Enhanced Thermal Conductivity of High Internal Phase Emulsions

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Enhanced Thermal Conductivity of High Internal Phase Emulsions with Ultra-Low Volume Fraction of Graphene Oxide Tanesh Gamot, Arup Ranjan Bhattacharyya, Tam Sridhar, Alex James Fulcher, Fiona Beach, Rico F. Tabor, and Mainak Majumder Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04116 • Publication Date (Web): 17 Jan 2019 Downloaded from http://pubs.acs.org on January 19, 2019

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Enhanced Thermal Conductivity of High Internal Phase Emulsions with Ultra-Low Volume Fraction of Graphene Oxide Tanesh D. Gamot†‡‖¥, Arup R. Bhattacharyya‡*, Tam Sridhar§, Alex J. Fulcher£, Fiona Beach¶, Rico F. Tabor┴ Mainak Majumder*‖¥ †IITB-Monash Research Academy, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India ‡Department of Metallurgical engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India ‖Nanoscale Science and Engineering Laboratory (NSEL), Department of Mechanical and Aerospace Engineering, Monash University, Clayton, VIC 3800, Australia §Department of Chemical Engineering, Monash University, Clayton, VIC 3800, Australia £Monash Micro Imaging (MMI), Monash University, Clayton, VIC 3800, Australia ┴School of Chemistry, Monash University, Clayton, VIC 3800, Australia ¶Orica Mining Services, George Booth Drive, NSW 2327, Australia

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¥ARC Research Hub for Graphene Enabled Industry Transformation, Monash University, Clayton, Victoria 3800, Australia *E-mail: [email protected], [email protected]


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ABSTRACT: Thermal conductivity enhancement in a multi-phase fluid such as water-in-oil (W/O) emulsion can substantially improve efficacies in a broad range of applications. However nano-particle additives, often used to do so can catastrophically destabilize a delicate emulsion system, in our case a high internal phase emulsion (HIPE), while large concentration of additives can adversely impact practical processing aspects. Therefore, means to enhance the thermal conductivity of emulsions with minute concentration of additives ( 0.74) in a low volume fraction of continuous phase, with many useful and interesting properties such as very large surface area per volume of the continuous phase2 and solid-like viscoelastic behavior3. High internal phase Pickering emulsions (HIPPE) are another type of emulsion which has been stabilized solely by colloidal particles, instead of surfactant molecules, and are reported to possess enhanced stability4. Heat transfer characteristics of fluids and liquids, including emulsions, are crucial to broad swathe of engineering applications and has been studied in a wide variety of fluidic systems such as two-phase fluids5, suspensions6, nanofluids7, electro- & magneto-rheological fluids8,9 and emulsions10. It is also worth noting that use of emulsions for enhancing heat transfer dates back to the work of Moore et al. as early as 19599 and is relevant even today.10 Inclusion of nanoparticles to a fluid phase have been reported to enhance thermal conductivity of fluids11; however, this approach becomes increasingly challenging in multi-phase fluids12, such as emulsions5,13, because this may compromise the stability of the emulsions, particularly HIPEs which are known to undergo phase inversion and catastrophic destabilization14. While the large surface to volume ratios can indeed increase the thermal energy transfer properties, similar scaling behavior also manifests in increased particle-particle interactions and complex rheological problems affecting practical aspects such as processing, piping and pumping.


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Graphene and graphene-like materials have generated considerable interest in fundamental and applied aspects of materials science because of their properties such as large surface area-tovolume ratio, good electrical and thermal conductivity15–17, and high strength. Graphene16–18 and reduced-GO19,20 possess high thermal conductivity due to their high in-plane thermal conductivity. GO, on the other hand, have lower thermal conductivity21,22 but comparable to that of certain types of Graphite23,24 GO sheets, being a flexible 2D carbon system with interesting interfacial properties and anisotropic thermal conductivity, have not been contemplated in the literature as a means to tailor the properties of emulsions. One may argue that the large aspect ratio and planar structure of GO may provide better surface coverage, lower thermal percolation limits, unidirectional heat flow and lower interfacial resistance. Oxide nanomaterials like GO has been reported to enhance the thermal conductivity of nanofluids25 and nanoemulsion fluids26; however, the challenges in the synthesis of HIPEs meant that the possibility of creating GO stabilised-HIPEs with long-term stability and enhanced thermal conductivity has not been reported in the literature. We27 have recently shown that reduced GO can be suitably engineered to prepare water-in-oil emulsions, but they lack long- term stability. In contrast to particle-based Pickering emulsions, given the molecular nature of GO, a significantly better alternative is to use suitably modified GO as a minor additive into a molecular-surfactant system, thereby forming a weakly interacting composite stabilizer. In this article, we report high internal phase water-in-oil emulsions containing minute quantities of GO and functionalized GO, enabling them to be distributed in the water or oil phase by design. This rationalized approach to emulsion preparation have shown the dramatic influence these minute additives have on the microstructure, stability, rheology and most importantly thermal properties which sheds new insights into thermal pre-percolation behavior in emulsions. ACS Paragon Plus Environment

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EXPERIMENTAL SECTION Materials and methods Methylated canola oil (i.e. a methyl ester of fatty acids obtained from canola oil triglycerides) and the emulsifier were provided by Orica Limited, Australia. The emulsifier is a blend of polymeric stabilisers such as poly-isobutylene succinic anhydride (PIBSA) and alkanolamine such as diethanolamine28. The emulsifier can be obtained by reacting PIBSA with diethenolamine in the presence of sulphuric acid29 and is designated as E476 and has been used elsewhere30. Other components for emulsion preparation such as ammonium sulphate (Amresco Inc.), ammonium chloride (Merck Pty. Ltd.), thionyl chloride and ethylene diamine were procured from Merck Pty. Ltd. were sourced commercially. GO was synthesized using Hummers’ method31. Functionalization of GO GO was functionalized using thionyl chloride and ethylene diamine. 1 g of GO was dispersed in 50 g of thionyl chloride in presence of 1 mL DMF. It was stirred for 24 h at 70 ºC. After the completion of the reaction the reaction mixture was washed, filtered and dried in vacuum oven for 6 h. 0.5 g of the functionalized material was mixed with 40 ml of ethylene diamine and stirred for 6 h at 60 ºC. The final reaction mixture was carefully washed, filtered and dried in oven. This fGO was dispersed in the canola oil along with E476 for emulsion synthesis. Synthesis of the highly concentrated emulsion with emulsifier, GO and fGO The aqueous phase was a dispersion of GO in DI water, while the oil phase in this case is a mixture of canola oil and E476. The emulsions typically constituted 93.5 wt% of aqueous phase and 6.5 wt% of the oil phase. In addition to GO the aqueous phase also contained significant amount of ammonium sulfate and ammonium chloride (42.3 wt%). For our parametric studies, the GO concentration was varied keeping the total weight ratio of the aqueous phase and oil phase constant.


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Emulsions were prepared by heating and stirring the aqueous phase at 70 ºC. The oil phase was prepared by heating the components at 60 ºC, separately. The hot aqueous phase was then slowly added to the oil phase at a rotational speed of 700 rpm using a Jiffy impeller mounted on a Caframo BDC1850 high shear mixer for 5–10 min until a viscous brown colored coarse emulsion formed for the GO containing emulsion. Thereafter, the formed emulsion was refined for next 15 min by mixing at 1400 rpm. For synthesis of fGO containing emulsions, the fGO was dispersed in the oil phase by mild sonication for 15 min, and the same procedure was followed.

Table 1. Typical emulsion samples with their components and their compositions are tabulated below. Salt composition

Aqueous phase Oil phase composition composition

(NH4)2SO4: NH4Cl

Salt: water

oil: emulsifier

4:1

42.3:51.2

4.2:2.3

(NH4)2SO4: NH4Cl

Salt: water: GO

oil: emulsifier

4:1

42.3:51.1:0.1

4.2:2.3

(NH4)2SO4: NH4Cl

Salt: water

oil: emulsifier: fGO

4:1

42.3:51.1

4.2:2.3:0.1

Emulsion Sample composition name 93.5:6.5

Emulsion O

93.5:6.5

Emulsion G

93.5:6.5

Emulsion F

Please note that emulsion O is the parent emulsion with E476 as the stabilizer, emulsion G is the emulsion with E476 + GO, and emulsion F is the emulsion with E476 + fGO. We have used this convention throughout the paper.


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Characterization Rheological measurements Rheological measurements were carried out at room temperature using an Anton Paar modular compact rheometer (Physica MCR 301). The data were collected using a parallel-plate geometry (diameter 25mm) and the gap between the plates was 1mm in the following deformation modes: (i) Amplitude sweep oscillations in the range of strains from 0.1 to 500% at the constant frequency of 1 Hz. The amplitude sweep method was used to ensure that the obtained values of dynamic elastic moduli in a linear regime of deformations, and (ii) Frequency sweep - oscillating regimes of deformations in the range of frequencies from 0.01 to 100 Hz. Thermal imaging Thermal imaging was undertaken using a FLIR-i7 IR camera. About 1 gm of the emulsion sample was placed uniformly on a flat plate spatula and heated on a hot plate at about 60 ºC. Thermal conductivity measurements The thermal conductivity of the emulsion was measured by using TCi C-Therm thermal conductivity analyzer at 60 ºC temp. A T-shaped TCi sensor was used for measurement. Before testing the emulsion sample, the sensor was first calibrated to room temperature as well as with standard sample such as water. For testing, a very small amount (0.5 g) of the emulsion sample was smeared onto the sensor such that the sensing area is covered entirely by the sample. The sample coated sensor was kept inside a furnace to ensure uniformity of temperature and the measurements were repeated ten times to obtain statistically significant measurements. Confocal fluorescence microscopic imaging Fluorescence microscopy imaging was carried out using a Leica SP8 confocal microscope equipped with a 63X oil objective and running LAS X software (Leica MicroSystems, Manheim,


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Germany). GO and fGO were non-covalently modified with Doxorubicin (DOX) before dispersing them into aqueous phase and oil phase respectively. DOX was loaded onto GO and fGO (predetermined amount) by sonication in DOX solution (0.01 wt %, 5 mL) in DI water, followed by overnight stirring in the dark for 12 h. Unreacted DOX was removed by centrifugation at 6000 rpm for 2 h. The residual was washed, filtered and dried in vacuum at 50 ºC for 6 h. The emulsions were prepared with the DOX-modified GO and fGO using the method as discussed earlier. The samples for microscopy were prepared by squeezing tiny amount of emulsion between the glass slides. The images were taken in the fluorescence mode by using the excitation wavelength 488 nm and emission wavelength 560 nm corresponding to DOX. Pendant drop tensiometry Interfacial tensions were measured using a custom pendant drop setup running OpenDrop software version 1.132. Measurements were made by growing a droplet of dispersed phase from 2.7 mm outer diameter stainless steel blunt-tipped needle, and the continuous phase was contained within a 10 mm path-length fluorescence cuvette. Polarized light imaging Polarized light imaging was done using a Leica DM IRB microscope with an LC-PolScope– Abrio imaging system from CRI Inc. For sample preparation, a tiny amount of the emulsion was placed on a glass slide and covered with a cover slip and squeezed for uniform distribution of the sample. This is to reduce the sample thickness in order to allow the light to transmit through the opaque sample. Before imaging the sample, an oil background was taken.


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RESULTS AND DISCUSSION

1. Lipophilisation of GO

a

c

b

d

Figure 1 Lipophilisation of GO. Schematic showing chemical structure of a, GO and b, fGO. c, FTIR spectra of the GO and fGO. The spectra for fGO shows amine and amide bonds on the GO sheets distinguishing fGO from GO d, Left-to-right: Pristine canola oil, undispersed GO in canola oil and homogeneously dispersed fGO in canola oil. To address this complex problem of enhancing the thermal transport properties without adversely impacting the inherent processability & stability of emulsion O, which is a HIPE stabilized with E476 and contains a small volume fraction continuous oil phase and large volume fraction discontinuous water phase, we rationalized that if we could include GO (or its derivatives)


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in the continuous oil phase we have a strong possibility of overcoming the challenge33. The chemical structure of GO and fGO are depicted in Figure 1a and 1b. The oxygen and amine functionalization in GO (non-lipophilic) and fGO (lipophilic) respectively is indicated by the FTIR spectroscopy in Figure 1c. In the spectrum of GO, the peak intensities at 1102 cm-1 and 1723 cm-1 wavenumbers corresponds to C-O and C=O stretching vibrations34. Further, the peak intensity at 1652 cm-1 wavenumber corresponds to C=C stretching vibrations while broad peak intensity at 3360 cm-1 wavenumber corresponds to –OH stretching vibrations. In the spectrum of fGO, amide functionality of oxygen groups is clearly indicated in the reduced transmitted intensity of C-ONH2 groups at 3470 cm-1 which generally corresponds to –OH groups in GO. Further, the spectrum shows amide formation indicated by peaks at 1546 cm-1 while presence of primary amines is indicated by shift at 3470 cm-1. The antisymmetric C-N peak and shoulder between 1255-1465 cm-1 can be attributed to free amine group of EDA whose one amine group attached to carbonyl via amide linkages34. The broad spectrum between 2000 cm-1 and 2600 cm-1 can be attributed to free NH4+ groups in the dispersion formed during the functionalization. The amine functionalization has been further confirmed by Raman and elemental analysis through EDS (see Supporting Information S1). The amine functionalization of GO increases its lipophilicity and forms a stable dispersion in oil as can be seen in Figure 1d. The stable dispersion forms within 5 minutes of sonication while the as-synthesized GO do not disperse in oil even after sonicating for an hour.


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2. Aminated-GO minimizes interfacial tension enabling stable emulsion

Figure 2 Interfacial tension of the oil-water interface. Interfacial tension measurement of the oil-water biphasic mixture in presence of surfactants in each of phases alternately. Interfacial tension measurements of various oil-water interfaces were carried out with individual phase are dispersed with GO/fGO/E-476 are shown in Figure 2, it shows that the oil-water interface has the lowest interfacial tension when fGO is dispersed in oil phase. In some cases of measurement, the measurements had to be stopped because the solution of E476 approaches the interface, changes the drop volume and destabilizes the measurements. While the emulsifier reduces the interfacial tension, the interfacial tension is still higher than the interface with fGO dispersed in oil. This large reduction in interfacial tension can be attributed to the role of lipophilic amine functionality in fGO minimizing interfacial energy35.


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3. Characteristics of the emulsion O, emulsion G and emulsion F: microscopy and droplet size distribution

a

b

c

d

e

f

Figure 3 Droplet size distribution of HIPEs. Confocal microscopic images and corresponding droplet size distribution of a, emulsion O b, emulsion G and c, emulsion F. Figure 3 shows confocal images and the corresponding droplet size distribution of the emulsions. As can be seen from Figure 3a, emulsion O (with E476) has fine droplets varying between 100 nm to 5 µm and the size distribution shows log-normal distribution with mean value 3.3 µm (Figure 3d). In emulsion G, (E476 + GO), with GO in the aqueous phase, a wider droplet size distribution is observed as shown in the Figure 3b and Figure 3e. This is due to the surfactant-like properties and inhibition action of GO at the interface competing with the emulsifier E476 for preferential sorption at the interface (see Supporting Information S2). For emulsion F (E476+fGO), lipophilic GO becomes partially hydrophobic, becomes less surface active than GO and this enables the formation of an emulsion similar to O, but with fGO incorporated into the microstructure. This can be seen from the Figure3c. The asACS Paragon Plus Environment

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prepared emulsion F has finer droplets with narrower log-normal droplet size distribution with mean value 4.3 µm. It also has excellent long term stability even after eight months from the date of preparation and show no signs of phase separation or droplet coalescence, quite similar to emulsion O. 4. Rheological properties of the emulsions

a

b

c

d

Figure 4 Viscoelastic properties of HIPEs. a, Amplitude sweep measurements of emulsions. Evolution of the storage modulus G’ (filled symbols) and loss modulus G” (open symbols) with strain amplitude. γ. b, Frequency sweep measurements shows variation of the storage modulus G’


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with the increase in the strain amplitude γ in the linear viscoelastic (LVE) Regime. c-d, Steadystate shear flow measurements shows variation in the complex viscosity |η*| and shear stress τ at increasing shear rate γ. All emulsions show viscoelastic behavior characterized by dynamic oscillatory shear rheological measurements. The amplitude sweep (strain sweep) plots of the emulsions are shown in Figure 4a and demonstrates the typical evolution of the storage modulus (G') and loss modulus (G") at a constant frequency of 1 Hz. The elastic-to-viscous transition (crossover) for the emulsions occurs at a specific strain amplitude, represented as γ*, and the value of γ* for emulsion O (49.2%) is significantly different from emulsion G (29.7%) , but similar to emulsion F (49 %)36,37 - indicating that emulsion F behaves similar to emulsion O despite the presence of fGO. For the emulsion O, as shown in Figure 4a, the elastic modulus and loss modulus are linear for a large amplitude of strain and is independent of the strain in an amplitude domain up to γ*=49.2% (the cross-over point). This high value of elastic-to-viscous transition are indicative of fine droplets providing a strong cohesive interaction.38 At values higher than γ*, deformation starts and the moduli no longer remains constant. With the incorporation of the GO in the aqueous phase, the deformation takes place at lower strain amplitude because the large droplets (vide infra Figure 3b) are prone to droplet break-up.39 On the other hand, with the incorporation of the fGO in the oil phase, the fine droplet structure of the parent emulsion is preserved (vide infra Figure 3a and 3c) and the deformation occurs at strain amplitude of γ* =49%, similar to emulsion O. We also note that in the amplitude sweep plots, the elastic modulus of emulsion O is significantly higher than that of emulsion F or emulsion G. For the emulsion G, the elastic modulus as well as the γ* is much lower than emulsion O, due to the presence of surfactant-GO at the interface40 (see Supporting Information S2). This is contrary to what was observed by Nesterenko et al.41 for W/O emulsions stabilized using silica particles along with a molecular surfactant. In case of particle and ACS surfactant-stabilized emulsion, strong network-like structure Paragon Plus Environment 15

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between the particles and the surfactant increases the storage modulus in comparison to emulsion stabilized by surfactant only. In our case, the GO sheets behaves as lubricants by the virtue of their wrinkle structure and forms weak network with the surfactant resulting in lower storage modulus. These observations for emulsion G & F, reaffirms the molecular-surfactant property of GO-based surfactant as described by Megan et al.42 Figure 4b shows the frequency dependence of the storage modulus within the linear viscoelastic -the plateau at lower frequencies, represents the value of shear modulus of the emulsion. Both G and F shows high shear modulus and exhibits quasi-solid behavior even if there is variation in the microstructure. There is some non-linearity in the frequency dependence of the emulsion F which could arise from some molecular scale reorganization. The low shear modulus of G is consistent with crossover value in amplitude sweep measurements36,37. Figure 4c shows the steady-state shear flow measurements in terms of complex viscosity versus shear rate. It was found that the complex viscosity decreases with increasing shear rate for all the emulsions. Emulsion O represents typical shear-thinning behavior of high internal phase emulsions over a wide range of shear stresses. Emulsion F typically follows the trend but with lower complex viscosity by the virtue of higher droplet size (vide infra Figure 3). Figure 4d shows the variation in shear stress with respect to increase in shear rate. It can be seen that for all the three emulsions, yield stress occurs with only slight variation in the applied shear rate, which is a typical characteristic of high internal phase emulsion. However, the value of yield for emulsion O is higher than that of emulsion F and emulsion G, which can be attributed to the presence of the additives inducing droplet break-up & modification of the particle-surfactant interaction in the emulsion structure. In summary, emulsion F exhibits viscoelastic behavior with elastic-to-viscous transitions


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at strain rates very similar to the parent emulsion, O, and contrary to other additives reported does in fact reduce the viscosity and elastic modulus indicating enhanced processability.

5. Thermal imaging and thermal conductivity of the emulsions

a b c d


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Figure 5 Thermal properties of HIPE. Time-dependent IR images of a, emulsion G, b, emulsion O and c, emulsion F at 0.1 wt% GO and fGO concentration respectively. The images were taken at 15 minute intervals. Inset - Photograph of the spatulas with the emulsions on a hot plate. d, Thermal conductivity enhancement with respect to fGO concentration measured with the two independent techniques.

Figure 5 shows the IR images of (a) emulsion G, (b) emulsion O and (c) emulsion F taken after every 15 minutes (left to right). With time, the emulsion sample heats-up and locus of the IR camera indicate the temperature of the surface. Blue indicates low temp while yellow indicates high temp of the surface. The surface with high thermal conductivity, heats-up rapidly, thus it can be inferred from the Figure 5, that the heat transfer rates of emulsions containing GO (emulsion G) and fGO (emulsion F) is enhanced over the emulsion O. Emulsion F and emulsion G were explored further for the determination of thermal conductivity at several fGO and GO concentration respectively. Table S1 (Supporting Information) shows the thermal conductivity of emulsions with varying concentrations of GO and fGO measured by two independent techniques: thermal imaging (see Supporting Information S3 and S4) and TCi Thermal Analyzer. Figure 5d shows the enhancement in the thermal conductivity of the emulsion F with respect to the increase in the fGO concentration- the two measurements techniques are in close agreement to each other. At a concentration of 0.1 wt%, the enhancement is about 21%. We note that within the constraints of our experiments, increasing the fGO concentration beyond 0.1 wt% destabilizes the HIPE.


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6. Distribution of fGO and GO sheets in the emulsion

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b

c

d

20 µm

20 µm

e

20 µm

h

g

f

20 µm

20 µm

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Figure 6 Distribution of fGO and GO in HIPE. Raman spectra of a, emulsion F and b, emulsion G at 0.1 wt% fGO and GO concentration respectively. The spectra were taken with incidence laser movement of 100 nm. The spectra show D band and G band corresponding to fGO and GO distribution inside the emulsion. c-d, are the Confocal fluorescence microscopy images of emulsion F and emulsion G at 0.1 wt% DOX-tagged fGO and GO respectively. The red color indicates the location of fGO and GO in the emulsions. e-g, are the polarized light microscopy images of emulsion O, emulsion F and emulsion G respectively h-j, are the cryo-SEM micrographs of the emulsion F, O and G respectively. Raman spectra of the emulsion F and G were taken to elucidate the distribution of fGO and GO inside the emulsion (Figure 6a, b). For this, Raman laser was incident at two positions in the emulsion viz. at the droplet edge and inside the droplet. Raman spectra of GO and fGO would represent D band and G band intensities at around ~1355 cm-1 and ~1620 cm-1 respectively (see Supporting Information S1). The Raman of emulsion F was found to give D band and G band at the droplet edge while for emulsion G was found to be inside the droplet. The G band intensity of fGO was higher than GO since functionalization will impart some graphitic character to the sheets. Additionally, no fGO signal was found inside the droplet instead C-H stretching peak which could attributed to the oil phase was observed. Figure 6c, d shows the fluorescence emerging from the fGO and GO within the emulsion. fGO being dispersed in the oil phase gives red fluorescence in the continuous phase while GO at the interface emits red fluorescence over the droplets, and the continuous phase becomes transparent to red fluorescence. These images are confirmatory proof of the distribution of fGO and GO in emulsion F and emulsion G, respectively. In emulsion F, fGO is distributed in the continuous oil phase of the water-in-oil high internal phase emulsion, whereas GO is consigned primarily to the dispersed water phase of emulsion G. More importantly, the fGO


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forms a network despite the fact that the oil phase is only a minor component in the HIPE. Figure 6f shows the presence of birefringent liquid-crystalline fGO along the droplet periphery in emulsion F in consistence with the fluorescence imaging in Figure 6c while birefringence in emulsion G has strong contribution from the droplet interiors. We also note that the emulsion O, i.e. only with the surfactant does not show any birefringence in the emulsion structure (Figure 6e). A little less clear, but the cryo-SEM images of emulsion F shows that the fGO sheets are located on the external surface of the water droplets, whereas in Figure 6i, j for emulsion O and G this is not seen. (See Supporting Information S5).


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7. Modeling of heat transfer in the emulsion

a

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c


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Figure 7 Modelling of heat transfer in the emulsion. The principal difference between the two models is the fact that in first case a, the nanoscale additive is incorporated in the oil phase (comparable to emulsion F) and in the second case b, the nanoscale additive is incorporated in the water phase (comparable to emulsion G). c, Thermal conductivity enhancement of emulsion samples as a function of increase in volume fraction of the fGO and GO in oil and aqueous phase respectively (log-log scale). The solid line indicates the predicted enhancement by the model while the symbols indicates the experimentally observed values. Given that an emulsion is a two-phase fluid, one can model the heat transport in such a fluid using a combination of series and parallel resistances. We assume that there are two parallel pathways: one directly through the continuous phase and another partially through the dispersed phase and the continuous phase as shown in Figure 7. A very simple model in consistence with predictive thermal conductivity models in liquid-liquid emulsions is utilized to analyze our data. 5 The model essentially uses the rule of mixture involving the thermal conductivity and amount of all ingredients in the individual phases. The equivalent thermal conductivity Ke can be expressed as:

�� = �� + [

��

�� + ��

]………. (1)

Where φo is the volume fraction of oil phase and φw is the volume fraction of the water phase. Ko is the thermal conductivity of the oil phase and Kw is the thermal conductivity of the water phase. For the cases of our emulsions, i.e. emulsion F and emulsion G, we essentially considered if the GO additives were in the oil (emulsion F) or water phase (emulsion G), and modified the thermal conductivity accordingly. We took KE476 ~ 0.18 W/m-K (from the materials datasheet by the supplier), Koil ~ 0.18 W/m-K (from the materials datasheet by the supplier) KfGO ~ 9.7W/m-K43,


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KGO ~18 W/m-K22 for our calculations and utilized the experimentally known volume fractions of fGO, GO, oil and water phases to estimate the increase in thermal conductivity. We also took the densities of the ingredients in calculating the volume fractions viz. ρE476 ~ 0.92 (material datasheet), ρoil (material datasheet) ~ 0.915, ρfGO ~ 2.243 and ρGO ~ 1.844. The values utilized for the calculations are shown in Table S2 and S3 in the supplemental section (see Supporting Information). We argue that fGO, with additional amine functionality, will have lower thermal conductivity than GO. Kim et al.43 have theoretically demonstrated that the interface between sp2 and sp3 carbons as well as clamping effect (phonon confinement between the hydrocarbon chains) arises due to additional mass, lowers the phonon modes and suppresses the thermal conductivity. Additionally, the amine functionality acts as the scattering centers on the surface and leads to loss of phonons, such behavior has been noted in GO and CNT based polymer nanocomposites45.


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8. Comparison to literature

Figure 8 Reported thermal conductivity enhancement of various emulsion systems. Comparison of thermal conductivity enhancement with an emulsion containing graphite nanoparticles. Inset: HIPE with aminated-GO shows a large enhancement in thermal conductivity at very low loading of the additive as compared to published literature till date.


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Figure 8 compares our work with the reported literatures. As will appear from the plot enhancements of upto 50% are achieved in various systems, but this required adding in excess of 5 vol % of additives which ranges from using non-ionic surfactant (Chiesa et al46), nano-water droplets (Yang et al47), alcohol (Xu et al26), oxide nano-particles (Xie et al48), nano-drops (Han et al49), graphite nano-particles (Wang et al50). The closest system which we can compare our results to is the work of Wang et al. which shows the enhancement of thermal conductivity of an emulsion system by the addition of graphite nano-particles. Our HIPE shows a pre-percolation thermal conductivity from the presence of thermallyconducting fGO which provides conductive sites in an otherwise lowly-conducting medium51–53.

This

kind

of

pre-percolation

behavior

has

observed

in

polymer

nanocomposites54,55 and other porous material systems.56

CONCLUSION We have shown that hydrophilic GO and lipophilic fGO (aminated-GO) can be incorporated easily in either the water or oil phase in HIPE stabilized by a typical emulsifier (E476 in this case). When dispersed in the oil phase, fGO forms a weak-network composite emulsion stabilizer, maintains the processability, stability and microstructure of the parent emulsion and forms a prepercolating liquid-crystalline network in the low volume fraction of the continuous phase. GO on the other hand, competes with the emulsifier, remains preferentially in the high volume fraction water phase and changes the microstructure of the HIPE to coarser droplets. Both these emulsions show enhanced thermal conductivities, enhancements can be modelled by simple heat transfer circuit diagrams. This is the first demonstration of HIPEs with enhanced thermal conductivities and will have implications in the design of heat transfer fluids, thermal phase change materials, ACS Paragon Plus Environment 26

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and high-performance emulsion explosives.


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ASSOCIATED CONTENT Supporting Information. State of the dispersion of GO and fGO in canola oil Barrier properties of GO at the interface Thermal imaging of the emulsion O and emulsion G Thermal conductivity measurements by using thermal imaging device Morphology of the emulsions: distribution of fGO and GO in the emulsion Modelling of the thermal transport in the emulsions

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]. ORCID Arup Ranjan Bhattacharyya: 0000-0002-2099-2655 Rico F. Tabor: 0000-0003-2926-0095 Mainak Majumder: 0000-0002-0194-9387

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources ORICA Mining Services, NSW, Australia has supported the research by funding for consumables and equipment purchase.


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Notes The authors declare the following competing financial interest(s): this work gave rise to a patent WO2018222138 (Gamot, Tanesh & Majumder, Mainak & Bhattacharyya, Arup Ranjan & Beach, Fiona G & Sridhar, Tamarapu & Robinson, Kelly M., Explosive Composition, Application No.PCT/SG2018/050267). ACKNOWLEDGMENTS The author would like to acknowledge SAIF and CRNTS facility at IIT Bombay for providing characterization facilities viz. Raman spectroscopy and HR-TEM. Also, the author would like to acknowledge Central Facilities at IIT Bombay for carrying out various characterizations viz. XPS, Broadband dielectric spectroscopy, confocal microscopy, cryo-SEM, cryo-TEM and Rheology. The authors would also acknowledge the facilities, scientific and technical assistance of Monash Micro Imaging, Monash University, Victoria, Australia. The work is supported financially by ORICA Mining Services, NSW, Australia. This research was conducted by the Australian Research Council Research Hub for Graphene Enabled Industry Transformation (project number H 150100003) and partially funded by this program.

ABBREVIATIONS GO Graphene Oxide, fGO amine-functionalized graphene oxide, HIPE high internal-phase emulsion, W/O water-in-oil


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Enhanced Thermal Conductivity of High Internal Phase Emulsions

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