data could be modeled with linear functions, as follow:

= 1 7.14 , nf m bf     (5) = 1 1.13 , nf m bf     (6)
for CB1-EG and CB2-EG respectively. The coefficient of determination 2 =1 R for both fitted curves.

The difference in surface tension could be related to average particle size which value for CB1 is almost twice higher than for CB2. Similar conclusions were obtained in [START_REF] Chinnam | Measurements of the surface tension of nanofluids and development of a new correlation[END_REF], where Chinnam et al. were analyzing the surface tension of four types of nanofluids in base fluid of 60%

propylene glycol and 40% water by mass. On the other hand it should be noted that the specific surface area of CB1 is also two times higher that in case of CB2, which could affect the surface tension.

A c c e p t e d M a n u s c r i p t

Dynamic viscosity

Dependence of the shear stress and dynamic viscosity on the shear rate of both examined nanofluids has been presented in Fig. 9. Both, CB1-EG and CB2-EG nanofluids exhibit a non-Newtonian shear thinning behavior. Similar nature of ethylene glycol based nanofluids containing carbon structures (nanohorns, graphite/diamond mixture, nanodiamonds) has been already presented in literature [START_REF] Selvam | Effective thermal conductivity and rheological characteristics of ethylene glycol-based nanofluids with single-walled carbon nanohorn inclusions[END_REF][START_REF] Żyła | The influence of ash content on thermophysical properties of ethylene glycol based graphite/diamonds mixture nanofluids, Diamond and Related A c c e p t e d M a n u s c r i p t[END_REF][START_REF] Żyła | Nanodiamonds-ethylene glycol nanofluids: Experimental investigation of fundamental physical properties[END_REF]. One might note that this kind of behavior could be modeled with Herschel-Bulkley (H-B) model as follow:

0 =, N K     (7)
where: 0  is yield stress, K -consistency, N -dimensionless flow index behavior. Figure 9 presents both the experimental results and model ( 7) fitting. Herschel-Bulkley rheological parameters together with correlation coefficient, R 2 , were summarized in Table 6. Table 6: Herschel-Bulkley rheological parameters, 0  , K , N , determined on the base of flow curves presented in Fig. 9, of nanofluids containing two types of carbon black nanoparticles at 298.15 K and correlation coefficient, R 2 , of fitting.

CB1-EG CB2-EG m  0  K N R 2 0  K N R 2  Pa Pa s N   Pa Pa s N   0.

Viscoelastic structure

Non-Newtonian shear thinning behavior could indicate that these nanofluids has viscoelastic structure. In the series of oscillatory experiments at constant frequency 1 Hz both the storage (G') and the loss (G") moduli were determined in function of deformation. Results of this study have been presented in Fig. 10. It could be noted that the viscoelastic structure is fragile and G' starts decreasing at low deformation value (0.01). However a short linear viscoelastic region could be observed for the lowest examined deformations (up to 0.01). In this region both G' and G" are constant and could be calculated as an average in this deformation range. Results of such evaluation has been presented in Table 7 and shown as solid lines in Fig. 10. explained by the "overshoot phenomenon" as it was described in details by Pastoriza-Gallego et al.

[62] and Hermida-Merino et al. [START_REF] Hermida-Merino | Evidence of viscoplastic behavior of exfoliated graphite nanofluids[END_REF].

Refractive index

Summary results of refractive index, n , measurements for examined ethylene glycol based carbon black nanofluids with different mass fractions of nanoparticles are shown in Fig. 11 and Table . 8. The refractive index of the studied nanofluids increases linearly with the mass fraction of the nanoparticles. The mathematical analysis carried out using the least squares fitting showed a correlation coefficient, R 2 , equal to 1 for both linear functions fitted experimental data.

The linear dependence describing increase of the refractive index value for both examined types of nanofluids could be presented as:

= 1 0.0914 , nf m bf n n   (8) = 1 0.1410 , nf m bf n n   (9)
where equation ( 8) refers to the CB1-EG and eq. ( 9) to CB2-EG nanofluids respectively. 1) and creation of conduction path for fluids with higher content of CB1 as schematically shown in Fig. 13. For low mass fraction of nanoparticles in ethylene glycol, electrical conduction can increase by formation of electrical double layer (EDL) around nanoparticles [START_REF] Chereches | Electrical conductivity of new nanoparticle enhanced fluids: An experimental study[END_REF]; for higher content of CB1, the conduction path can start to create.

Electrical conductivity enhancement of both type of CB-EG nanofluids can be described with good accuracy with the power function. It was found that results for CB1-EG nanofluids can be fitted by following equation (with R 2 =0.97247): [START_REF] Khodadadi | A comprehensive review on rheological behavior of mono and hybrid nanofluids: effective parameters and predictive correlations[END_REF] and for CB2-EG nanofluids (with R 2 =0.99999):

Conclusions

The paper presents a comprehensive experimental characterization about thermal conductivity, isobaric heat capacity, mass density, nanofluid-air surface tension, dynamic viscosity and viscoelastic structure of ethylene glycol based nanofluids containing two types of carbon black nanoparticles. The electric properties like refractive index and electrical conductivity were also evaluated. This experimental study was performed at well controlled temperature 298.15 K and varying the mass content in CB nanoparticles up to 2 wt.%.

It was presented that the specific surface area of particles in nanofluids has great influence on the fundamental physical properties. From the application point of view, the nanofluids containing nanoparticles with higher specific surface area seems to be more interesting according to their higher thermal conductivity and lower surface tension and isobaric heat capacity. On the other hand one must note that the dynamic viscosity increases significantly for these materials, which make it harder to apply in real-life heat exchanging systems.

The remarkable electrical conductivity enhancement reported in this paper (according to the best authors knowledge one of the highest reported in literature) in CB1-EG nanofluids could be interesting from the viewpoint of potential application of these materials in ink jet printed electronics, where currently expensive silver nanoparticles are used. More potential applications of the nanofluids with enhanced electrical conductivity could be found in recent review paper [START_REF] Minea | A review on electrical conductivity of nanoparticle-enhanced fluids[END_REF].

A

  constant depending on the reflection plane [-]B full width at half-maxiumum (fwhm)[-] Corresponding authorEmail address: gzyla@prz.edu.pl (Gaweł Żyła)

A c c e p t e d M a n u s c r i p t containing carbon nanoparticles

  with ethylene glycol as base fluid was not experimentally studied before.

A c c e p t e d M a n u s c r i p t

Figure 1 :

 1 Figure 1: Scanning electron microscope (SEM) pictures of dry nanoparticles: A) and B) present CB1 (Degussa AG (Hanau-Wolfgang, Germany)) nanoparticles with different magnification, C) and D) show CB2 (PlasmaChem GmbH (Berlin, Germany)) with different magnification.

Figure 2 :

 2 Figure 2: A) and C) the scanning electron microscope pictures of observed area of CB1 (Degussa AG (Hanau-Wolfgang, Germany)) and CB2 (PlasmaChem GmbH (Berlin, Germany)) respectively. B) and D) energy-dispersive X-ray spectroscopy microanalysis spectra of CB1 and CB2 carbon black nanopowders respectively.

A c c e p t e d M a n u s c r i p t

Figure 3 :

 3 Figure 3: X-Ray diffractogram for the nanopowders used in this study. Black line refers to the CB1 (Degussa AG (Hanau-Wolfgang, Germany)) nanoparticles and gray to the CB2 (PlasmaChem GmbH (Berlin, Germany)).

Figure 4 :

 4 Figure 4: Results of dynamic light scattering (DLS) examination of the apparent size of particles in ethylene glycol based nanofluids containing 0.02 wt.% carbon black nanoparticles. A) and B) Apparent size distribution for nanofluids containing CB1 and CB2 nanoparticles, respectively; different colors refers to time after preparation of the samples. C) Average Z-size as a function of time after preparation of the samples for the CB1-EG ( ) and CB2-EG ( ) nanofluids at 298.15 K; dotted lines indicate the value after preparation (0h).

A c c e

  p t e d M a n u s c r i p t conductivity data of liquids in agreement with ASTM D7896-14 standard[START_REF]Standard test method for thermal conductivity, thermal diffusivity, and volumetric heat capacity of engine coolants and related fluids by transient hot wire liquid thermal conductivity method[END_REF]. This device is coupled with an EchoTerm Dry Bath which maintains the sample at the test temperature. The wire welded in a sensor has a length of 60 mm with a diameter of 0.1 mm. It is vertically inserted in the sample, and the whole set is introduced in the dry bath. Before starting the measurements, the used software performs a power test for each family of fluids under study and from this test, a heating power and a short power input time are selected avoiding natural convection phenomena in the measurements. Five measurements were carried out to ensure the repeatability of the experimental values, the duration of each measurement being 10 seconds. The declared accuracy in thermal conductivity values by the manufacturer is 5%.

A c c e p t e d M a n u s c r i p t shape of the

  drop and based on the Young-Laplace equation allows to determine the surface tension of the fluid.

A c c e p t e d M a n u s c r i p t 3410

 3410 conductivity meter (WTW GmbH, Weilheim, Germany) working with conductivity probe

A c c e p t e d M a n u s c r i p t approximately 1 %

 1 . Results of the experimental study of thermal conductivity of CB2-EG nanofluids could be also modeled with linear function in form of:

Figure 5 :

 5 Figure 5: Dependence of thermal conductivity, k , and thermal conductivity ratio,

Figure 6 :

 6 Figure 6: Dependence of heat capacity, p c , and heat capacity ratio, , p nf c / , p bf c , on the mass

A c c e p t e d M a n u s c r i p t

Figure 9 :

 9 Figure 9: Dependence of shear stress ( ), and dynamic viscosity ( ) on shear rate (flow and viscosity curves respectively) for ethylene glycol based nanofluids containing two types of carbon black nanoparticles at 298.15 K. Symbols represent measuring points collected with different concentration of nanoparticles, as follow: triangle (,) -0.50 wt.%, diamond (◊,♦) -1.00 wt.%, squares (□, ■) -2.00 wt.%; solid (red) linesresults of the (7) model, dotted lines -pure ethylene glycol according to Ref. [61]. Empty and solid symbols refers to CB1-EG and CB2-EG nanofluids respectively.

Figure 10 :

 10 Figure 10: Storage (G') and loss (G") moduli as a function of deformation (  ) for various mass fractions of nanoparticles in ethylene glycol based nanofluids containing two types of carbon black nanoparticles at temperature 298.15 K and oscillation frequency of 1.0 Hz. Symbols represent measuring points collected with different concentration of nanoparticles, as follow: triangle (,) -0.50 wt.%, diamond (◊, ♦) -1.00 wt.%, squares (□, ■) -2.00 wt.%; solid linesaverage G' and G" in the range of deformations from 0.001 to 0.01. Empty and solid symbols refers to CB1-EG and CB2-EG nanofluids respectively.

A c c e p t e d M a n u s c r i p t

Figure 12 :

 12 Figure 12: Dependence of electrical conductivity,  , and electrical conductivity ratio, nf  /

Figure 13 :

 13 Figure 13: Presentation of idea of creating conduction path in CB1-EG nanofluids. A) pure base fluid, B) nanofluid with low content of CB1, C) nanofluid with middle content of CB1, D) nanofluid with high content of CB1. Black dots present dispersed CB1 nanoparticles, red dots present creation of potential conduction path through dispersed CB1 nanoparticles.

A c c e p t e d M a n u s c r i p t

  al. carried out measurements in which they studied ethylene glycol based nanofluid, with single-walled carbon nanotube (SWNT) using the transient hot wire technique. It was found out that the maximum enhancement is 14.8% for nanofluid containing 0.21% volume concentration[START_REF] Harish | Enhanced thermal conductivity of ethylene glycol with single-walled carbon nanotube inclusions[END_REF]. Investigation on the thermal properties of diamond-ethylene glycol (DNP-EG) nanofluid was conducted by Yu et al. in Ref[START_REF] Yu | Experimental investigation on the thermal transport properties of ethylene glycol based nanofluids containing low volume concentration diamond nanoparticles[END_REF]. The thermal conductivity enhancement of DNP-EG nanofluids for 1.0 vol.% at 303.15 K is up to 17.23%. However, their study has been expanded to observe if thermal conductivity of nanofluids is correlative with pH values. The data shows that pH value has a direct effect on the stability of the samples -according to the measurements for DNP-EG nanofluids when pH is above value of 8.5, no sedimentation is observed within 6 months[START_REF] Yu | Experimental investigation on the thermal transport properties of ethylene glycol based nanofluids containing low volume concentration diamond nanoparticles[END_REF].

	M a n u s c r i p t
	A c c e p t e d

K. Experimental study of thermal conductivity for nanofluid containing graphene-carbon nanotube (Gr-CNT) hybrid material in ethylene glycol was performed by Trinh et al. The sample with 0.07 vol.% Gr-CNT at 303.15 K has 18 % enhancement and in 323.15 K -50% [19]. Harsh et In the last years, nanofluids have achieved attention in terms of thermal conductivity studies. However properties such as viscosity of nanofluids containing carbon nanoparticles was also experimental studied. Yu et al. has conducted measurements of diamond nanoparticles in ethylene glycol at 303.15 K [21]. The data depicts that with the increase of temperature, the viscosity of nanofluids decreases rapidly. His conclusions demonstrates Newtonian behaviour due to linear relation between shear stress and shear rate.

Table 1 :

 1 Basic properties of the carbon black nanoparticles used in the study.

	M a n u s c r i p t
	A c c e p t e d
	In this study two different types of carbon black nanoparticles were used. Both of them are

Table 2 :

 2 Experimental values of the thermal conductivity,

	k , of CB-EG nanofluids at 298.15 K.
	nf

Table 3 :

 3 Experimental values of the isobaric heat capacity, ,

	p nf c , of CB-EG nanofluids at 298.15

Table 4 :

 4 Experimental values of the mass density, nf  , of CB-EG nanofluids at 298.15 K.

		CB1-EG		CB2-EG
					nf  / bf 
	-	g cm 3 	-	g cm 3 	-
	0.000	1.1095	1.0000	1.1095	1.0000
	0.005	1.1121	1.0023	1.1099	1.0004
	0.010	1.1137	1.0038	1.1134	1.0035
	0.020	1.1156	1.0055	1.1189	1.0085
	Figure 7: Dependence of mass density,  , and mass density ratio, nf  / bf  , on the mass fraction,

m  nf  nf  / bf  nf  m

Table 5 :

 5 Experimental values of the surface tension, nf  , of CB-EG nanofluids at 298.15 K.

			CB1-EG				CB2-EG
		m		nf	nf  /		bf		nf		nf  /		bf
	-		mN m 1 	-			mN m 1 		-
	0.000	47.53	1.0000	47.53		1.0000
	0.005	46.92	0.9872	47.83		1.0063
	0.010	45.57	0.9588	47.12		0.9914
	0.020	39.76	0.8365	46.25		0.9731
	Figure 8: Dependence of surface tension,  , and surface tension ratio,	 /	 , on the mass
										nf	bf
	fraction,	 , of the particles for the CB1-EG (○) and CB2-EG (•) nanofluids at 298.15 K. Points
			m
	represent experimental data, dotted and solid line -linear function (

Table 7

 7

			: Values of storage (G'), loss (G") modulus of CB1-EG and CB2-EG nanofluids at 298.15
	K. Values determined on the base of experimental results presented in Fig. 10 in region of
	deformation (lower than 0.010) where both G' and G' were constant.
			CB1-EG		CB2-EG
		m	G'	G"	G'	G"
	-		Pa	Pa	Pa	Pa
	0.005	0.397	0.360	0.601	0.458
	0.010	23.004	2.585	5.717	1.449
	0.020	93.647	8.671	55.726	6.579

Existence of the maximum in loss moduli observed for both examined nanofluids could be

Table 8 :

 8 Experimental values of refractive index, nf n , of CB-EG nanofluids at 298.15 K.The electrical conductivity of two types of carbon black ethylene glycol based nanofluids was measured at 298.15 K and its results were presented in Fig.12and Table9. The obtained results clearly indicate that both types of carbon black have impact on electrical properties of ethylene glycol. Increase in mass fraction (in whole examined range) of nanoparticles in EG causes enhancement of electrical conductivity for CB1-EG nanofluid as well as CB2-EG nanofluid. Observed changes in values of electrical conductivity for CB2-EG nanofluids are similar to those presented for ethylene glycol based nanofluids containing another carbon nanostructures (graphite/diamond with low ash content) presented elsewhere[START_REF] Żyła | The influence of ash content on thermophysical properties of ethylene glycol based graphite/diamonds mixture nanofluids, Diamond and Related A c c e p t e d M a n u s c r i p t[END_REF]. On the other hand electrical conductivity enhancement for CB1-EG nanofluids is high and according to the best authors knowledge it is the highest increase in electrical conductivity reported for nanofluids based on ethylene glycol for such low load of nanoparticles. The similar in electrical conductivity enhancement of nanofluids was achieved for glycol with 7.5 wt.% silicon nitride nanoparticles (Si 3 N 4 ) at 298.15 K, which is almost four times higher mass fraction than used in this study. Such huge increase in electrical conductivity of CB1-EG nanofluids can be caused by extremely high specific surface area of used carbon black nanoparticles (Table

	M a n u s c r i p t
	CB1-EG nf n -1.43028 1.43092 1.43138 1.43301 Figure 11: Dependence of refractive index, n , and refractive index ratio, nf CB2-EG m  nf n / bf n nf n ---0.000 1.0000 1.43028 0.005 1.0004 1.43167 0.010 1.0008 1.43238 0.020 1.0019 1.43419 n / bf nf n / bf n -1.0000 1.0010 1.0015 1.0027 n , on the mass fraction, m A c c e p t e d

 , of the particles for the CB1-EG (○) and CB2-EG (•) nanofluids at 298.15 K. Points represent experimental data, dotted and solid linelinear function (

8

) and (

9

) respectively.

Table 9 :

 9 Experimental values of electrical conductivity, nf  , of CB-EG nanofluids at 298.15 K.

	M a n u s c r i p t
	A c c e p t e d

Acknowledgments

Research was partially supported by EU COST Action CA15119: Overcoming Barriers to Nanofluids Market Uptake and Rzeszow University of Technology grant no. UPB.FE.20.001. Additionally, authors acknowledge the financial support by Spanish "Ministerio de Economía, Industria y Competitividad" and EU FEDER programme through ENE2017-86425-C2-1-R project. JS acknowledges EU COST for the STSM grants ref. COST-STSM-CA15119-46214 and COST-STSM-CA15119-46737. JPV acknowledges FPI Program of "Ministerio de Economía y

Competitividad". PE acknowledges the European Union through the European Regional Development Fund (ERDF), the Ministry of Higher Education and Research, the French region of Brittany and Rennes Métropole for the financial support related to the device used in this study for surface tension measurement. The authors wishes to thank M. Pasturel from Université Rennes 1 and B. Vigolo from IJL, Nancy for XRD experiments and analysis.

Competing interests

The authors declare that they have no competing interests.

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

The authors declare the following financial interests/personal relationships which may be considered as potential competing