14th European Conference on Mixing

Warszawa, 10-13 September 2012

INFLUENCE OF NONIONIC SURFACTANT

ON NANOFLUID PROPERTIES

Michał Drzazga, Grzegorz Dzido, Marcin Lemanowicz, Andrzej Gierczycki

Silesian University of Technology, Faculty of Chemistry, Department of Chemical and

Process Engineering, ul. ks. M. Strzody 7, 44-100 Gliwice, Poland

michal.drzazga@polsl.pl

Abstract. In this study the properties of nanofluids composed of commercially available copper(II)

oxide nanopowder and water were analyzed. The influence of two nonionic surfactants addition, i.e.

Rokanol K7 and Rokacet O7, ranging from 500 to 4000 ppm on the Particle Size Distribution (PSD),

zeta potential, nanofluid flow and thermal properties were examined. Flow resistance was determined

in a closed loop system equipped with centrifugal pump, differential pressure transducer and

electromagnetic flow meter for 4 mm inner diameter copper pipe for two different pipe lengths in

order to eliminate possible errors due to the inlet and outlet disturbances. The friction factors were

determined for eleven different flow rates of each investigated nanofluid-surfactant system. It was

found that addition of small amount of nonionic surfactant reduces Darcy friction factor of nanofluids,

especially for the smallest investigated values of Reynolds number ca. 10000. In case of Rokanol K7

the effect was stable throughout the whole investigated range (9000 – 50000), whereas for Rokacet O7

the effect diminishes as the Reynolds number increases.

Keywords: nanofluid, drag reduction, nonionic surfactant

1. INTRODUCTION

Fluids composed of nanoparticles (i.e. particles smaller than 100 nm) suspended in base

liquids are called the nanofluids. They are currently in the field of interest of many

researchers, among the others, due to their enhanced thermal properties. Nanofluid may find

application in electronics [1-3], automotive industry [4-5] or industrial cooling [6].

Studies concerning suspensions of solids in liquids as a way of improving their thermal

properties are known since more than 100 years [7-9]. However, the application of microscale

and larger particles meets several difficulties, like high concentration of solid phase required

for satisfactory properties improvement and therefore abrasion of installation parts or

instability and settling of suspension.

Development of nanotechnology and new methods of nanoparticles preparation allowed

one to overcome these problems. In order to achieve satisfying enhancement of thermal

properties nanoparticles concentration of ca. 4% vol. or even smaller are required. Addition of

13 nm γ-Al2O3 enhanced thermal conductivity of water by ca. 30% at 4% vol [10-11]. Even

better results were obtained for nanosuspensions of metal particles. Thermal conductivity of

ethylene glycol was improved by 40% after addition of 0.3% vol. copper nanoparticles

smaller than 10 nm [12]. Application of nanofluids improves also convective heat transfer.

Nanosuspension composed of Cu nanoparticles smaller than 100 nm in water shown ca. 60%

improvement of convective heat transfer at concentration of 2% vol [13]. Enhancement of

convective heat transfer was observed also for metal oxides nanofluids. Addition of 6% vol.

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Al2O3 nanoparticles to ethylene glycol-water mixture improved convective heat transfer

coefficient by 50%, and 10% vol. even by 80% [14].

On the other hand, solid phase content increases the dynamic viscosity coefficient of the

nanofluids and thus increases the pressure loss during their flow through a pipeline system.

Viscosity of nanofluids depends on concentration of nanoparticles. E.g., it was proved that in

the case of 0.4% vol. carbon nanotubes in water viscosity of nanosuspension was similar to

one of base liquid [15]. Whereas for Al2O3-water nanofluid with concentrations of 1, 4, 9 and

12% vol. dynamic viscosity coefficient increased by 12, 60, 300 and 530% respectively in

comparison to the base fluid [16]. For small volume concentrations of nanoparticles nanofluid

viscosity increases by ca. 10% for each. 1% vol. of solid phase content.

Unfortunately higher viscosity has also implications on flow properties. E.g., the pressure

drop for 10% vol. Al2O3 in ethylene glycol-water mixture was 4.7 times larger than in the

case of pure base fluid [14]. Therefore, it would be advantageous to find ways of reduction

flow resistance of nanofluids. One of the possibilities is the application of drag reducing

agents (DRA), such as polymers and surfactants. In closed-loop systems surfactants are

a better choice, because they do not lose their drag reduction effect after contact with high

shear forces (e.g. in pumps). Addition of cationic surfactant CTAC to carbon nanotubes-water

nanofluid reduced pressure drop by 45% [17]. This effect is comparable with drag reduction

for pure water. There is lack of information about nonionic surfactant influence on properties

of metal oxide nanofluids, like viscosity, pressure drop, zeta potential and nanoparticle size.

2. EXPERIMENTAL

In the presented study the nanofluid composed of commercially available copper(II)

oxide nanopowder (Nanostructured & Amorphous Materials Inc., USA) of 30 – 50 nm

diameter and RO water (Hydrolab, Poland) was analyzed. The appropriate amount of dry

powder was weighted using a laboratory balance with an accuracy of 10-4

g and added to

a thermostated vessel filled with RO water with an addition of triammonium citrate as the

stabilizing factor. Next the suspension was sonicated using 130 W ultrasonic mixer (Sonics &

Materials Inc., USA) of 13 mm tip diameter for at least one hour. Finally, received nanofluid

was stirred using high power homogenizer for at least half an hour. The influence of two

nonionic surfactants addition, i.e. Rokanol K7 and Rokacet O7 (Rokita S.A., Poland), were

investigated in this study. They were added to the nanofluid in the form of water solution and

then mixed using magnetic stirrer. In all experiments the volume fraction of nanoparticles was

equal to 1% whereas the concentration of surfactants was ranging from 500 to 4000 ppm.

Thermal conductivity was measured using the hot wire technique. A thermostat vessel

was placed on a special anti-vibration table for analytical balances and filled with an

appropriate amount of nanofluid. The KD2 PRO thermal properties analyzer (Decagon

Devices, USA) probe was placed within the sample. The time between consecutive

measurements was not less than 15 minutes in order to stabilize samples temperature. For

each sample at least 5 measurements were made for which R2 was higher than 0.9995.

The dynamic viscosity coefficient was measured using a thermostated digital viscometer

Viscolab 410 (Cambridge Applied Systems Inc., UK). For each sample at least 10 consecutive

measurements were made with time interval equal to 5 minutes.

The particle zeta potential was determined using Malvern Zetasizer Nano Z (Malvern

Instruments Ltd., UK) whereas the particle size distribution was measured using Malvern

Zeta Sizes Nano S90. In both cases at temperature of 25°C at least five measurements were

made in order to eliminate random errors.

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3. RESULTS

Due to the lack of space only the results for Rokacet O7 surfactant are presented in this

section .

Influence of the surfactant addition on relative (nanofluid to water) thermal conductivity

is shown in Fig. 1. Thermal conductivity of the nanofluid is slightly higher then the

conductivity of pure water. The best enhancement of thermal conductivity (2.5%) was

obtained for the highest investigated temperature, i.e. 20°C. For 5°C the enhancement was the

smallest (less than 1%). Generally, the addition of Rokacet O7 had no major influence on the

conductivity of the nanofluid. For the temperature above 15°C and increasing nonionic

surfactant concentration slight enhancement of thermal conductivity, not exceeding 1%, was

observed. For lower temperatures this effect is not clear.

Fig. 1. Impact of Rokacet O7 surfactant concentration on the relative thermal conductivity coefficient of

the nanofluids.

Fig. 2. Impact of Rokacet O7 surfactant concentration on the nanofluid dynamic viscosity coefficient.

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As it should be expected the addition of surfactants raised dynamic viscosity coefficient

of copper(II) oxide nanofluids (Fig. 2). In the case of Rokacet O7 it was equal to

1.43·10-3

Pa·s and 1.51·10-3

Pa·s for 0 ppm and 4000 ppm concentrations respectively at 50C

and 0.73·10-3

Pa·s and 0.78·10-3

Pa·s for 0 ppm and 4000 ppm concentrations respectively at

350C. On the other hand, for the investigated case the drag reduction effect was not obtained

by viscosity reduction but by the change of flow mechanism, i.e. creation of micelles which

reduce the friction between the fluid and pipe wall. As it will be proved below, the pressure

drop arising from the line losses during nanofluid flow through a pipe was decreased.

The particle size distribution of copper(II) oxide nanofluid is presented in Fig. 3a.

Although the declared diameter of dry particles varied between 30 and 50 nm the measured

hydrodynamic diameter ranged from 20 and 1000 nm. Surprisingly, the PSD was bimodal –

the first peak was at 47.71 nm and the second one at 302.3 nm. It may have been caused by

strong agglomeration of nanoparticles in the powder. The breakage of agglomerates by

sonication may not have been sufficient due to low power of ultrasonic mixer or not enough

sonication time. The addition of surfactants had no greater impact on the particle size

distribution for doses equal or smaller than 2000 ppm (Fig. 3b, 3c). In the case of Rokacet O7

the z-average size for that concentration range varied between 174 and 225 nm (Table 1). In

the case of the highest investigated dose i.e. 4000 ppm of Rokacet O7 the third peak appeared

at 4996 nm (Fig. 3d). However, it may have been caused by impurities, i.e. larger particles

that disturbed PSD analysis.

The addition of surfactant influenced to some extent the zeta potential value (Table 1). In

case of Rokacet O7, for doses equal or smaller than 1000 ppm, the zeta potential was equal

ca. -45 mV. The higher doses increased that value - for the highest investigated concentration

it was equal to ca. -37 mV. This fact could explain the appearance of the third peak of the

particle size distribution (Fig. 3d). The Lower absolute value of zeta potential had negative

influence on the stability of the suspension.

Fig. 3. Particle size distribution of cooper (II) oxide nanofluid for different surfactant concentrations:

a) 0 ppm, b) 1000 ppm, c) 2000 ppm, d) 4000 ppm.

Table 1. Nanofluids z-average size and zeta potential

Rokacet O7 concetration [ppm] 0 500 1000 2000 4000

Z-average size [nm] 174.4 173.9 225.5 194.4 208.2

Zeta potential [mV] -45.58 -44.92 -44.32 -40.72 -37.38

Pressure drop vs. Reynolds number for the nanofluid is shown in Fig. 4. Drag reduction

was observed for the smallest investigated Re, i.e. between 10000-30000. For Re higher than

30000 the effect diminished. The highest drag reduction was obtained for the lowest

investigated concentration of Rokacet O7, i.e. 500 ppm. Also concentration of 1000 ppm

showed comparable results. The smallest extend of drag reduction effect was observed for the

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highest examined concentration of the surfactant, i.e. 4000 ppm. Application of Rokacet O7

caused drag reduction effect in copper(II) oxide nanofluid. The best results were obtained for

the smallest applied concentration, which may suggest that for slightly smaller surfactant

concentrations effect might be better. Increase of Rokacet O7 concentration had negative

influence on pressure drop. It was caused by higher viscosity of the nanosuspension, which

was not balanced by drag reduction phenomenon.

Fig. 4. Log-log diagram of surfactant concentration influence on nanofluid pressure drop.

4. CONCLUSION

In this study the properties of copper(II) oxide – water nanofluids were analyzed. The

influence of two nonionic surfactants addition on the particle size distribution, zeta potential,

nanofluid flow and thermal properties were examined. Basing on received results the

following conclusions can be drawn:

1. The addition of surfactant had no significant impact on the relative thermal

conductivity coefficient value.

2. As it should be expected the addition of surfactant increased the value of dynamic

viscosity coefficient. The lower the temperature, the more visible were the changes.

3. Small concentrations of surfactants had no influence on the PSD. Large aggregates

appeared only at the highest dose of polymers.

4. The zeta potential of nanofluids was constant for surfactant doses below 2000 ppm.

Higher doses resulted in reduced its absolute value.

5. The addition of Rokacet O7 had noticeable influence on pressure drop of nanofluid.

The best effect were obtained for Re smaller than 30000 and the smallest investigated

surfactant concentration.

This work was supported by the State Committee for Scientific Research (Poland) under grant no. N N209 764040.

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